Estimate from NEC Table 310.16 (75 C column), 240.4(D) and Chapter 9 Table 8. NM-B cable uses the 60 C column and may need one size larger.
This is an estimate for planning. Electrical work needs an expert electrician and, in most places, a permit.
How the calculator works
The tool checks two things and picks the size that passes both. First is ampacity: how many amps the wire can carry. It uses the 75 C column of NEC Table 310.16. Small wire also has a lower limit in NEC 240.4(D): 14 AWG copper is held to 15 A, 12 AWG to 20 A and 10 AWG to 30 A. For aluminum, 12 AWG is held to 15 A and 10 AWG to 25 A.
Second is voltage drop. Wire has resistance, so some voltage is lost along the run. The tool uses the resistance of each size from NEC Chapter 9 Table 8 (ohms per 1,000 ft at 75 C) and this formula for a single-phase circuit: voltage drop = 2 x length x resistance x amps / 1,000. The length is one way, from the panel to the load. The 2 counts the trip out and back.
If the wire that carries the amps drops more than your target, the tool steps up one size at a time until the drop is at or under the target. The result tells you which check set the size.
What changes the answer
Continuous loads. A load that runs 3 hours or more, like an EV charger or a water heater, is sized at 125% of its amps. Choose Yes for that question and the tool adds 25%.
Cable type. NM-B cable (often called Romex) must use the 60 C ampacity column under NEC 334.80. Many breakers and devices rated 100 A or less also use the 60 C column under NEC 110.14(C). That can mean one size larger than this tool shows. Wire in conduit, like THHN, can often use the 75 C column.
Heat and crowding. Hot attics, hot rooftops and many wires bundled together lower ampacity. This tool does not apply those corrections.
Breaker basis. If you enter a breaker size, the tool sizes the wire to the breaker and checks voltage drop at the full breaker amps. That is a worst case. Entering the real load amps gives a smaller drop.
When to call an expert electrician
Call an electrician for any new circuit, sub-panel, EV charger, or long run to a garage or shed. They check things this tool does not: the cable type, the breaker and terminal ratings, heat, conduit fill, and the ground wire. When the hot wires are made larger for voltage drop, NEC 250.122(B) says the ground wire must be made larger in proportion.
Bigger wire may not fit the screws or lugs on a breaker or outlet. An electrician picks parts that fit. Electrical work also needs a permit in most places.
The code basis
Ampacity comes from NEC Table 310.16 (75 C column) and the small-conductor rule in NEC 240.4(D). Resistance comes from NEC Chapter 9 Table 8. The 3% voltage drop default comes from an informational note to NEC 210.19(A). A second note, to 215.2(A), suggests no more than 5% in total for the feeder and branch circuit together. These notes are advice, not rules, unless your local code adopts them.
You can read the NEC for free on the NFPA website. Wire makers also publish ampacity charts, like this Cerrowire chart.
| Size | Copper amps | Aluminum amps | Copper ohms per 1,000 ft | Aluminum ohms per 1,000 ft |
|---|---|---|---|---|
| 14 AWG | 15 (240.4(D)) | not listed | 3.14 | not listed |
| 12 AWG | 20 (240.4(D)) | 15 (240.4(D)) | 1.98 | 3.25 |
| 10 AWG | 30 (240.4(D)) | 25 (240.4(D)) | 1.24 | 2.04 |
| 8 AWG | 50 | 40 | 0.778 | 1.28 |
| 6 AWG | 65 | 50 | 0.491 | 0.808 |
| 4 AWG | 85 | 65 | 0.308 | 0.508 |
| 2 AWG | 115 | 90 | 0.194 | 0.319 |
| 1/0 AWG | 150 | 120 | 0.122 | 0.201 |
| 2/0 AWG | 175 | 135 | 0.0967 | 0.159 |
| 4/0 AWG | 230 | 180 | 0.0608 | 0.100 |
Worked example: a 16 A load 150 feet away
- The load is 16 A at 120 V, copper wire, 150 ft one way, 3% target.
- Ampacity: 14 AWG copper is held to 15 A, which is less than 16 A. 12 AWG copper is good for 20 A, so 12 AWG carries the amps.
- Voltage drop on 12 AWG: 2 x 150 x 1.98 x 16 / 1,000 = 9.50 V. That is 9.50 / 120 = 7.92%, well over 3%.
- Try 10 AWG: 2 x 150 x 1.24 x 16 / 1,000 = 5.95 V, or 4.96%. Still over 3%.
- Try 8 AWG: 2 x 150 x 0.778 x 16 / 1,000 = 3.73 V, or 3.11%. Just over 3%.
- Try 6 AWG: 2 x 150 x 0.491 x 16 / 1,000 = 2.36 V, or 1.96%. Under 3%.
- Estimate: 6 AWG copper. Voltage drop, not ampacity, set the size. The breaker stays at 20 A, so check that 6 AWG fits its terminal.
Wire Size Calculator questions
Short answers to common questions.
What size wire do I need for a 20 amp circuit?
Most 20 A circuits use 12 AWG copper. That is the smallest copper size allowed on a 20 A breaker under NEC 240.4(D). On a long run, a larger wire may be needed to keep voltage drop near 3%.
What size wire for a 50 amp breaker?
With wire rated for the 75 C column, like THHN in conduit, 8 AWG copper carries 50 A. NM-B cable uses the 60 C column, where 8 AWG copper carries only 40 A, so 6 AWG NM-B is common for 50 A. Long runs may need larger wire.
Is the length one way or round trip?
Enter the one-way length, from the panel to the load. The formula already doubles it to count the current going out and coming back.
Why does a long run need bigger wire?
Every foot of wire has resistance, so a longer run loses more voltage. Lights may dim and motors may run hot. A bigger wire has less resistance and loses less.
Can I use aluminum wire instead of copper?
Aluminum carries fewer amps than copper of the same size, so it needs to be about one or two sizes larger. It is common for feeders and service wire. It needs parts rated for aluminum and careful connections, so an electrician should do it.
Is 3% voltage drop a code rule?
No. The 3% branch circuit and 5% total figures are informational notes in the NEC. They are advice for good performance. Some local codes and some equipment makers set firmer limits.
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