Short answer: Conductors in parallel are joined at both ends so two or more wires share the current of one phase, neutral or grounded conductor. They are generally permitted in sizes 1/0 AWG and larger (NEC 310.10(G)(1)). The wires that make up each phase, neutral or grounded conductor all have to be the same length, the same metal, the same size, the same insulation type, and terminated the same way. The raceways or cables that carry them also need the same number of conductors and the same electrical characteristics (NEC 310.10(G)). Each current-carrying conductor in a raceway, including each conductor of a parallel set, counts toward ampacity adjustment (NEC Table 310.15(C)(1)). Whether the neutral counts follows 310.15(E), and equipment grounding conductors do not count. If the overcurrent device is 800 A or less, you can sometimes go up to the next standard size when the conditions in 240.4(B) are met. Once the device is over 800 A, the total ampacity has to be at least the device rating, with no rounding up (NEC 240.4(C)).
PSI lists Branch Circuits and Conductors (10 scored NEC Knowledge and 4 scored Calculations questions) and Feeders (3 and 3) as subject areas of the Texas Journeyman exam, and names conductors and feeder overcurrent protection in them. PSI does not say how many questions cover parallel conductors. The Texas Journeyman Practice Pack includes 1,000 original questions and 10 timed mock exams in the real two-part format, plus step-by-step solutions for every calculation. You can try free practice questions with no sign-up. The full pack is US$39, paid once.
Tsai Electro is an independent learning site. Not affiliated with TDLR, PSI or NFPA. We explain the rules in plain language and do not copy NEC text. Check every rule and table value in your own 2026 NEC, and follow your supervisor and the local inspector.
Sizing a feeder with parallel sets
The conductor ampacity page covers Table 310.16, temperature correction and adjustment for a single conductor. For a parallel feeder, find one conductor’s ampacity after correction, adjustment and the termination temperature limit. Multiply by the number of conductors per phase, then check the total against the overcurrent device under 240.4.
Why feeders are run in parallel
A 1,200 A feeder in one conductor per phase would need a very large wire that is hard to pull, bend and terminate. Three or four smaller sets per phase are easier to pull and terminate, as long as the equipment has terminals listed for that number and size of conductors. If one path has more impedance than the others, the current won’t divide evenly. That’s why the conductors have to match.
The matching rules (310.10(G))
| Item | What has to match | Example |
|---|---|---|
| Size | 1/0 AWG and larger, all conductors of one phase the same size | Three 500 kcmil per phase, not two 500 kcmil and one 400 kcmil |
| Within a phase | Same length, conductor material, size (circular mil area), insulation type and termination method | All phase A conductors are 60 ft copper THHN on the same lug type |
| Between raceways or cables | Same number of conductors and the same electrical characteristics | Each conduit carries A, B, C and the neutral |
| Between phases | The matching applies within each phase, not between phases | Phase B can use a different size than phase A, as long as each phase meets its own ampacity and installation rules. Almost all installations use one size |
An exam question might give you three conductors of different lengths and ask whether they can run in parallel. The longer conductor has more impedance, so it carries less current and the shorter ones carry more than their share. The rule calls for the same length.
For most exam questions, use 1/0 AWG as the minimum unless a specific exception applies.
Ampacity adjustment with parallel sets
- Separate raceways, one set each. In this 3-phase, 4-wire example, each raceway holds one conductor from each phase and one neutral. Whether the neutral counts as current-carrying follows 310.15(E). For balanced, linear loads on a 3-phase 4-wire circuit it generally does not, so each raceway has 3 current-carrying conductors and no adjustment.
- All sets in one raceway. Two sets of A, B and C in one conduit is 6 current-carrying conductors. Table 310.15(C)(1) applies an 80 percent adjustment for 4 to 6 conductors.
- Don’t split the phases into different metal conduits. Article 300 requires all phase conductors, the neutral and the equipment grounding conductor of a circuit to be in the same metal raceway, so induced currents do not heat the steel. For feeders like the ones on this page, keep each full set together in its own raceway. Putting all the phase A conductors in one metal conduit and all the phase B conductors in another causes inductive heating. Article 300 has specific provisions for the few arrangements that differ.
Why the 800 A cutoff matters
| Device rating | Rule | What it means |
|---|---|---|
| 800 A or less | 240.4(B) | If the total ampacity falls between two standard sizes, the next higher standard size is allowed when the conditions in 240.4(B) are met |
| Over 800 A | 240.4(C) | The total ampacity must be equal to or greater than the device rating, with no rounding up |
Worked example 1: a 400 A feeder
Two sets of 3/0 AWG copper THHN/THWN-2, one set per conduit, 75°C terminations, balanced linear 3-phase load, no ambient correction. Each conduit has 3 current-carrying conductors.
- Ampacity per conductor: Table 310.16, 75°C column, 3/0 copper = 200 A.
- Adjustment: each conduit has 3 current-carrying conductors, so none.
- Total: 2 × 200 = 400 A. That passes the ampacity check for a 400 A device.
Worked example 2: the same sets in one conduit
Now put both sets in one conduit instead.
- Count: 6 current-carrying conductors, so the factor is 80 percent.
- Start from the insulation rating: 3/0 copper at 90°C = 225 A. 225 × 0.80 = 180 A.
- Termination limit: the 75°C value of 200 A does not limit it, because 180 A is lower.
- Total: 2 × 180 = 360 A, so these conductors won’t carry a 400 A load in one conduit. You’ll need separate conduits or larger wire. (If the calculated load does not exceed 360 A, a 400 A device may be permitted under 240.4(B), but only when the installation meets every condition of that section, including its limits on circuit type and device rating.)
Worked example 3: a 1,200 A feeder
Copper, 75°C terminations, one set per conduit.
| Option | Per conductor (Table 310.16, 75°C) | Total | Result |
|---|---|---|---|
| 3 sets of 500 kcmil | 380 A | 1,140 A | Too small. Over 800 A, you cannot round up to 1,200 A |
| 3 sets of 600 kcmil | 420 A | 1,260 A | OK |
| 4 sets of 350 kcmil | 310 A | 1,240 A | OK |
The equipment grounding conductor in parallel runs
When wire-type equipment grounding conductors are used with parallel sets in separate raceways, each raceway gets one, sized from the rating of the overcurrent device ahead of the feeder using Table 250.122. It is not divided by the number of sets. A qualifying metal raceway can serve as the equipment grounding path instead, and multiconductor cables have their own provisions. For the Table 250.122 lookup, see the grounding conductor sizing page.
Four quick questions
- Smallest conductor size generally permitted in parallel? 1/0 AWG.
- Three sets of 500 kcmil copper (380 A each) on a 1,200 A breaker. Compliant? No. 1,140 A is less than 1,200 A, and rounding up is not allowed above 800 A.
- Phase A conductors are 60 ft, 60 ft and 65 ft. Compliant? No. Paralleled conductors of one phase are the same length.
- Two parallel sets in one conduit: adjustment factor? 80 percent for 6 current-carrying conductors.
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Three NEC spots worth marking
NFPA 70, National Electrical Code, 2026 Edition (softcover). Highlight Table 310.16, Table 310.15(C)(1) and the 800 A line in 240.4 in the same color. You’ll use all three when you size a parallel feeder. The Texas exam is open book. PSI allows a soft-bound NEC with highlighting, underlining and notes made in advance. Loose-leaf, spiral-bound and ring-bound copies are not allowed, and only permanent index tabs made or supplied by the NEC publisher are permitted; homemade and third-party tabs are not.
FAQ
What is the smallest wire you can run in parallel?
Generally 1/0 AWG. The section lists limited exceptions for smaller sizes.
Do parallel conductors have to be the same length?
Yes. The conductors of each phase, neutral or grounded conductor have the same length, material, size, insulation type and termination method.
Can you round up the breaker on a parallel feeder?
At 800 A or less, the next higher standard size is allowed under the conditions in 240.4(B). Above 800 A, the total ampacity has to equal or exceed the device rating.
Does putting parallel sets in one conduit change the ampacity?
Yes. Every current-carrying conductor counts, so two sets of three phases is six conductors and an 80 percent adjustment.
Does each conduit need its own ground wire?
When wire-type equipment grounding conductors are used with parallel runs in separate raceways, each raceway gets one, sized from Table 250.122 using the full overcurrent device rating. A qualifying metal raceway can serve as the equipment grounding path instead.
Sources checked October 2026
- PSI Candidate Information Bulletin, Texas electrician examinations (updated September 15, 2026): https://test-takers.psiexams.com/api/content/bulletin/1682
- NFPA 70, National Electrical Code, 2026 Edition, Articles 240, 250, 300 and 310
