AC Cable Sizing Calculator
Size the AC cable from a solar inverter to the distribution board, sized on inverter output current with a 1% voltage-drop target.
Inputs
Grid inverters normally run at unity
Formula
- I_ac = P / (V × pf) for 1-ph or P / (√3 × V × pf) for 3-ph
- Protective device I_n ≥ 1.25 × I_ac
- A_min = k × ρ × L × I / (limit% × V)
The inverter AC cable is a continuous-duty circuit, so IEC practice applies a 1.25 continuous-load factor to the protective device and requires the cable to carry the full inverter rated output indefinitely. Because inverters trip on grid over-voltage, keeping the AC-side drop at or below 1% also prevents nuisance disconnection on sunny days when the point of connection is already near the upper voltage limit.
Step-by-step calculation
AC current
P / (√3 V pf)
13.912 A
Continuous rating
1.25 × I
17.39 A
Area for voltage drop
√3 ρ L I / (limit × V)
2.5 mm²
Selected
max(ampacity size, drop size)
2.5 mm²
Worked example
- 10 kW three-phase inverter, 415 V, 25 m copper run, 1% target.
- I_ac = 10 000 / (1.732 × 415 × 1) = 13.9 A; device = 20 A.
- A_min = 1.732 × 0.0172 × 25 × 13.9 / (0.01 × 415) = 2.5 mm² → 4 mm² after derating check.
Assumptions
- Unity power factor grid-tied inverter and balanced three-phase output.
Tips
- Where the run exceeds 40 m, go one size up: the cost is small against lost generation.
Warnings
- Grid over-voltage tripping is very often caused by an undersized inverter AC cable — never exceed 1%.
- The AC isolator and RCD must be rated for the selected device current.
Standards & references
- IEC 62109
- IEC 60364-7-712
- IS 732:2019
Frequently asked questions
What cable size does a 10 kW inverter need?
About 13.9 A at 415 V three phase, so 4 mm² copper is typical for runs up to about 30 m with a 1% voltage-drop target.
Why keep the AC voltage drop to 1%?
Grid inverters shut down when the terminal voltage exceeds the statutory limit. A high cable drop pushes the inverter terminal voltage up during export and causes nuisance tripping.
Do I size on inverter rating or panel capacity?
On the inverter's maximum continuous AC output, because that is the highest current the cable will ever carry.
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