How voltage drop cable sizing works for solar and battery systems

Voltage drop cable sizing for solar PV and battery systems is one of the most consequential parts of system design yet it’s also one of the most frequently misunderstood. If a cable is inappropriately sized, best case scenario it is an overpriced commodity. However, if the cable conductor is not sized correctly to handle current flowing through it and heats up, it could burn the insulation around it and start a fire. Therefore, selecting an optimal cable size is quite important.

When you are sizing a cable, the parameters you should consider are voltage drop, operational current, and fault current. In this article we will deep dive into one of the parameters – voltage drop/rise, and see how it affects cable sizing. Specifically we are trying to make things easier for you, and identify at which cable lengths you can reasonably ignore voltage drop and when you must pay attention. What we will show in the article is PV cables <40m, Battery cables < 10m, Single phase inverter cables < 5m, and three phase inverter cables <15m can reasonably ignore voltage drop as operational current will take precedence. 

Looking at voltage in cable sizing, we will encounter two terms quite often – voltage drop and voltage rise. Voltage drop is performance loss due to the cable’s internal resistance, while voltage rise is the increase in voltage from the MSB to the inverter to allow for export of energy to the grid. The limits on permitted voltage rise and drop are governed by AS/NZS 4777.1:2024, which should be read alongside any applicable network provider requirements. This should be complied with, alongside any network provider limits. Under AS/NZS 5033:2021, a recommended 3% voltage drop is permitted on the DC side between the PV array or DC battery and the inverter. On the AC side between the point of attachment to the grid and the Inverter, a 2% voltage rise limit is permitted which includes a 1% rise from the MSB to the inverter.

The voltage drop/rise in a cable is determined by:

• Conductor area
• Length of the conductor
• Current flow through the conductor

And the equation for calculating voltage drop/rise is:

V_D = (L x I x V_C) / 1000

Where:

• V_D = actual voltage drop (in V)
• L = route length (in m), defined as the distance measured along a run of wiring from the origin of the circuit to the point of consideration
• I = current flow (in A). For PV DC cables, the ISC current (at STC) should be used.
• V_C = millivolt drop per amp-metre route length (in mV/Am)

Repurposing the above equation, we can find the length constraint for each cable size and this will help us get a good idea of how long a cable you can lay for the most conservative cable size (in this case smaller cables) which will still meet the 3% voltage drop and 2% voltage rise limits.

We can rearrange the above voltage drop/rise equation as follows:

L = (V_D x 1000) / (I x V_C)

Where:

• V_D = Loss% permitted x System Voltage
• V_C = Values can be found in AS/NZS 3008.1.1:2025 Standard Tables
• I = Assumptions based on the most conservative components in the market currently

For a detailed breakdown of what changed in this standard and how it affects solar cable design, see our AS/NZS 3008.1.1:2025 guide.

PV System DC cables

For voltage drop cable sizing in solar PV systems, a commonly used cable is a Copper (Cu) conductor with XLPE insulation and cable size of 4mm² (unless your PV strings are paralleled). Let’s assume a single string for this as it is much more common, and a conservatively high short-circuit current (I) value of 15A.

We need to consider one final assumption for the equation – voltage of the PV string. Here, again we go for the more conservative value (in this case leading to a lower system voltage). Say the V_MP value of the PV Module is 32V and there are 8 of them connected together in series. In that case the length of the cable would be as follows:

Input:

VMP = 32V

No. of modules = 8

I = 15A

Loss% = 3%

Cable size = 4mm2

Output:

               V_D = Loss% x System Voltage 

              V_D = Loss% x V_mp x No. of modules 

              V_D = 0.03 x 32 x 8 = 7.68 V

              V_C = 11.8 (for 4mm² cable as per Table 4.16 of AS/NZS 3008.1.1:2025)

              L = (V_D x 1000) / (I x V_C)

             L = (7.68 x 1000) / (15 x 11.8)

             L = 43.39m ~ 43m

Based on the above calculations, any cable length of less than 43m meets the voltage drop requirements for the PV DC cables of sizes greater than 4mm². If it exceeds this length, you should look at voltage drop calculations to find the optimal cable size. 

If you’re working through cable sizing step by step, our cable sizing correction factors worked example covers how to apply these tables in practice.

Battery System DC cables

Now, let’s look at another set of DC cables. We’ll look at DC battery systems or Section 5 Batteries (Pre-assembled battery systems) as per AS/NZS 5139:2019.

Continuing with the most conservative approach, we will look at the maximum cable length for a system composed of 2 stacked battery modules with a nominal battery voltage of 128V and operating current value of 30A. Based on the battery operating current we need a 32A Circuit Breaker (CB). The cable size needs to have Current Carrying Capacity (CCC) which is greater than this value. Assuming the cable is a Copper (Cu) conductor with XLPE insulation and looking at Table 3.22 of AS/NZS 3008.1.1:2025 we get a cable size of 4mm². Now let’s take a look at the maximum length allowed for a 3% voltage drop in the cables:

Input:

Nominal Voltage = 128V

I = 30A

Loss% = 3%

Cable size = 4mm2

Output: 

            V_D= Loss% x System Voltage

            V_D = 0.03 x 128 = 3.84 V

            V_C = 11.8 (for 4mm² cable as per Table 4.16 of AS/NZS 3008.1.1:2025)

            L = (V_D x 1000) / (I x V_C)

            L = (3.84 x 1000) / (30 x 11.8)

            L = 10.84m ~ 10m

Continuing the calculations with the above assumptions except for upsizing the cable size to 6mm² (most conservative cable size currently), the length constraint now is:

Input:

Nominal Voltage = 128V

I = 30A

Loss% = 3%

Cable size = 6mm2

Output: 

            V_D = Loss% x System Voltage 

            V_D = 0.03 x 128 = 3.84 V

            V_C = 7.85 (for 6mm² cable as per Table 4.16 of AS/NZS 3008.1.1:2025)

            L = (V_D x 1000) / (I x V_C)

            L = (3.84 x 1000) / (30 x 7.85)

            L = 16.31m ~ 16m

The conclusion from the above is that, for battery DC cables with a cable size of 4mm² a 10m length and for cable size 6mm² a 16m length meets the 3% voltage drop limits. Any lengths exceeding this will require a higher cable size.

Inverter AC cables

Finally, let’s do a deep dive into the AC cable size lengths based on the permitted 1% voltage rise allowed between the MSB and the inverter. We will take a look at both single and three-phase inverter system AC cables to the MSB for a range of sizes to determine the acceptable cable lengths.

This 1% allocation from the MSB to the inverter sits within the broader voltage rise framework explained in our AS/NZS 4777.1:2024 overview.

Single-phase Inverters (System Voltage = 230V)

L = (V_D x 1000) / (I x V_C)                          ; where V_D = Loss% x System Voltage = 0.01 x 230

L = (0.01 x 230 x 1000) / (I x V_C)

Current: I = (P / V) = P / 230

Cable size is based on CCC requirements (Table 3.16 of AS/NZS 3008.1.1:2025) which is calculated based on CB rating (based on current).

V_C value is found using Table 4.17(B) of AS/NZS 3008.1.1:2025. A factor of 1.155 is multiplied to the value as the standard table 4.17 (B) only has three-phase V_C values.

Power (kW)	Current (A)	Cable size (mm2)	VC	Length (m)
5	21.74	2.5	16.4 x 1.155	5.58 ~ 5
8	34.78	4	10.2 x 1.155	5.61 ~ 5
10	43.48	6	6.8 x 1.155	6.74 ~ 6
15	65.22	16	2.54 x 1.155	12.02 ~ 12

Three-phase Inverters (System Voltage = 400V)

L = (V_D x 1000) / (I x V_C)                          ; where V_D = Loss% x System Voltage = 0.01 x 400

L = (0.01 x 400 x 1000) / (I x V_C)

Current: I = (P / V) = P / 400

Cable size is based on CCC requirements (Table 3.19 of AS/NZS 3008.1.1:2025) which is calculated based on CB rating (based on current).

V_C value is found using Table 4.17(B) of AS/NZS 3008.1.1:2025.

Power (kW)	Current (A)	Cable size (mm2)	VC	Length (m)
10	25	4	10.2	15.69 ~ 15
15	37.5	6	6.8	15.68 ~ 15
20	50	10	4.04	19.80 ~ 19

Conclusion from the above calculations leads to cable lengths maximum of 5m for a single-phase system and 15m for three-phase system.

Maximum Cable Length Summary

As one can see, cable sizing is quite nuanced and very important. But from the conservative estimates taken above, it is reasonable to assume that when below the thresholds identified in Table 3 you will not need to consider voltage drop in your cable sizing process. The table below summarises the voltage drop cable sizing thresholds for solar PV and battery systems at a glance.

Cable	Max. Cable Length
PV DC Cable	40 m
Battery DC Cable	10 m
Single-phase Inverter AC Cable	5 m
Three-phase Inverter AC Cable	15 m

Cable sizing is one of the more technically demanding parts of solar and battery system design and getting it right requires more than just running the numbers. If you’re looking to build a solid foundation in compliant system design, GSES offers accreditation courses in Grid-Connected Solar PV Systems and Grid-Connected Battery Storage Systems, covering design principles, Australian standards, and everything in between. Enrol today and gain the accreditation to back your expertise.