How to use this calculator
- Split the load. Continuous = expected to run 3+ hours (lighting, EVSE, heaters); noncontinuous = the rest of the demand-factored load.
- Pick a candidate conductor. Enter its table ampacity at the insulation rating (often the 90°C column) and its termination-column value (usually 75°C).
- Enter the derating factors. Ambient correction for the route temperature, bundling adjustment for the number of current-carrying conductors.
- Read both margins. Both paths must be non-negative; the governing note tells you which constraint is binding.
- Finish the design. Check overcurrent protection, voltage drop on long runs, and conduit fill for the chosen conductors.
How it works
Conductor sizing fails in the field for one reason more than any other: people run one of the two required checks. The code structure (NEC 215.2(A) for feeders, 210.19(A) for branch circuits) demands both:
Path 1 — terminations: I_term ≥ 1.25 × I_cont + I_non (no derating)
Path 2 — insulation: I_table × k_amb × k_adj ≥ I_cont + I_non
Path 1 protects lugs and breakers from continuous heating — evaluated at the termination temperature column (75°C for most equipment) with no correction factors. Path 2 protects the insulation along the run — evaluated from the conductor's own rating column (90°C for THHN) with every applicable correction and adjustment. A hot, crowded conduit makes Path 2 govern; a heavily continuous load makes Path 1 govern. The calculator reports both margins and names the binding path.
Companion checks live on their own pages: the ampacity derating calculator builds the k-factors when conditions stack, the parallel conductor calculator covers multi-set feeders, the voltage drop calculator handles long runs, and the conduit fill calculator sizes the raceway for whatever this check selects.
Worked example
Verified against the live calculator
A feeder carries 40 A continuous and 60 A
noncontinuous through a conduit with 4-6 current-carrying conductors
(k_adj = 0.8) at standard ambient (k_amb = 1.0).
Candidate: a copper conductor with 130 A at its 90°C
insulation column and 115 A at the 75°C termination column.
Path 1: 1.25 × 40 + 60 = 110 A ≤ 115 A → passes, 5 A margin
Path 2: 130 × 1.0 × 0.8 = 104 A ≥ 100 A → passes, 4 A margin
Both paths pass, each with only a few amps to spare — the corrected path
is the binding one at 96% utilization. Add two more current-carrying
conductors to the same conduit (k_adj drops toward 0.7) and
Path 2 fails at 91 A < 100 A: the same wire that
satisfies the 125% rule no longer survives the derating. That is exactly
the case the two-path check exists to catch.
Frequently asked questions
How do I size a feeder conductor?
Two checks, both must pass. First, the conductor ampacity at the termination temperature column must be at least 1.25 × continuous load + noncontinuous load, with no derating applied. Second, the conductor table ampacity multiplied by the ambient correction and bundling adjustment factors must be at least the total (100%) load. The worse of the two governs.
Why is continuous load multiplied by 125%?
The 125% factor protects terminations and overcurrent devices that are not listed for continuous duty at full rating — a load running 3 hours or more heats them beyond their test conditions. It applies at the termination check; the derated-insulation check uses 100% of the load.
Why are there two different ampacities for the same wire?
The conductor insulation may be rated 90°C (THHN/XHHW-2), letting you start correction and adjustment from the higher 90°C table column — but the lugs it lands on are usually only rated 75°C, so the final answer can never exceed the 75°C column value. Enter both and the calculator applies each where it belongs.
Where do the correction and adjustment factors come from?
From the adopted code edition: the ambient-temperature correction table for your insulation rating, and the more-than-three current-carrying-conductors adjustment table for the raceway or cable bundle (e.g. 0.8 for 4-6 CCC). This page deliberately does not embed those tables — enter the values from your code book.
Does this calculator pick the breaker size?
No. It checks the conductor. Overcurrent device selection, the next-standard-size-up rule, small-conductor caps and tap rules are separate code steps, and motor circuits follow their own article entirely.
Method & assumptions
- Implements the sizing structure of NEC 215.2(A)(1)/210.19(A) without embedding any code tables — ampacities, correction and adjustment factors come from the adopted edition you enter.
- The 125% continuous factor is omitted only for listed 100%-rated assemblies (verify the listing); this page always applies it.
- Motor, HVAC motor-compressor, welder and capacitor loads carry article-specific multipliers not modeled here.
- Overcurrent device selection, next-size-up allowance, small-conductor caps (240.4(D)), tap conductors and voltage drop are separate checks.
- This is a design screen, not an engineering judgment or an inspection outcome; the adopted local code governs.