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How to Size Wire for a Circuit
Start with the Load
Wire sizing is one of the most code-dependent calculations in construction. If you’re working with plumbing instead, see drain pipe sizing with DFU tables for the IPC equivalent. Before you pick a wire gauge, you need to know two things: the amperage of the circuit and the distance from the panel to the load. If you haven’t determined the amperage yet, start with an electrical load calculation to size the whole service first.
A 20-amp kitchen circuit at 15 feet is a different wire than a 20-amp shop circuit at 150 feet. Same amps, different wire. Distance changes everything because of voltage drop.
NEC 310.16 Ampacity Table (Copper)
NEC 310.16 lists conductor ampacities in three temperature columns. Which column you can use depends on the wiring method and the terminals, not just the wire’s insulation:
- NM-B cable (Romex) is limited to the 60C column, even though the conductors inside are rated 90C (NEC 334.80).
- Circuits of 100 amps or less use the 60C column unless every termination is listed for 75C (NEC 110.14(C)(1)(a)).
- Wire in conduit with 75C-rated breakers and lugs can use the 75C column.
| AWG | 60C | 75C | 90C |
|---|---|---|---|
| 14 | 15A | 20A | 25A |
| 12 | 20A | 25A | 30A |
| 10 | 30A | 35A | 40A |
| 8 | 40A | 50A | 55A |
| 6 | 55A | 65A | 75A |
| 4 | 70A | 85A | 95A |
| 3 | 85A | 100A | 115A |
| 2 | 95A | 115A | 130A |
| 1 | 110A | 130A | 145A |
| 1/0 | 125A | 150A | 170A |
| 2/0 | 145A | 175A | 195A |
| 3/0 | 165A | 200A | 225A |
| 4/0 | 195A | 230A | 260A |
That’s why 8 AWG NM-B tops out at 40 amps while 8 AWG THHN in conduit on 75C terminals can go to 50.
And if 14 AWG shows 20 amps in the 75C column, why does a 20-amp circuit need 12 AWG?
Because NEC 240.4(D) sets maximum overcurrent protection at 15A for 14 AWG, 20A for 12 AWG, and 30A for 10 AWG, regardless of the ampacity table. The breaker size determines the minimum wire size for branch circuits, not the raw ampacity number.
So for branch circuits: 15A circuit = 14 AWG minimum. 20A circuit = 12 AWG minimum. 30A circuit = 10 AWG minimum.
Voltage Drop
Here’s where most people stop, and where problems start. You’ve sized the wire for ampacity. Now you need to check voltage drop.
The NEC doesn’t require a maximum voltage drop for ordinary branch circuits. An informational note in NEC 210.19 recommends no more than 3% on a branch circuit and 5% total for feeder plus branch. Treat it as good practice, not a code requirement.
The formula:
VD = (2 x K x I x D) / CM
Where:
- VD = voltage drop in volts
- K = resistivity constant (12.9 for copper, 21.2 for aluminum)
- I = current in amps
- D = one-way distance in feet
- CM = circular mils of the wire (from NEC Chapter 9, Table 8)
Circular mils by gauge:
| AWG | Circular Mils |
|---|---|
| 14 | 4,110 |
| 12 | 6,530 |
| 10 | 10,380 |
| 8 | 16,510 |
| 6 | 26,240 |
| 4 | 41,740 |
| 3 | 52,620 |
| 2 | 66,360 |
Running a Real Calculation
You’re wiring a detached garage workshop. 20-amp circuit, 120V, single phase. The panel is 125 feet from the garage sub-panel.
Start with 12 AWG (minimum for 20A per NEC 240.4(D)).
VD = (2 x 12.9 x 20 x 125) / 6,530
VD = 64,500 / 6,530
VD = 9.88 volts
Percent voltage drop: 9.88 / 120 = 8.2%
That’s way over the 3% recommendation. Your tools will run hot, your lights will be dim, and motors might not start reliably.
Try 10 AWG:
VD = (2 x 12.9 x 20 x 125) / 10,380 = 6.22 volts = 5.2%
Still over 3%. Try 8 AWG:
VD = (2 x 12.9 x 20 x 125) / 16,510 = 3.91 volts = 3.3%
Close. For a shop circuit running power tools, I’d bump to 6 AWG for some margin, especially if you might run a welder or air compressor later.
When to Upsize
The ampacity table gives you the minimum. Voltage drop gives you the real size. Here are the situations where you’ll almost always end up upsizing:
Long runs to outbuildings, detached garages, or pole barns. Anything past 100 feet on a 20-amp circuit needs at least 10 AWG and often 8 AWG.
Motor loads. NEC 430.22 requires conductors sized at 125% of the motor’s full-load current. A 10-amp motor needs wire sized for 12.5 amps minimum. Motors also have high inrush current at startup, and undersized wire can cause the voltage to sag enough that the motor struggles to start.
Continuous loads (running 3+ hours). NEC 210.20(A) says the circuit must be rated at 125% of the continuous load. A 16-amp continuous load on a 20-amp circuit technically passes, but the wire needs to carry 20 amps (16 x 1.25) continuously without issue.
Conduit fill with multiple current-carrying conductors. NEC 310.15(C)(1) requires derating when you have more than 3 current-carrying conductors in a raceway. With 4-6 conductors, derate to 80%. With 7-9, derate to 70%. THHN can be derated from its 90C value: 12 AWG at 30 amps becomes 24 amps at 80% or 21 amps at 70%. The result still can’t exceed what the terminals and 240.4(D) allow. And once you’ve sized the wire, you still need to verify it physically fits in the conduit — see the conduit fill calculator for NEC Chapter 9 table lookups.
High ambient temperatures. Working in a hot attic in Phoenix? NEC Table 310.15(B)(1) has correction factors. At 40C ambient (104F), your 75C wire loses about 12% of its ampacity.
Common Mistakes
Using the 75C or 90C column for NM cable. NM-B is held to the 60C column. The 90C rating is for derating math only, and on circuits of 100 amps or less you size from 60C unless every termination is listed for 75C.
Forgetting the “2” in the voltage drop formula. The current travels to the load AND back. The “2” accounts for the round-trip distance. Miss it and your voltage drop calculation is half of reality.
Sizing aluminum wire the same as copper. Aluminum has higher resistance (K = 21.2 vs 12.9). You typically need to go up two wire sizes when switching from copper to aluminum. 4 AWG copper becomes 2 AWG aluminum.
Skip the Math on Site
Trussly’s wire size calculator does the ampacity lookup for your wiring method and upsizes for voltage drop. Punch in the amps, distance, and voltage. It picks the gauge, shows the drop, and explains which ampacity column it used. Derating for bundled conductors and hot locations has its own calculator. Use voice input to enter dimensions hands-free on the ladder. Faster than flipping through the NEC book.