Difference between revisions of "Voltage drop"

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All wires have a certain amount of resistance that corresponds to their diameter which will cause a certain amount of voltage in the circuit to be lost when current is flowing. High voltage drop in a circuit will cause a lower than expected voltage which can create performances isues, but does not represent a safety hazard in itself. If a wire is improperly sized it can lead to significant loses in voltage that can cause [[:Category:Loads|loads]] to stop functioning or improper battery charging. Voltage drop must examined for all circuits in an off-grid PV system.  
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[[Category:Detailed system design basics]]
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All wires have a certain amount of resistance that corresponds to their diameter which will cause a certain amount of voltage in the circuit to be lost when current is flowing. High voltage drop in a circuit will cause a lower-than-expected voltage which can create performances issues but does not represent a safety hazard in itself. If a wire is improperly sized it can lead to significant loses in voltage that can cause [[Special:MyLanguage/Energy efficient loads|loads]] to stop functioning or improper battery charging. Voltage drop must be examined for all circuits in an off-grid PV system.  
  
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The calculation can be performed easily by hand, but there are many different free calculators available online that are much quicker - see [[wire sizing resources]]. The factors that determine voltage drop in an off-grid PV system are:
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The calculation can be performed easily by hand, but there are many different free calculators available online that are much quicker - see [[Special:MyLanguage/Resources|Resources]]. The factors that determine voltage drop in an off-grid PV system are:
 
:*The maximum circuit current.
 
:*The maximum circuit current.
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:*The resistance of the wire.
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:*The resistance of the conductor (wire) based upon its size.
 
:*The length of the circuit.
 
:*The length of the circuit.
 
:*The operating voltage of the circuit.
 
:*The operating voltage of the circuit.
  
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The formula for calculating voltage drop is:
 
The formula for calculating voltage drop is:
  
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{| class="wikitable" border=1 style="width: 80%;"
 
{| class="wikitable" border=1 style="width: 80%;"
 
! style="width: 20%"|Voltage drop
 
! style="width: 20%"|Voltage drop
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! style="text-align:left;"| = 2 x Circuit current x One-way distance (m) x  [[#Conductor resistance values table|Resistance Ω/Km]] ÷ 1000
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! style="text-align:left;"| = 2 x Circuit current x One-way circuit length (m) x  [[#Conductor resistance values|Resistance (Ω/Km)]] ÷ 1000
 
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The most important value is the percentage voltage drop for the circuit. This is calculated using the formula:
 
The most important value is the percentage voltage drop for the circuit. This is calculated using the formula:
  
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{| class="wikitable" border=1 style="width: 80%;"
 
{| class="wikitable" border=1 style="width: 80%;"
 
! style="width: 20%"|Percentage voltage drop
 
! style="width: 20%"|Percentage voltage drop
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! style="text-align:left;"| = Voltage drop ÷ Circuit operating voltage x 100
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! style="text-align:left;"| = Voltage drop ÷ Circuit nominal voltage x 100
 
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The table below has recommended maximum voltage drop values for vaious circuits. It is important to note that other components in a circuit - terminals, fuses, breakers - add resistance and will increase voltage drop as well, therefore it is important to be conservative.
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See [[Wire, overcurrent protection, and disconnect sizing and selection#Phase 4: Voltage drop|Wire, overcurrent protection, and disconnect sizing and selection - Phase 4: Voltage drop]] for recommended voltage drop values for circuits in an off-grid PV system.
  
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'''Note:''' Not all PV systems are wired the same. These voltage drop values are for total circuit length between one component and another. It is common to have DC [[Busbars|busbars]] that have one single set of wires that runs to the [[:Category:Energy storage|energy storage system]] and connect to a positive and negative DC busbar that serve as a point of connection for the [[Inverter|inverter]] and [[Charge controller|charge controller]]. In this case it is necesary to calculate the total voltage drop between the charge controller and the energy storage system, rather than just to the busbar.
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==Conductor resistance values== <!--T:8-->
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{| class="wikitable" border=1
 
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!Circuit
 
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!Maximum recommended voltage drop
 
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|PV source to charge controller
 
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|2%
 
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|-
 
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|Charge controller to energy storage
 
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|1.5%
 
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|-
 
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|Energy storage to inverter
 
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|1.5%
 
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|-
 
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|DC lighting circuits
 
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|5%
 
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|-
 
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|DC load circuits
 
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|3%
 
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|-
 
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|AC load and lighting circuits
 
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|2%
 
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|}
 
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==Conductor resistance values==
 
 
A simplified chart with resistances for copper wires.<ref name="NEC4"> NFPA 70 - National Electrical Code 2017: Chapter 9, Table 8 </ref>
 
A simplified chart with resistances for copper wires.<ref name="NEC4"> NFPA 70 - National Electrical Code 2017: Chapter 9, Table 8 </ref>
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[[Category:System design]]
 
 
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|0.2050 Ω
 
|0.2050 Ω
 
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==Notes/references== <!--T:9-->
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Latest revision as of 17:36, 15 March 2021

Other languages:
English • ‎español

All wires have a certain amount of resistance that corresponds to their diameter which will cause a certain amount of voltage in the circuit to be lost when current is flowing. High voltage drop in a circuit will cause a lower-than-expected voltage which can create performances issues but does not represent a safety hazard in itself. If a wire is improperly sized it can lead to significant loses in voltage that can cause loads to stop functioning or improper battery charging. Voltage drop must be examined for all circuits in an off-grid PV system.

The calculation can be performed easily by hand, but there are many different free calculators available online that are much quicker - see Resources. The factors that determine voltage drop in an off-grid PV system are:

  • The maximum circuit current.
  • The resistance of the conductor (wire) based upon its size.
  • The length of the circuit.
  • The operating voltage of the circuit.

The formula for calculating voltage drop is:

Voltage drop = 2 x Circuit current x One-way circuit length (m) x Resistance (Ω/Km) ÷ 1000

The most important value is the percentage voltage drop for the circuit. This is calculated using the formula:

Percentage voltage drop = Voltage drop ÷ Circuit nominal voltage x 100

See Wire, overcurrent protection, and disconnect sizing and selection - Phase 4: Voltage drop for recommended voltage drop values for circuits in an off-grid PV system.

Conductor resistance values

A simplified chart with resistances for copper wires.[1]

Copper resistance at 75°C
Size AWG Area Metric equivalent Ω/km
18 0.823 mm² 1 mm² 26.5 Ω
16 1.31 mm² 1.5 mm² 17.3 Ω
14 2.08 mm² 2.5 mm² 10.7 Ω
12 3.31 mm² 4 mm² 6.73 Ω
10 5.261 mm² 6 mm² 4.226 Ω
8 8.367 mm² 10 mm² 2.653 Ω
6 13.30 mm² 16 mm² 1.671 Ω
4 21.15 mm² 25 mm² 1.053 Ω
3 26.67 mm² — 0.833 Ω
2 33.62 mm² 35 mm² 0.661 Ω
1 42.41 mm² 50 mm² 0.524 Ω
1/0 53.49 mm² — 0.415 Ω
2/0 67.43 mm² 70 mm² 0.329 Ω
3/0 85.01 mm² 95 mm² 0.2610 Ω
4/0 107.2 mm² 120 mm² 0.2050 Ω

Notes/references

  1. ↑ NFPA 70 - National Electrical Code 2017: Chapter 9, Table 8
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