Ampacity & MCA Calculator
Engineering Specification Sheet
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Generated: August 10, 2026
Ampacity & MCA Calculator
Work out an HVAC nameplate's minimum circuit ampacity under NEC 440.33, or read what a conductor you already have can carry. Free, with a printable Table 310.16 chart.
Conductor & Conditions
Grounds and unloaded neutrals don't count. Above 3, 310.15(C)(1) adjustment applies.
Sizing a circuit from a load?
This page runs the lookup in one direction: you name the conductor, it tells you what the conductor can carry. Going the other way — load first, then conductor, with a voltage-drop check over the run — is a different tool.
Wire Size & AWG Calculator →Allowable Ampacity
Derivation
The termination rating is binding here. Both constraints apply independently, so the allowable ampacity is the lower of the two — the equipment terminals, not the wire, are the limit.
Ampacity is not the same as breaker size
#12 copper carries 25.0 A, but NEC 240.4(D) never permits it to be protected above 20 A — whatever the ampacity works out to.
Size the overcurrent device →Calculations based on NEC 2023 guidelines. Results are for estimation purposes — verify against local code before installation.
Verified against NEC 2023 · Table 310.16, 310.15(B)(1), 310.15(C)(1), 110.14(C)
NEC 2023 Table 310.16 — allowable ampacities
Not more than three current-carrying conductors in a raceway, cable or earth, at an ambient of 30°C (86°F). These are the base values every correction above starts from — print this page for a field copy.
| Size | Copper | Aluminum | ||||
|---|---|---|---|---|---|---|
| 60°C | 75°C | 90°C | 60°C | 75°C | 90°C | |
| #14 | 15 | 20 | 25 | — | — | — |
| #12 | 20 | 25 | 30 | 15 | 20 | 25 |
| #10 | 30 | 35 | 40 | 25 | 30 | 35 |
| #8 | 40 | 50 | 55 | 30 | 40 | 45 |
| #6 | 55 | 65 | 75 | 40 | 50 | 60 |
| #4 | 70 | 85 | 95 | 55 | 65 | 75 |
| #3 | 85 | 100 | 110 | 65 | 75 | 85 |
| #2 | 95 | 115 | 130 | 75 | 90 | 100 |
| #1 | 110 | 130 | 150 | 85 | 100 | 115 |
| #1/0 | 125 | 150 | 170 | 100 | 120 | 135 |
| #2/0 | 145 | 175 | 195 | 115 | 135 | 150 |
| #3/0 | 165 | 200 | 225 | 130 | 155 | 175 |
| #4/0 | 195 | 230 | 260 | 150 | 180 | 205 |
| 250 kcmil | 215 | 255 | 290 | 170 | 205 | 230 |
| 300 kcmil | 240 | 285 | 320 | 195 | 230 | 260 |
| 350 kcmil | 260 | 310 | 350 | 210 | 250 | 280 |
| 400 kcmil | 280 | 335 | 380 | 225 | 270 | 305 |
| 500 kcmil | 320 | 380 | 430 | 260 | 310 | 350 |
A dash means the NEC assigns no ampacity to that combination. 14 AWG aluminum is the only one in this range: it is not a recognised conductor, which is why 240.4(D) lists no overcurrent cap for it either.
How this works
This page does two different jobs that both come back to the same table. Enter an HVAC compressor's rated-load current and its fan loads, and it works out minimum circuit ampacity, the smallest conductor a circuit is allowed to have, using NEC 440.33: every load at 100%, plus another 25% of whichever motor is largest. Or pick a conductor you already have, and it reads back what that wire can actually carry once ambient temperature, bundling, and the termination rating are factored in against NEC Table 310.16.
Most of the time you shouldn't be calculating MCA at all. NEC 440.4(B) requires manufacturers to mark unitary equipment with its minimum circuit ampacity and maximum overcurrent rating, and where that plate exists, it governs over any field calculation. This tool is for unmarked or field-assembled equipment, or for double-checking a plate that looks off.
MCA sizes the wire. It doesn't size the breaker. NEC 440.22(A) caps an air-conditioning breaker at 175% of the compressor's rated-load current, a ceiling you round down from, and that's a separate number from MCA that routinely lands somewhere else entirely.
Worked example
A #12 THHN copper conductor, run with 6 other current-carrying conductors in a raceway sitting in a 40°C attic, terminating on a 75°C-rated breaker.
Derating alone brings this #12 down to 21.84A before the termination cap even applies, 8.16A less than its 30A table rating.
A condenser with an 18.7A compressor and a 1.2A fan, unmarked and needing a field-calculated MCA.
MCA comes to 24.575A, not the 19.9A the equipment actually draws, and it's what sizes the wire: a #12 AWG conductor here.
Frequently Asked Questions
What is minimum circuit ampacity (MCA), and how is it different from the equipment's running amps?↓
MCA is the smallest conductor ampacity a circuit is permitted to have — not what the equipment actually draws. Under NEC 440.33 it is the sum of every load on the circuit at 100%, plus an extra 25% of the largest motor. A condenser with an 18.7 A compressor and a 1.2 A fan draws 19.9 A but has an MCA of 24.575 A. The 25% adder attaches to the largest motor only: a strip heater that is bigger than the compressor still counts at 100% and does not move the adder onto itself.
My unit's nameplate already lists an MCA. Should I calculate my own?↓
No. NEC 440.4(B) requires the equipment to be marked with its minimum circuit ampacity and maximum overcurrent device rating, and where it is marked, those values govern. The manufacturer knows the rated-load current, the branch-circuit selection current and every factory-installed load. Calculate an MCA only for unmarked or field-assembled multi-motor equipment, or to check a plate you have reason to doubt.
Why does this tool refuse to show 14 AWG aluminum?↓
Because there is no such conductor in the code. Table 310.16 assigns 14 AWG aluminum no ampacity at 60°C, 75°C or 90°C, and NEC 240.4(D) lists no overcurrent cap for it — its aluminum entries start at 12 AWG. Calculators that offer the size and then print "0 A" are showing you a hole in their data rather than a rule. The smallest aluminum branch-circuit conductor is 12 AWG.
Does MCA tell me what breaker to install?↓
No, and treating it as though it does is a common and expensive mistake. MCA sizes the conductor. The device is a separate rule: NEC 440.22(A) caps an air-conditioning branch-circuit breaker or fuse at 175% of the compressor's rated-load current — a maximum you round down from, not a minimum you round up to. The two numbers come from different inputs and routinely disagree.
Learn more
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