Wire Size and Voltage Drop Calculator
AWG size from load, distance and voltage using the NEC ampacity tables — including the termination rule that makes the 90 °C column the wrong answer.
Every size, both constraints
Green passes both. The smallest row that does is the answer.
| Size | Derated | Termination | Usable | Drop |
|---|
The 90 °C column is not an ampacity
Almost all modern building wire is rated 90 °C, and the 90 °C column of NEC Table 310.16 gives generous numbers. It is also almost never the figure you are allowed to use. NEC 110.14(C) limits you to the weakest link in the circuit, which is the terminations — and breakers, panels and receptacles rated 100 A or less are 60 °C unless the equipment is marked otherwise. Most residential equipment is not. The 90 °C column exists so you have a starting point to derate from when correcting for heat or bundling. A calculator that hands you the 90 °C number is telling you to put a 30 A breaker on 12 AWG.
- 14, 12 and 10 AWG are capped regardless. NEC 240.4(D) limits them to 15, 20 and 30 A whatever the ampacity table says. A 12 AWG copper conductor good for 25 A at 75 °C still gets a 20 A breaker.
- Voltage drop is not code. It is an informational note recommending 3% on a branch circuit and 5% overall, and notes are not enforceable. It is still what decides the wire size on any long run — a circuit can be perfectly legal and still leave a motor stalling.
- Continuous loads are sized at 125%. Anything running three hours or more: EV chargers, shop lighting, heating. A 48 A charger becomes a 60 A circuit, which is why the wire is bigger than the nameplate suggests.
- Corrections compound. A hot attic and a full conduit multiply together. Six conductors at 45 °C leaves you 70% of the table figure, which turns a comfortable run into a marginal one without anything visibly changing.
- Dwelling services get an allowance. NEC 310.12 permits 83% of the rating on a single-dwelling service or main feeder, so a 200 A house runs on 2/0 copper rather than 3/0. This tool does not assume it, which is why an electrician may quote you smaller — and they are not wrong.
This is a planning aid, not a design. Local amendments, the specific equipment listing and the inspector all govern. Anything at a service, in a commercial building, or that you are not certain about wants a licensed electrician regardless of what any calculator says.
How to use
- Enter the load current, system voltage and one-way run length.
- Set the termination rating — 60 °C unless the equipment is marked otherwise.
- Add ambient temperature and conductor count if the run is hot or crowded.
- Read the recommended size, then check whether ampacity or voltage drop set it.
Frequently asked questions
Why does this give a bigger wire than other calculators?
Almost certainly because they read the 90 °C column and this does not. Modern building wire is rated 90 °C, but NEC 110.14(C) limits you to the temperature rating of the terminations, and breakers and panels rated 100 amps or less are 60 °C unless the equipment is marked otherwise. The 90 °C column exists as a starting point for derating, not as an ampacity. A calculator that hands you the 90 °C figure is telling you to put a 30 amp breaker on 12 AWG.
My 12 AWG is rated 25 amps, so why only a 20 amp breaker?
Because of NEC 240.4(D), the small-conductor rule. It caps overcurrent protection at 15, 20 and 30 amps for 14, 12 and 10 AWG copper regardless of what the ampacity table allows. The rule exists because these sizes are used in enormous quantities in places nobody inspects again, and the margin is deliberate. It is not negotiable by picking a hotter insulation.
Is voltage drop actually required by code?
No. It appears in the NEC only as an informational note recommending 3 percent on a branch circuit and 5 percent overall, and informational notes are not enforceable. It is still what decides the wire size on any long run. A circuit can be entirely legal and still leave a motor stalling on startup or lights visibly dimming, so treat the ampacity answer as the floor and the drop answer as the real one.
What counts as a continuous load?
Anything expected to run for three hours or more, which must be sized at 125 percent of its current. EV chargers, shop and warehouse lighting, electric heating and most commercial loads all qualify. A 48 amp charger becomes a 60 amp circuit and needs the wire to match, which is why the conductor is a size larger than the nameplate current suggests.
How do the derating factors combine?
They multiply, which catches people out. Six current-carrying conductors in a raceway is 80 percent, and a 45 °C attic on 90 °C wire is 87 percent — together that is 70 percent of the table figure. Both corrections start from the insulation column and the result is then capped by the termination rating, so a full calculation has two separate steps and doing only one of them gives an answer that looks carefully worked out and is wrong.
Can I use aluminium instead?
Yes, and it is markedly cheaper on large feeders, but it needs terminations listed for it — marked AL/CU or CO/ALR — and an antioxidant compound on the connections. Mixing aluminium conductors into terminals rated only for copper is a genuine fire risk rather than a paperwork technicality. Aluminium also needs roughly two sizes larger than copper for the same load.
Why does my electrician say a smaller wire is fine?
Possibly NEC 310.12, which permits 83 percent of the rating on the service or main feeder of a single dwelling. That allows a 200 amp house service on 2/0 copper rather than 3/0. It applies only to dwelling services and main power feeders, so this tool does not assume it — but if that is your situation, the smaller conductor is correct and the difference is the allowance rather than a mistake.
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