Wire Size Calculator
Screen a branch circuit for conductor size. Enter the load in amps, the conductor material, the termination temperature column, and the one-way run length to get an AWG size from NEC Table 310.16, its table ampacity, and the voltage drop at that size. Long runs are automatically stepped up to stay under 3%.
Your project
Estimate
10AWG
Suggested conductor size · 0 and below mean 1/0, 2/0, 3/0, 4/0
- Table ampacity at that size
- 30A
- Voltage drop at that size
- 1.86%
- Upsized for voltage drop?
- 0
- Carries the full load?
- 1
Estimates are guidance only and exclude tax, labor, and site conditions unless stated. Confirm with a local pro.
How it's calculated
No black boxes — here is the exact math behind your estimate, so you can check it or adapt it for your own quote.
- 1
Pick the ampacity table: copper or aluminum, 60 °C or 75 °C column (NEC Table 310.16) - 2
Suggested size = the smallest conductor whose table ampacity is at least the load amps - 3
Voltage drop % = 2 × K × amps × one-way length ÷ circular mils ÷ voltage × 100 (K = 12.9 copper, 21.2 aluminum) - 4
While the drop is over 3%, step up one AWG size and recheck - 5
Upsized flag = 1 if the drop check forced a size larger than ampacity alone required - 6
Carries the full load = 0 when even 4/0 is under-rated for the amps entered
Worked example
A 30 A, 240 V circuit feeding a detached garage subpanel 60 feet away in copper NM-B.
Inputs
- Load current
- 30 A
- Conductor material
- 1
- Temperature column
- 60
- One-way run length
- 60 ft
- Circuit voltage
- 240
Result
- Suggested conductor size
- 10 AWG
- Table ampacity at that size
- 30 A
- Voltage drop at that size
- 1.86 %
- Upsized for voltage drop?
- 0
- Carries the full load?
- 1
Tools & materials for this project
As an Amazon Associate we may earn from qualifying purchases, at no extra cost to you.
- Electrician wire strippers Strip 14 through 8 AWG cleanly without nicking the conductor.
- 12/2 NM-B cable The standard 20 A branch-circuit conductor for interior dry locations.
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Frequently asked questions
What size wire do I need for 15, 20, and 30 amps?
The household basics are 14 AWG copper on a 15 A breaker, 12 AWG copper on a 20 A breaker, and 10 AWG copper on a 30 A breaker. Those are not rules of thumb — NEC 240.4(D) caps the overcurrent device for those three copper sizes at 15, 20, and 30 A regardless of what the 75 °C ampacity column shows. Aluminum branch conductors are capped at 15 A for 12 AWG and 25 A for 10 AWG. A long run can push you above these minimums, but never below them.
Do the breaker and the wire have to match?
Yes, and the reason is often misunderstood: the breaker protects the conductor, not the appliance. A 20 A breaker on 14 AWG lets the wire run past its rated temperature long before the breaker trips, which is a classic source of scorched receptacles and boxes. The breaker must be sized at or below the conductor ampacity. Going the other direction is fine and common — a 12 A tool on a 20 A circuit wired in 12 AWG is entirely normal, and oversizing the conductor on a long run is good practice.
Why does the 60 °C column usually apply to house wiring?
Terminations govern, not the insulation. NEC 110.14(C)(1) limits the ampacity to the lowest temperature rating in the circuit, and breakers, receptacles, and most residential equipment on circuits of 100 A or less are listed for 60 °C. NM-B cable has 90 °C insulation, but 334.80 requires you to use the 60 °C ampacity anyway. So even THHN pulled in conduit lands in the 60 °C column unless every lug, breaker, and device at both ends is marked 75 °C.
What is derating and when does it change the answer?
Two corrections in NEC 310.15 reduce a table ampacity. Ambient correction applies above 30 °C: a hot attic at 45 °C multiplies a 75 °C conductor by about 0.82. Adjustment applies when more than three current-carrying conductors share a raceway or cable — 4 to 6 conductors is 80%, 7 to 9 is 70%, 10 to 20 is 50%. Stack both and a 12 AWG conductor rated 20 A can fall below 20 A. This calculator does not apply either correction, so bundled or hot installations need a real review.
When does voltage drop force a bigger wire than ampacity does?
Roughly past 100 feet on a 120 V circuit and past 200 feet on 240 V, for typical branch loads. A 20 A, 120 V circuit on 12 AWG copper is under 3% only to about 45 feet one way. Doubling the voltage halves the current for the same watts and cuts the drop by four, which is why detached garages, shops, and well pumps are usually fed at 240 V. The tool steps the size up automatically and flags it in the last result.
Is this enough to wire a circuit?
No. Treat it as a planning estimate. Real conductor sizing starts with a load calculation under NEC Article 220, then applies derating for ambient temperature and conduit fill (310.15), termination temperature limits (110.14(C)), continuous-load factors, conductor type and wet-location ratings, and the equipment grounding conductor from Table 250.122. The tool also stops at 4/0, since loads past roughly 200 A move into parallel conductor sets or larger kcmil under an engineered design. The locally adopted code edition and a licensed electrician govern the final design, and permits and inspection are required for almost all new circuits.