Detailed DC system design

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Physical evaluation

Location: Pampachiri, Apurimac, Peru
GPS coordinates: -14.1870656, -73.5445298
Altitude: 3378 m
Description: A two story adobe home in the Peruvian Andes with only DC power needs. The home is used all year and the loads are used consistently throughout the year. Load usage is primarily during the evening/night. The homeowners do not intend on adding any major appliances in the near future.

  • Photo of the home.

  • Location of the home.

Contents

Load evaluation

Step 1: Fill out DC load chart

April - September October - March
# Load Quantity Watts Total watts Duty cycle Hours per day Days per week Average daily DC watt-hours Hours per day Days per week Average daily DC watt-hours
1 LED light 6 5 W 30 W 1 3 hours 7 days 90 Wh 3 hours 7 days 90 Wh
2 Radio 1 6 W 6 W 1 5 hours 7 days 30 Wh 5 hours 7 days 30 Wh
3 Cell phone 2 10 W 20 W 1 1 hours 7 days 20 Wh 1 hours 7 days 20 Wh
  • Load: The make and model or type of load.
  • Quantity: The number of the particular load.
  • Watts: The power rating in watts of the load.
  • Total watts = Quantity × Watts
  • Duty cycle = Rated or estimated duty cycle for the load. If the load has no duty cycle a value of 1 should be used. A load with a duty cycle of 20% would be inputted as .2
  • Hours per day: The maximum number of hours the load(s) will be operated per day. If the load has a duty cycle 24 hours should be used.
  • Days per week: The maximum number of days the load(s) will be operated per week.
  • Average daily DC watt-hours = Total watts × Duty cycle × Hours per day × Days per week ÷ 7 days

Step 2: Determine DC energy demand

Total average daily DC watt-hours (April - September) = sum of Average daily DC watt-hours for all loads for April - September
= 140 Wh
Total average daily DC watt-hours (October - March) = sum of Average daily DC watt-hours for all loads for October - March
= 140 Wh

Total average daily energy demand

The total energy demand for the system is the added Average daily DC-watt hours and Average daily AC watt-hours for each time period.

Average daily watt-hours required (April - September) = Total average daily DC watt-hours (April - October) + Total average daily AC watt-hours (April - September)
= 140 Wh
Average daily watt-hours required (April - September) = Total average daily DC watt-hours (October - March) + Total average daily AC watt-hours (October - March)
= 140 Wh

Weather and solar resource evaluation

Maximum ambient temperature = 23°C
Minimum ambient temperature = 2°C
Maximum indoor temperature = 20°C
Minimum indoor temperature = 10°C

  • Retrieving PVGIS monthly insolation data.

  • Retrieving PVGIS monthly insolation data.

  • Retrieving PVGIS weather data.

  • Retrieving PVGIS weather data.

Load and solar resource comparison

Step 1: Determine monthly ratio of energy demand to solar resource

Month Average monthly insolation Total average daily energy demand Ratio
January 193.85 kWh/m² 140 Wh .722
February 162.2 kWh/m² 140 Wh .86
March 179.81 kWh/m² 140 Wh .78
April 174.98 kWh/m² 140 Wh .8
May 214.31 kWh/m² 140 Wh .65
June 200.05 kWh/m² 140 Wh .7
July 210.35 kWh/m² 140 Wh .67
August 229.96 kWh/m² 140 Wh .61
September 126.87 kWh/m² 140 Wh 1.1
October 214.82 kWh/m² 140 Wh .65
November 212.91 kWh/m² 140 Wh .66
December 176.98 kWh/m² 140 Wh .79

Step 2: Determine design values

Design daily insolation = Average monthly insolation from month with the highest ratio ÷ 30
= 126.87 kWh/m² ÷ 30 = 4.23 kWh/m²
Design daily watt-hours required = Total average daily energy demand from month with the highest ratio
= 140 Wh

Design parameters

System voltage parameter = 12 V

  • The system, based upon the load evaluation, will be relatively small. A 12 V system makes the most sense.

Irradiance safety parameter = 1.25

  • The irradiance safety parameter is always the same.

Continuous duty safety parameter = 1.25

  • The continuous duty safety parameter is always the same.

Low voltage disconnect parameter = 11.5 V

  • A simple charge controller with a pre-programmed low voltage disconnect will be used.

Energy storage sizing and selection

Step 1: Determine depth of discharge parameter
For this project a depth of discharge of .4 (40%) is a good compromise.

  • Depth of discharge = .4

Step 2: Determine days of autonomy parameter
The home is used daily and providing lighting is very important, but at the same time the budget for the project is limited. The users are willing to adjust their consumption during periods of poor weather according to the state of charge of the energy storage system.

  • Days of autonomy = 2

Step 3: Determine battery temperature correction factor
The minimum indoor temperature was determined to be 10°C. An AGM battery will be used to avoid regular maintenance.

  • Battery temperature correction factor = 1.08

Correction factors for various battery types:[1]

Temperature FLA AGM Gel
25°C 1.00 1.00 1.00
20°C 1.06 1.03 1.04
15°C 1.13 1.05 1.07
10°C 1.19 1.08 1.11
5°C 1.29 1.14 1.18
0°C 1.39 1.20 1.25
-5°C 1.55 1.28 1.34
-10°C 1.70 1.35 1.42

Step 4: Calculate total Ah required

Total Ah required = Average daily Watt-hours required ÷ System voltage parameter × Battery temperature correction factor (Step 3) × Days of autonomy parameter (Step 2) ÷ Depth of discharge parameter (Step 1)
= 140 Wh ÷ 12 V × 1.08 × 2 days ÷ .4 = 43 Ah

Step 5: Calculate number of batteries in series
A 12 V battery is ideal for a system of this size.

Batteries in series = System voltage parameter ÷ Chosen battery voltage
= 12 V ÷ 12 V
= 1 × 12 V battery is sufficient

Step 6: Calculate number of batteries in parallel
An MK Deka 12V 55Ah AGM battery will be used with this design. Specifications sheet

Batteries in parallel = Total Ah required (step 4) ÷ Chosen battery Ah rating
= 43 Ah ÷ 55 Ah = .78
= Round up to 1 × 55 Ah battery.

Step 7: Calculate final Ah capacity

Final Ah capacity = Number of batteries in parallel (Step 7) × Chosen battery Ah rating
= 1 battery in parallel × 55 Ah = 55Ah

Minimum PV source size

Step 1: Deteremine PV source loss parameters

The PV module(s) will be mounted on a pole mount system.

  • Module degradation parameter = .94
  • Shading loss parameter = .95
  • Soiling loss parameter = .97
  • Wiring loss parameter = .96
  • Module mismatch parameter = 1
  • Mounting system temperature adder = 20°C for a pole mount
  • PV source temperature loss parameter = -.48%/°C
PV source temperature loss parameter = 1 + (Maximum ambient temperature + Mounting system temperature adder - 25°C) x Temperature coefficient of max power %/°C ÷ 100
= 1 + (23°C + 20°C - 25°C) x -.48%/°C ÷ 100 = .91
Total PV source loss parameter = Module degradation parameter × Shading loss parameter × Soiling loss parameter × Wiring loss parameter × Module mismatch parameter × PV source temperature loss parameter
= .94 × .95 × .97 × .96 × 1 × .91 = .76

Step 2: Charge controller efficiency parameter

All charge controllers lose a certain percentage of all energy that is produced as heat as it is transferred to the energy storage system and loads. For both PWM and MPPT charge controllers a value of .98 (98% efficient) can be used.

Step 3: Energy storage efficiency parameter

The system will use an AGM battery, which is a VRLA battery.

  • Valve-regulated lead acid (VRLA) battery efficiency = .85 (85% efficient)

Step 4: Deteremine minimum size of the PV source

Minimum PV source size = Design daily watt-hours required ÷ Design daily insolation ÷ Total PV source loss parameter (Step 1) ÷ Charge controller efficiency parameter (Step 2) ÷ Energy storage efficiency parameter (Step 3)
= 140 Wh ÷ 4.23 kWh/m² ÷ .76 ÷ .98 ÷ .85 = 52 W

Step 5: Determine charge controller type

A PWM charge controller is a reliable, low-cost option for a small system like this.

PWM charge controller sizing and selection

Step 1: Determine PV module power rating

The chosen system voltage limits the choices of modules and configurations that are possible with a PWM charge controller. A 12 volt system requires 1 × 36-cell module per string. The minimum size was determined to be 52 W.
A Dasol 80 W, 36-cell polycrystalline PV module will be used for the design. Specifications sheet

PV module power rating = 80 W
Number of modules in series = 1 module

Step 2: Determine proposed module configuration

This calculation will give a minimum number of PV modules - the result should always be rounded up. Different modules sizes and configurations can be explored to find the optimal design.

Minimum number of PV modules = Minimum PV source size ÷ PV module power rating (Step 1)
= 52 W ÷ 80 W = .65
= 1 × 80 W module.
Minimum number of modules in parallel = Minimum number of PV modules ÷ Number of modules in series (Step 1)
= 1 ÷ 1 = 1

Step 3: Verify excess production

Proposed PV source low insolation production = PV module power rating (Step 1) × Minimum number of PV modules (Step 2) × Total PV source loss parameter × Design daily insolation × Charge controller efficiency parameter × Energy storage efficiency parameter
= 80 W × 1 × .76 × 4.23 kWh/m² × .98 × .85 = 214 Wh
Daily excess production in Ah = (Proposed PV source low insolation production - Design daily watt-hours required) ÷ System voltage parameter
= (214 Wh - 140 Wh) ÷ 12 volts = 6.2 Ah
Ah used at full depth of discharge = Final Ah capacity × Depth of discharge parameter
= 55Ah × .5 = 27.5 Ah
Time to reach full state of charge = Ah used at full depth of discharge ÷ Daily excess production in Ah
= 27.5 Ah ÷ 6.2 Ah = 4.4 days

The battery will be able to reach full state of charge while using loads in 4.4 days, which is less than the maximum of 7 days. The design is okay.

Step 4: Verify charging current

This system will use an AGM battery, so that charge current from the PV source should be between .05 (5%) and .2 (20%) of its C/20 rating.

It is necessary to check the minimum required charge current against the available charge current from the proposed PV source power rating.

Available charging current = Maximum power current (Imp) × Minimum number of PV modules in parallel
= 4.4 A × 1 = 4.4 A
Percentage of C/20 rate = Available charging current ÷ Final Ah capacity
= 4.4 A ÷ 55 Ah = .08

The PV source can supply .08 (8%) of the C/20 current rating of the energy storage system, which is more than .05 (5%) and less than .2 (20%). The PV source configuration is okay.

Step 5: Determine final number of PV modules

Determine a final number of modules and a series/parallel configuration that can meet the requirements of Step 1, Step 2, Step 3, and Step 4.

Final number of PV modules = 1
Final number of PV modules in series = 1
Final number of PV modules in parallel = 1

Step 6: Total PV source current

This calculation will give a minimum current rating to use as a basis for selecting the charge controller. The Isc rating of the PV module can be found on its specifications sheet.

Total PV source current = Final number of PV modules (Step 5) × Isc rating of chosen module (Step 1) × Irradiance safety parameter
= 1 × 4.85 A × 1.25 = 6.1 A

Step 7: Select a charge controller

The final chosen charge controller should:

  1. Function at the system voltage.
  2. The charge controller(s) should have a total current rating that is larger than the minimum current rating (Step 6).

A Morningstar SHS-10 PWM charge controller will be used. Specifications sheet Multi-lingual user manual

The result of the following equation should always be rounded up.

Number of charge controllers = Total PV source current (Step 6) ÷ Chosen charge controller current rating
= 6.1 A ÷ 10 A = .61 = 1 (round up to 1 charge controller)

Step 8: Determine final PV source power rating

PV source power rating = PV module power rating (Step 1) × Final number of PV modules in parallel (Step 5)
= 80 W × 1 = 80 W

Wire, overcurrent protection, and disconnect sizing and selection

The chosen wire for a circuit must meet the requirements set out in each phase of this process. The wire size must be increased if it fails to meet any of these phases and then the process must be performed again with the new wire size.

PV source circuit

Phase 1: Maximum circuit current

Maximum circuit current = PV module Isc × Irradiance safety parameter
= 4.85 A × 1.25 = 6.1 A

Phase 2: Wire ampacity

There will only be two current-carrying conductors in the conduit. The maximum ambient temperature 23°C. 90°C rated PV wire will be used for this circuit.

The minimum wire size for a circuit can be using the following steps:

  1. Determine the ambient temperature correction factor based upon the maximum ambient temperature using the ambient temperature correction factor table. The ambient temperature correction factor for this circuit is 1.04 because it is a 90°C rated wire with a maximum ambient temperature of 23°C.
  2. Determine the conduit fill correction factor based upon the number of number of conductors in the conduit using the conduit fill correction factor table There will only be two current-carrying conductors in the conduit, so the conduit correction fill factor is 1.
  3. Determine the total wire correction parameter based upon the smaller of: the Ambient temperature correction multiplied by the Conduit fill correction factor or .8 (a safety factor from the US National Electrical code).
    4. Total wire correction parameter = Smaller of (Ambient temperature correction factor × Conduit fill correction factor) or .8
    = Smaller of (1.04 × 1) or .8 = .8
  4. Determine minimum wire ampacity. Divide the maximum circuit current (Phase 1) by the total wire correction factor (Step 3)
    5. Minimum wire ampacity = Maximum circuit current (Phase 1) ÷ Total wire correction parameter (Step 3)
    = 6.1 A ÷ .8 = 7.6 A
  5. Select a wire size with a maximum rated ampacity equal to or above the minimum wire ampacity calculated in the previous step using the allowable wire ampacity table. A 4 mm² PV wire with an ampacity rating of 30 A at 90°C will be used for this circuit as this is a commonly used wire type for a circuit of this type. 30 A is larger than the required 7.6 A ampacity.

Phase 3: Overcurrent protection and disconnects

This circuit, since it only has 1 PV module, does not require an OCPD, but one will be located near the PV equipment to provide a PV power source disconnect.

The appropriate overcurrent protection device size can be determined by:

  1. Determine the minimum size
    2. Minimum OCPD size = Maximum circuit current (Phase 1) × 1.25
    = 7.6 A × 1.25 = 9.5 A
  2. A standard 10 A DC breaker will be used. This is breaker is larger than the required minimum OCPD size, but smaller than the rated 30 A ampacity of the wire that it will protect.
  3. Verify that the chosen OCPD size from Step 2 will protect the wire size chosen in Phase 2 from excessive current under the conditions of use. The current rating of the chosen breaker size (Step 2) must be less than the calculated maximum current under conditions of use unless the calculated maximum current under conditions of use is between standard OCPD values, in this case the next largest breaker can be chosen.
    Maximum current under conditions of use = Wire ampacity from allowable wire ampacity table × Total wire correction parameter (Phase 2).
    = 30 A × .8 = 24 A
    Verify OCPD under conditions of use = Maximum current under conditions of use must be greater than or equal to the current rating of the chosen OCPD (Step 2)
    = 24 A is greater than 10 A.

The OCPD size is okay.

Phase 4: Voltage drop

If the voltage drop for the wire chosen in Phase 2 for a particular circuit is not within the recommended values, then using a larger sized wire should be considered. Increasing the wire size will not affect any other part of this process; the calculated OCPD size can remain the same.

This circuit will be 6 meters long one-way. It is 4 mm² wire with a resistance value in (Ω/Km) of 6.73 Ω. It is recommended that the voltage drop between the PV source and the charge controller be kept below 2%. The circuit current is the Imp of the PV module: 4.44 A. The nominal circuit voltage is the Vmp of the PV module: 18 V.

Voltage drop = 2 x Circuit current x One-way circuit length (m) x Resistance (Ω/Km) ÷ 1000
= 2 × 4.44 A × 6m × 6.73 Ω ÷ 1000 = .36 V
Percentage voltage drop = Voltage drop ÷ nominal circuit voltage x 100
= .36 V ÷ 18 V × 100 = 2%

2% voltage drop for this circuit is acceptable. The wire size is okay.

Phase 5: Terminal temperature ratings

The PV wire is rated at 90°C, but the terminals on the charge controller and breaker are rated at 75°C. It must be verified that the OCPD will still protect these terminals at their lower temperature rating. If the wire is not protected by the OCPD at the temperature rating of the terminals in the circuit, then the wire size must be increased until the circuit passes this test. No other calculations need to be performed again. Increasing the wire size will have the added benefit of decreasing voltage drop.

Step 1: Maximum ampacity based upon terminal temperature rating

Maximum ampacity based upon terminal temperature rating = Wire size (Phase 2) ampacity at terminal temperature rating from allowable wire ampacity table × Total wire correction parameter (Phase 2)
= 25 A × .8 = 20 A

Step 2: Verify OCPD protection of terminals

Verify OCPD protection of terminals = Maximum current considering terminal temperature rating (Step 1) must be greater than or equal to the current rating of the chosen OCPD (Phase 3)
= 20 A is greater than the 10 A current rating of the breaker

Wire and OCPD size are okay.

Charge controller output circuit/battery circuit

This same size wire will be used from the charge controller to the battery with one overcurrent protection device which will also serve as a power source disconnect.

Phase 1: Maximum circuit current

Maximum circuit current = Current rating of the charge controller
= 10 A

Phase 2: Wire ampacity

There will only be two current-carrying conductors in the conduit. The maximum indoor temperature 20°C. 90°C dry / 75° wet rated wire will be used for this circuit.

The minimum wire size for a circuit can be using the following steps:

  1. Determine the ambient temperature correction factor based upon the maximum ambient temperature using the ambient temperature correction factor table. The ambient temperature correction factor for this circuit is 1.08 because it is a 90°C dry / 75° wet rated wire with a maximum indoor temperature of 20°C.
  2. Determine the conduit fill correction factor based upon the number of number of conductors in the conduit using the conduit fill correction factor table There will only be two current-carrying conductors in the conduit, so the conduit correction fill factor is 1.
  3. Determine the total wire correction parameter based upon the smaller of: the Ambient temperature correction multiplied by the Conduit fill correction factor or .8 (a safety factor from the US National Electrical code).
    4. Total wire correction parameter = Smaller of (Ambient temperature correction factor × Conduit fill correction factor) or .8
    = Smaller of (1.08 × 1) or .8 = .8
  4. Determine minimum wire ampacity. Divide the maximum circuit current (Phase 1) by the total wire correction factor (Step 3)
    5. Minimum wire ampacity = Maximum circuit current (Phase 1) ÷ Total wire correction parameter (Step 3)
    = 10 A ÷ .8 = 12.5 A
  5. Select a wire size with a maximum rated ampacity equal to or above the minimum wire ampacity calculated in the previous step using the allowable wire ampacity table. A 4 mm² 90°C dry / 75° wet rated wire with an ampacity rating of 30 A at 90°C will be used for this circuit as this is a commonly used wire type for a circuit of this type. 30 A is larger than the required 12.5 A ampacity.

Phase 3: Overcurrent protection and disconnects

This circuit requires an OCPD/disconnect as it is connected directly to the battery which is a power source.

The appropriate overcurrent protection device size can be determined by:

  1. Determine the minimum size
    2. Minimum OCPD size = Maximum circuit current (Phase 1) × 1.25
    = 10 A × 1.25 = 12.5 A
  2. A standard 13 A DC breaker will be used. This is breaker is larger than the required minimum OCPD size, but smaller than the 30 A rated ampacity of the wire that it will protect.
  3. Verify that the chosen OCPD size from Step 2 will protect the wire size chosen in Phase 2 from excessive current under the conditions of use. The current rating of the chosen breaker size (Step 2) must be less than the calculated maximum current under conditions of use unless the calculated maximum current under conditions of use is between standard OCPD values, in this case the next largest breaker can be chosen.
    Maximum current under conditions of use = Wire ampacity from allowable wire ampacity table × Total wire correction parameter (Phase 2).
    = 30 A × .8 = 24 A
    Verify OCPD under conditions of use = Maximum current under conditions of use must be greater than or equal to the current rating of the chosen OCPD (Step 2)
    = 24 A is greater than 13 A.

The OCPD size is okay.

Phase 4: Voltage drop

If the voltage drop for the wire chosen in Phase 2 for a particular circuit is not within the recommended values, then using a larger sized wire should be considered. Increasing the wire size will not affect any other part of this process; the calculated OCPD size can remain the same.

This circuit will be 1.5 meters long one-way. It is 4 mm² wire with a resistance value in (Ω/Km) of 6.73 Ω. It is recommended that the voltage drop between the charge controller and the energy storage system be kept below 1.5%. The circuit current is the Imp of the PV module: 4.44 A. The nominal circuit voltage is the system voltage: 12 V.

Voltage drop = 2 x Circuit current x One-way circuit length (m) x Resistance (Ω/Km) ÷ 1000
= 2 × 4.44 A × 1.5m × 6.73 Ω ÷ 1000 = .09 V
Percentage voltage drop = Voltage drop ÷ Nominal circuit voltage voltage x 100
= .09 V ÷ 12 V × 100 = .75%

.75% voltage drop for this circuit is acceptable. The wire size is okay.

Phase 5: Terminal temperature ratings

The wire is rated at 90°C dry / 75° wet, but the terminals on the charge controller and breaker are rated at 75°C. It must be verified that the OCPD will still protect these terminals at their lower temperature rating. If the wire is not protected by the OCPD at the temperature rating of the terminals in the circuit, then the wire size must be increased until the circuit passes this test. No other calculations need to be performed again. Increasing the wire size will have the added benefit of decreasing voltage drop.

Step 1: Maximum ampacity based upon terminal temperature rating

Maximum ampacity based upon terminal temperature rating = Wire size (Phase 2) ampacity at terminal temperature rating from allowable wire ampacity table × Total wire correction parameter (Phase 2)
= 25 A × .8 = 20 A

Step 2: Verify OCPD protection of terminals

Verify OCPD protection of terminals = Maximum current considering terminal temperature rating (Step 1) must be greater than or equal to the current rating of the chosen OCPD (Phase 3)
= 20 A is greater than the 13 A current rating of the breaker

Wire and OCPD size are okay.

Charge controller load circuit

Phase 1: Maximum circuit current

Maximum circuit current = Current rating of the charge controller lighting/load circuit
= 10 A

Phase 2: Wire ampacity

There will only be two current-carrying conductors in the conduit. The maximum indoor temperature 20°C. 90°C dry / 75° wet rated wire will be used for this circuit.

The minimum wire size for a circuit can be using the following steps:

  1. Determine the ambient temperature correction factor based upon the maximum ambient temperature using the ambient temperature correction factor table. The ambient temperature correction factor for this circuit is 1.08 because it is a 90°C dry / 75° wet rated wire with a maximum indoor temperature of 20°C.
  2. Determine the conduit fill correction factor based upon the number of number of conductors in the conduit using the conduit fill correction factor table There will only be two current-carrying conductors in the conduit, so the conduit correction fill factor is 1.
  3. Determine the total wire correction parameter based upon the smaller of: the Ambient temperature correction multiplied by the Conduit fill correction factor or .8 (a safety factor from the US National Electrical code).
    4. Total wire correction parameter = Smaller of (Ambient temperature correction factor × Conduit fill correction factor) or .8
    = Smaller of (1.08 × 1) or .8 = .8
  4. Determine minimum wire ampacity. Divide the maximum circuit current (Phase 1) by the total wire correction factor (Step 3)
    5. Minimum wire ampacity = Maximum circuit current (Phase 1) ÷ Total wire correction parameter (Step 3)
    = 10 A ÷ .8 = 12.5 A
  5. Select a wire size with a maximum rated ampacity equal to or above the minimum wire ampacity calculated in the previous step using the allowable wire ampacity table. A 4 mm² 90°C dry / 75° wet rated wire with an ampacity rating of 30 A at 90°C will be used for this circuit as this is a commonly used wire type for a circuit of this type. 30 A is larger than the required 12.5 A ampacity.

Phase 3: Overcurrent protection and disconnects

This circuit does not require an OCPD if the available current is limited by another OCPD or the charge controller - in this case there is both a 13 A OCPD on the connection to the battery and the charge controller itself is current limited to 10 A - to a maximum current that is below the ampacity rating of the wire. It is necessary to check to make sure that the terminals and wire will be protected by the other OCPD under the conditions of use - the calculation from Phase 5 can be used here.

Step 1: Maximum ampacity based upon terminal temperature rating

Maximum ampacity based on terminal temperature rating = Wire size (Phase 2) ampacity at terminal temperature rating from allowable wire ampacity table × Total wire correction parameter (Phase 2)
= 25 A × .8 = 20 A

Step 2: Verify OCPD protection of terminals

Verify OCPD protection of terminals = Maximum current considering terminal temperature rating (Step 1) must be greater than or equal to the current rating of the chosen OCPD (Phase 3 or the maximum charge controller output)
= 20 A is greater than the 13 A current rating of the breaker on the charge controller output circuit and the 10 A rating of the charge controller load circuit

No OCPD required

Phase 4: Voltage drop

If the voltage drop for the wire chosen in Phase 2 for a particular circuit is not within the recommended values, then using a larger sized wire should be considered. Increasing the wire size will not affect any other part of this process; the calculated OCPD size can remain the same.

This circuit will be .25 meters long one-way. It is 4 mm² wire with a resistance value in (Ω/Km) of 6.73 Ω. It is recommended that the voltage drop between the charge controller be kept below 5% for any lights and below 3% for any loads. The circuit current is the current required by The circuit nominal voltage is the Nominal circuit voltage is the system voltage: 12 V.

Charge controller load circuit current = Power rating of all DC loads ÷ Nominal circuit voltage
= 56 W ÷ 12 V = 4.67 A
Voltage drop = 2 x Circuit current x One-way circuit length (m) x Resistance (Ω/Km) ÷ 1000
= 2 × 4.67 A × .25m × 6.73 Ω ÷ 1000 = .016 V
Percentage voltage drop = Voltage drop ÷ Nominal circuit voltage x 100
= .016 V ÷ 12 V × 100 = .13%

.13% voltage drop for this circuit is acceptable. The wire size is okay.

Phase 5: Terminal temperature ratings

This check was performed in Phase 3 to verify that no OCPD is required.

DC branch circuit example

All DC branch circuits can be calculated in the same way as this example. This circuit has 3 × 5 W light bulbs.

Phase 1: Maximum circuit current

Maximum circuit current = Power rating of all DC loads on the circuit ÷ Nominal circuit voltage
= (3 × 5 W) ÷ 12 V = 1.25 A

Phase 2: Wire ampacity

There will only be two current-carrying conductors in the conduit. The maximum indoor temperature 20°C. 90°C dry / 75° wet rated wire will be used for this circuit.

The minimum wire size for a circuit can be using the following steps:

  1. Determine the ambient temperature correction factor based upon the maximum ambient temperature using the ambient temperature correction factor table. The ambient temperature correction factor for this circuit is 1.08 because it is a 90°C dry / 75° wet rated wire with a maximum indoor temperature of 20°C.
  2. Determine the conduit fill correction factor based upon the number of number of conductors in the conduit using the conduit fill correction factor table There will only be two current-carrying conductors in the conduit, so the conduit correction fill factor is 1.
  3. Determine the total wire correction parameter based upon the smaller of: the Ambient temperature correction multiplied by the Conduit fill correction factor or .8 (a safety factor from the US National Electrical code).
    4. Total wire correction parameter = Smaller of (Ambient temperature correction factor × Conduit fill correction factor) or .8
    = Smaller of (1.08 × 1) or .8 = .8
  4. Determine minimum wire ampacity. Divide the maximum circuit current (Phase 1) by the total wire correction factor (Step 3)
    5. Minimum wire ampacity = Maximum circuit current (Phase 1) ÷ Total wire correction parameter (Step 3)
    = 1.25 A ÷ .8 = 1.56 A
  5. Select a wire size with a maximum rated ampacity equal to or above the minimum wire ampacity calculated in the previous step using the allowable wire ampacity table. A 2.5 mm² 90°C dry / 75° wet rated wire with an ampacity rating of 25 A at 90°C will be used for this circuit as this is a commonly used wire type for a circuit of this type. 25 A is larger than the required 1.56 A ampacity.

Phase 3: Overcurrent protection and disconnects

This circuit does not require an OCPD if the available current is limited by another OCPD or the charge controller - in this case there is both a 13 A OCPD on the connection to the battery and the charge controller itself is current limited to 10 A - to a maximum current that is below the ampacity rating of the wire. It is necessary to check to make sure that the terminals and wire will be protected by the other OCPD under the conditions of use - the calculation from Phase 5 can be used here.

Step 1: Maximum ampacity based upon terminal temperature rating

Maximum ampacity based on terminal temperature rating = Wire size (Phase 2) ampacity at terminal temperature rating from allowable wire ampacity table × Total wire correction parameter (Phase 2)
= 20 A × .8 = 16 A

Step 2: Verify OCPD protection of terminals

Verify OCPD protection of terminals = Maximum current considering terminal temperature rating (Step 1) must be greater than or equal to the current rating of the chosen OCPD (Phase 3 or the maximum charge controller output)
= 16 A is greater than the 13 A current rating of the breaker on the charge controller output circuit and the 10 A rating of the charge controller load circuit

No OCPD required

Phase 4: Voltage drop

If the voltage drop for the wire chosen in Phase 2 for a particular circuit is not within the recommended values, then using a larger sized wire should be considered. Increasing the wire size will not affect any other part of this process; the calculated OCPD size can remain the same.

This circuit will be 8m meters long one-way. It is 2.5 mm² wire with a resistance value in (Ω/Km) of 10.7 Ω. It is recommended that the voltage drop between the charge controller be kept below 5% for any lights and below 3% for any loads. The circuit current is operating current and voltage of the charge controller output. Current is total current required by all of the loads on the circuit (same as calculated in Phase 1). Operating voltage is nominal voltage of the system: 12 V.

Voltage drop = 2 x Circuit current x One-way circuit length (m) x Resistance (Ω/Km) ÷ 1000
= 2 × 1.25 A × 8m × 10.7 Ω ÷ 1000 = .21 V
Percentage voltage drop = Voltage drop ÷ Circuit operating voltage x 100
= .09 V ÷ 12 V × 100 = 1.75%

1.75% voltage drop for this circuit is acceptable. The combined voltage drop of this circuit and the charge controller load circuit voltage drop is 1.88%, which is less than the recommended maximum of 3%. The wire size is okay.

Phase 5: Terminal temperature ratings

This check was performed in Phase 3 to verify that no OCPD is required.

Design summary

A three-line wiring diagram for this system.
  • DC nominal voltage: 12 V
  • Mounting system: Pole mount
  • Tilt angle: 14°
  • Azimuth: 0°

Components

Circuits

  • PV source circuit: 4 mm² 90°C PV wire. 10 A breaker as a power source disconnect.
  • Charge controller output circuit/energy storage circuit: 4 mm² 90°C dry / 75° wet rated wire. 13 A breaker as a power source disconnect.
  • Charge controller load circuit: 4 mm² 90°C dry / 75° wet rated wire. No overcurrent protection device required.
  • DC branch circuit: 2.5 mm² 90°C dry / 75° wet dry/wet rated wire. No overcurrent protection device required.

Grounding system

  • Grounding system: No grounding system due to cost and the simplicity of the system.

Notes/references

  1. ↑ Trojan Battery Company - Battery Sizing Guidelines https://www.trojanbattery.com/pdf/TRJN0168_BattSizeGuideFL.pdf
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