Solar DC Voltage Drop Calculator
Check PV d.c. cable volt drop against the AS/NZS 5033 limit.
Vmp and Imp from the module datasheet at STC - AS/NZS 5033 measures volt drop against the string's Vmp at STC under maximum load. Cable temperature: AS/NZS 5033 Clause 4.4.2.2 says to allow at least the hottest expected air temperature + 40°C for cable near or touching the modules, so 75°C suits a 35°C day. Warmer cable means more resistance and more volt drop.
Measure from the most remote module, along the route the cable takes (including the module leads between modules), to the inverter. Enter the one-way length - the calculator doubles it for the positive and negative. Add an array cable if strings join at a combiner before the inverter.
For guidance only. These calculators are planning aids for licensed electrical workers and inspectors. All results must be independently verified against the current editions of AS/NZS 3000:2018, AS/NZS 3008.1.2, and other applicable standards before being relied on for any installation. CertMate accepts no responsibility or liability for any work carried out, decisions made, or certification issued in reliance on these results. The certifying electrical worker remains solely responsible for compliance with the Electricity (Safety) Regulations 2010 and all applicable New Zealand law.
From the furthest module to the inverter
AS/NZS 5033 measures PV d.c. voltage drop over the whole path from the most remote module in the array to the inverter's input - the module leads, the home run and any array cable after a combiner all count. It's d.c., so there's no reactance or power factor to worry about: the drop is simply the current times the resistance of the positive and negative conductors.
This calculator uses the maximum conductor resistance from IEC 60228 - the standard PV cable conductors are made to - corrected for how hot the cable runs, then checks the total against the AS/NZS 5033 limit and recommendation and shows the smallest cable that would bring it within each.
Worked examples
Roof array straight to the inverter
One string of 10 modules (Vmp 41.5 V, Imp 10.6 A) gives a 415 V string. A 30 m route in 4 mm² PV cable at 75°C drops about 0.95% - comfortably within both the 3% recommendation and the 5% limit.
Two strings combined for a long run to a shed
The same strings, 15 m of 4 mm² to a combiner, then both strings together (21.2 A) on 60 m of 6 mm² to the inverter. The total is just over 3% (3.01%) - within the 5% limit but above the recommendation. Moving the array cable up to 10 mm² brings the total back to about 1.93%.
Frequently asked questions
What is the voltage drop limit for solar PV d.c. cables in New Zealand?
AS/NZS 5033 limits the voltage drop from the most remote PV module to the inverter input to 5% of the string's Vmp at STC for arrays operating above 120 V, and recommends keeping it to 3% for performance. At 120 V or less, AS/NZS 3000 applies instead.
Why use Vmp and Imp rather than Voc and Isc?
AS/NZS 5033 sets the limit as a percentage of the Vmp at STC, under maximum load. At maximum power the string runs at its Vmp and carries its Imp, so that's the voltage and current the drop is worked out at.
Why does the cable temperature matter?
Copper's resistance rises with temperature, so a hot cable drops more voltage. Cable on a roof near the modules runs far hotter than the air - AS/NZS 5033 says to allow at least the hottest expected air temperature plus 40 degrees for cable near or touching the modules.
What's the smallest cable allowed for PV d.c. wiring?
AS/NZS 5033 requires PV d.c. cable conductors of at least 4 mm², so the calculator starts at 4 mm².
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