Why is 1/0 AWG the minimum for parallel conductors?

NEC 310.10(H)(1) prohibits paralleling conductors smaller than 1/0 AWG. Smaller paralleled conductors share current too unevenly because slight differences in length and termination cause large proportional imbalances, risking overload on one conductor.

Do the parallel conductors have to be identical?

Yes. NEC 310.10(H)(2) requires every conductor in a parallel set to be the same length, material, size, insulation type, and termination method. This forces the impedance of each path to match so current divides equally.

How is current divided among the sets?

Because the sets are electrically identical, the total current divides equally. Each set must be sized for total current divided by the number of sets. Two sets each carry half, three sets each carry a third, and so on.

Which temperature column should I use?

Use the column matching the lowest-rated termination in the circuit, usually 75 C for modern lugs, or 60 C for older or small terminations. Conductors may have 90 C insulation, but you cannot load them above the termination rating per NEC 110.14(C).

Does this account for conduit fill and ambient derating?

No. The ampacities shown are base Table 310.16 values at 30 C ambient with three current-carrying conductors. Apply ambient correction and conductor-bundling adjustment factors to the table value before comparing it to your per-set load.

Parallel Conductor Sizing Calculator (NEC 310.10(H))

Very large feeders are often run as multiple parallel sets of smaller conductors rather than one impractically huge cable. NEC 310.10(H) governs how this is done: each conductor must be at least 1/0 AWG, and every conductor in a parallel set must be identical so the current divides equally. This calculator finds the smallest compliant conductor for a given feeder.

How it works

The total feeder current is divided equally among the parallel sets because the sets are electrically identical:

per-set ampacity required = total feeder current / number of sets

The tool then scans NEC Table 310.16 (for the chosen material and temperature column) and returns the smallest conductor — never below 1/0 AWG — whose ampacity meets that per-set requirement. The combined ampacity is the per-set ampacity multiplied by the number of sets.

The 310.10(H) rules

Paralleled conductors must share these characteristics within each set: same length, same conductor material, same size, same insulation type, and the same termination method. Each parallel set is run in its own raceway (or all in one raceway with appropriate fill derating), and an equipment grounding conductor sized per 250.122 is run with each set.

Example and notes

A 600 A feeder run as two parallel sets requires each set to carry 300 A. In 75 C copper that calls for 350 kcmil per set; in aluminum it would be 500 kcmil per set. Running three sets instead drops the per-set load to 200 A, allowing 3/0 copper. Always apply ambient and fill derating to the table ampacity before you compare it against the per-set requirement.