Solar & battery
Solar Cable Size Calculator
Sizes cable, where voltage drop usually governs rather than ampacity.
Result
DC circuits are held to a tighter drop than AC because the voltage is lower, so the same lost volts are a much larger share. This sizes on voltage drop only; the cable must independently satisfy ampacity, conduit fill, temperature derating and any code requirement for the specific conductor type. Solar DC circuits also have their own NEC article with additional rules.
Get a Free Business AuditHow the calculation works
Two separate requirements: ampacity sufficient for the current with the code's safety factors and any derating for temperature and bundling, and a conductor large enough to keep voltage drop within an acceptable percentage over the run length. The larger of the two governs, and on long low-voltage runs that is almost always voltage drop.
What moves the number
- Current, with code safety factors applied.
- One-way run length, which doubles in the drop calculation.
- System voltage, since drop matters proportionally more at low voltage.
- Ambient temperature and conduit fill, which derate ampacity.
Where this one goes wrong
Voltage drop is proportional to current and length and inversely proportional to voltage, which is why a low-voltage DC run of any distance needs conductors far larger than ampacity alone would suggest, and why raising system voltage is often cheaper than upsizing cable. Code minimum ampacity does not address drop at all. Conductor sizing for PV also involves specific code multipliers and derating rules, and the adopted edition and the inspector govern rather than a remembered figure.
Before you quote
- Size for voltage drop as well as ampacity, and take the larger.
- Use the one-way length doubled in the drop calculation.
- Apply temperature and conduit derating from the adopted code.
Where the rules come from
Questions
About this calculator
Why is my cable bigger than the ampacity table says?
Because voltage drop usually governs on anything but a short run. Drop is proportional to current and length and inversely proportional to voltage, so a low-voltage DC run needs far larger conductors than ampacity alone would require.
Is it cheaper to raise system voltage or upsize cable?
Frequently raising the voltage, since drop falls proportionally while conductor cost rises steeply with size. That is why larger off-grid systems use 48 volt banks rather than 12, and why long array runs are configured at high voltage.
Which code rules apply to PV conductors?
Article 690 of NFPA 70 plus the general conductor rules, with specific multipliers for continuous current and derating for temperature and conduit fill. The edition adopted locally and the inspector govern, so the current adopted text is the authority.
The formula
circular mils = 2 x K x amps x distance / allowable volts dropped
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How to use this calculator
- Current (A). Enter your own figure; the page starts at 30.
- System voltage. Enter your own figure; the page starts at 48.
- One-way run (ft). Enter your own figure; the page starts at 25.
- Acceptable voltage drop (%). DC runs are held tighter than AC, usually 2% or less.
- Conductor. Enter your own figure; the page starts at Copper.
The result updates as you type, so you can try a figure, see what it does to cable size, and change it back. Nothing you enter is sent anywhere or saved.
A worked example
These are the figures the calculator opens with. Every number in the results column below is produced by the same calculation the page runs, so you can follow it through before putting your own figures in.
What goes in
| Input | Value |
|---|---|
| Current (A) | 30 |
| System voltage | 48 |
| One-way run (ft) | 25 |
| Acceptable voltage drop | 2% |
| Conductor | Copper |
What comes out
| Result | Value |
|---|---|
| Cable size main result | 6 |
| Circular mils needed | 20,156 |
| Drop at that size | 1.54% |
| Volts lost | 0.7 |
6 AWG keeps the drop to 1.54% over 25 feet, losing 0.7 volts.
How sensitive the answer is
Holding everything else steady and changing only current (a), this is what happens to cable size. It is worth knowing which of your inputs the answer actually hinges on, and which barely move it.
| Current (A) | Cable size |
|---|---|
| 15 | 10 |
| 22.5 | 8 |
| 30 example above | 6 |
| 45 | 4 |
| 60 | 4 |
Every other input is held at the example value while this one changes, which is not how a real job behaves. Treat it as a guide to which number matters most, not as five separate quotes.
What each figure means
- Current (A)
- A number, between 0 and 2000, starting at 30.
- System voltage
- A number, between 0 and 1500, starting at 48.
- One-way run (ft)
- A number, between 0 and 2000, starting at 25.
- Acceptable voltage drop (%)
- DC runs are held tighter than AC, usually 2% or less.
- Conductor
- Choose from Copper, Aluminium.
What the results tell you
| Result | What it is |
|---|---|
| Cable size | The headline result. A category or planning selection, rather than a measured quantity. |
| Circular mils needed | A number. |
| Drop at that size | A percentage. |
| Volts lost | A decimal figure. |
A design still needs a site visit
Output and sizing figures use the assumptions you enter, not measured irradiance, shading or roof orientation at your address. Real production depends on all three, plus inverter behaviour and local interconnection rules.
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