Parallel Conductor Sizing Calculator

Why conductors are run in parallel

Above roughly 400 A, a single conductor becomes hard to pull, hard to terminate, and disproportionately expensive per ampere. Running two or more conductors per phase in parallel, electrically joined at both ends, splits the current among smaller, more workable conductors. An 800 A service is typically two sets of 500 kcmil copper rather than one oversized conductor. NEC 310.10(G) is the section that makes parallel conductors acceptable, and it sets the two rules this tool enforces: a 1/0 AWG floor and identical conductors within each phase.

The total ampacity is per conductor times sets

Each conductor in a parallel set carries its own share of the current, so the ampacity of the paralleled phase is the ampacity of one conductor multiplied by the number of sets. One 500 kcmil copper conductor at the 75 degrees C column of Table 310.16 is 380 A, so two in parallel carry 760 A and three carry 1140 A. The per-conductor value already reflects any ambient correction, conductor-count adjustment, and the 110.14(C) termination cap, so the multiplication is the last step, not the first.

Worked example: two sets of 500 kcmil copper

1. 500 kcmil copper is 1/0 AWG or larger, so it is eligible to be paralleled.
2. Its 90 degrees C insulation base ampacity is 430 A, but the 75 degrees C terminations cap it at 380 A per conductor under 110.14(C).
3. With one set per raceway, each raceway has three current-carrying conductors, so no Table 310.15(C)(1) adjustment applies.
4. Total ampacity is 380 A times two sets, which is 760 A.
5. If both sets shared one raceway, six current-carrying conductors would cut each conductor to 80 percent: 344 A each, 688 A total.

The shared-raceway penalty

The most common mistake with parallel conductors is forgetting the conduit-fill derating when several sets go into one raceway. Table 310.15(C)(1) counts every current-carrying conductor in the raceway, and paralleling multiplies that count. Two three-phase sets in one raceway is six conductors, which drops each conductor to 80 percent; three sets is nine conductors at 70 percent. Pulling each set in its own raceway keeps a normal three-wire set at three conductors and avoids the adjustment entirely. This tool models both arrangements so the penalty is visible before it becomes an undersized feeder.

Grounding a parallel run

When parallel conductors run in more than one raceway, NEC 250.122(F) requires a full-size equipment grounding conductor in each raceway, sized from Table 250.122 on the circuit overcurrent device. You do not divide one EGC among the raceways. An 800 A feeder run as two parallel sets needs a full 1/0 copper EGC in each of the two raceways. Enter the overcurrent device rating above and the tool returns that per-raceway size. Where a voltage-drop upsize on the ungrounded conductors also increases the EGC under 250.122(B), use the dedicated Equipment Grounding Conductor calculator.

Frequently asked questions

What is the smallest conductor that can be run in parallel?

NEC 310.10(G) permits paralleling only in sizes 1/0 AWG and larger. Smaller conductors must be a single run of adequate ampacity, except for a few narrow 310.10(H) applications outside the scope of ordinary feeders and services.

How do you calculate the ampacity of parallel conductors?

Take the derated, termination-capped ampacity of one conductor from Table 310.16 and multiply by the number of parallel sets. Two 500 kcmil copper conductors at 75 degrees C are 380 A each, so the phase carries 760 A.

Do parallel conductors have to be identical?

Yes. Within each phase group, the paralleled conductors must match in length, material, circular-mil size, insulation type, and termination method under 310.10(G)(1). Different phases may differ from each other, but the conductors making up a single phase may not.

Do you derate parallel conductors for conduit fill?

Only when the sets share a raceway. One set per raceway keeps a three-wire set at three current-carrying conductors with no adjustment; all sets in one raceway counts every conductor and applies the Table 310.15(C)(1) factor.

What size equipment grounding conductor do parallel conductors need?

A full-size Table 250.122 EGC, sized on the overcurrent device, in each raceway under 250.122(F), not the total split among the raceways.

Related calculators

• Wire Ampacity Calculator: the derated Table 310.16 ampacity of a single conductor, the per-conductor value this tool multiplies.
• Wire Size Calculator: size a single conductor for a load by ampacity and voltage drop together.
• Equipment Grounding Conductor Size Calculator: the full Table 250.122 EGC with the 250.122(A) cap and 250.122(B) voltage-drop upsize.
• Ampacity Correction & Adjustment Calculator: the ambient and conductor-count factors that derate each conductor.