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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.
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, 75C Column)
This is the table that matters for most residential and commercial work. NEC 310.16 lists ampacities for insulated conductors at different temperature ratings. The 75C column covers THWN-2, the most common wire you’ll pull through conduit or use in NM-B cable.
| AWG | 75C Ampacity |
|---|---|
| 14 | 20A |
| 12 | 25A |
| 10 | 35A |
| 8 | 50A |
| 6 | 65A |
| 4 | 85A |
| 3 | 100A |
| 2 | 115A |
| 1 | 130A |
| 1/0 | 150A |
| 2/0 | 175A |
| 3/0 | 200A |
| 4/0 | 230A |
Wait. If 14 AWG is rated for 20 amps, why does the NEC require 12 AWG for 20-amp circuits?
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.
NEC doesn’t mandate a maximum voltage drop, but it recommends no more than 3% on a branch circuit and 5% total (feeder + branch combined) in NEC 210.19(A) Informational Note No. 4.
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%. Your 12 AWG that was good for 25 amps now handles only 20 or 17.5 amps.
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 90C column from NEC 310.16 for standard installations. The 90C column applies to the wire’s insulation rating, but you still terminate at 75C-rated equipment. For most residential work, size from the 75C column.
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
All of these calculations (ampacity lookup, voltage drop, derating, upsizing for distance) are built into SiteCalc’s wire sizing calculator. Punch in the amps, distance, and voltage. It picks the wire gauge and shows the voltage drop percentage with the NEC code reference right there. Use voice input to enter dimensions hands-free on the ladder. Faster than flipping through the NEC book.