Checked against primary sources 2026-08-24

Paralleled sets share current only if they are identical, so the code makes them identical

Two conductors joined at both ends divide current by impedance, not by intention. Every identity condition in 310.10(G) exists to stop one of them taking more than its share.

Test yourself: 25 free questions on this topic

What paralleling means, and where the permission starts

Conductors in parallel are joined at both ends and treated as one conductor of the combined size. It is how a service carries more than any single conductor could, and how a run gets into a building that nobody could bend a single conductor into.

Conductors of each phase, polarity, neutral, or grounded circuit conductor shall be permitted to be connected in parallel only in sizes 1/0 AWG and larger (NEC 310.10(G)(1)). That floor is higher than most candidates guess, and guessing low is the single cheapest point to lose in this topic.

Below 1/0 AWG the section names a short list of specific applications and nothing outside that list qualifies. Read the list in your own book before test day, because an item that parallels a small conductor is testing that boundary and nothing else on this page.

The floor applies to the circuit conductors. Equipment grounding conductors and bonding jumpers in a parallel run are sized under Article 250 instead and are not held to 1/0 AWG, which is why a small grounding conductor in a parallel run is not automatically a violation.

Notice which direction the permission runs. Paralleling is not a general technique with a few exclusions bolted on. It is a restricted permission with a few extensions, and reading it the wrong way round produces confident wrong answers on the easiest items in the set.

What has to be identical

The comparison runs inside one set, not between sets. Every conductor of one phase matches every other conductor of that same phase, and the set on A owes nothing to the set on B. 310.10(G)(3) says as much outright, and an item that offers cross-phase matching as the answer is offering the inversion.

Each group is compared against itself and only itself: each phase, each polarity, the neutral, the grounded circuit conductor, the equipment grounding conductors, and the equipment bonding jumpers. Inside a group the conductors match in five respects, and each one is there for a reason you can reconstruct rather than memorize.

• The same length, because length is resistance and resistance is what divides the current.
• The same conductor material, because copper and aluminum of one size do not have one resistance.
• The same size in circular mil area, for the same reason again.
• The same insulation type, because insulation sets the temperature the conductor may reach, and a mismatched pair would be governed by the weaker one anyway.
• Terminated in the same manner, because a poor termination is resistance added at one end of one path and nothing downstream will ever reveal it.

Four of those five are about resistance. The fifth is about what the conductor is allowed to reach once the resistance has done its work. Say that out loud once and the list stops being a list.

A separate condition lands on the enclosures rather than the conductors. Where the sets run in separate cables or raceways, those cables or raceways carry the same number of conductors and have the same electrical characteristics (NEC 310.10(G)(3)). That is the clause that stops one set running in steel while its partner runs in PVC.

Why an unequal set is a hazard rather than an untidiness

Two paths joined at both ends see the same voltage across them, so the current divides in inverse proportion to impedance. That is a fact about circuits rather than a rule about wiring, and good intentions do not move it.

The numbers in this paragraph are invented for the example. Suppose two sets ought to be carrying 132 amperes each. Make one set 20 percent higher in impedance than the other, by running it a little longer or landing it a little worse, and the split becomes 144 and 120 instead.

Then look at what is protecting them. The overcurrent device is sized for the pair, so it sees the total, and the total has not changed. The device is perfectly content. It cannot see the split, nothing else in the circuit can see the split, and one conductor now runs above its share indefinitely under a device that will never operate.

That is the whole answer to any question asking why the identity conditions exist. No protective device can find an unequal division between two conductors it is protecting as one, so the code removes the causes of the division instead of trying to detect it.

The grounding conductor in a parallel run

Run a circuit in parallel through more than one raceway and each raceway takes its own wire-type equipment grounding conductor, where one is used, installed in parallel with the others and sized on the rating of the single overcurrent device ahead of the whole run (NEC 250.122(H)). Not a share of that rating. Not divided by the number of raceways. This is the part that gets missed.

The reason follows from where a fault happens. A ground fault inside one raceway returns through that raceway's own grounding conductor, and it returns at the fault current the system can deliver rather than at one raceway's fraction of the load. A grounding conductor sized by division would be undersized for the only job it will ever be asked to do.

The subdivision is titled Conductors in Parallel and it is worth reading in place rather than reconstructing, because it also covers what happens inside a multiconductor cable and when the metal raceway itself is doing the work. Find it by that title. It is 250.122(H) in the 2026 edition and it was 250.122(F) in the 2023 one, because three rules ahead of it gained letters this cycle and pushed everything behind them along.

The same geometry appears again on the supply side of the service, and it is handled by a different section with a different answer. Where the ungrounded supply conductors run in parallel in two or more raceways or cables, 250.102(C)(2) offers two ways of sizing the supply-side bonding jumper. One is an individual jumper for each raceway, selected from Table 250.102(C)(1) on the largest ungrounded supply conductor in that raceway. The other is a single jumper for two or more raceways, selected on the sum of the circular mil areas of the largest ungrounded conductors from each set. Read both, because an item can give you the geometry for one and the answer for the other.

Items in this area are usually a single sentence. Conductors run in parallel in four raceways, what size is the equipment grounding conductor in each. Every wrong answer on offer will be a fraction.

Each raceway carries a complete set

300.3(B) keeps the conductors of a circuit together in one raceway or cable, and it applies that separately to each portion of a paralleled installation. So each raceway of a parallel run holds a complete set, one conductor of every phase plus the neutral, rather than all of one phase.

Put every A phase conductor into one steel raceway and the magnetic fields stop canceling. The raceway becomes the core of an inductor wrapped around a current it cannot balance, and it heats. The same arrangement in a nonmetallic raceway still produces unequal impedance between the sets, which puts you back in the previous section.

It makes a clean exam item because the wrong answer looks tidier than the right one. Sorting conductors by phase across raceways is what an organized person would do. It is the exact thing the rule exists to stop.

The derating nobody expects

Paralleling multiplies the conductor count in whatever raceway you put it in. Two complete three-phase sets plus their neutrals is eight conductors in one pipe.

Six of those are current-carrying before anyone asks about the neutrals, so the adjustment in 310.15(C)(1) already applies. What the neutrals decide is which band of Table 310.15(C)(1) you land in, and the bands step down hard enough that two conductors move a conductor size.

So paralleling into a single raceway can hand back the ampacity you paralleled to obtain. That is one of the reasons parallel runs are normally pulled into separate raceways, and it is a useful sanity check on any answer that appears to gain nothing for the trouble.

Work the count before you open the table, the same as with any adjustment problem. Whether the neutrals count turns on the system and on the kind of load, and that decision is worth a page of its own rather than a line here.

Reading the question

• Gives you a conductor size and asks whether it may be paralleled: the 1/0 AWG floor, and the short list of applications underneath it.
• Gives you two runs of different lengths: the identity conditions, and the installation fails them rather than merely being untidy.
• Gives you separate raceways and asks about the grounding conductor: one in each raceway, sized on the device ahead of the run.
• Gives you one raceway and a conductor count: this is an adjustment factor question in a parallel costume.
• Asks why the conductors have to match: current divides by impedance, and the device protects the pair.
• Describes conductors sorted by phase between raceways: the circuit has been split, and the answer is about induced heating.

Two sentences carry this entire topic. Current divides by impedance, so the code makes the paths identical. Fault current comes back through its own raceway, so every raceway gets its own grounding conductor sized on the whole device. Everything else on this page follows from one of those two, which is why it is worth learning as physics rather than as a list.

Questions people ask

What does paralleling conductors mean?

Two or more conductors electrically joined at both ends and treated as one conductor of the combined size. NEC 310.10(G) is the permission, and it is written for each ungrounded conductor, the grounded conductor and the neutral conductor. It is how a service carries more current than any single conductor could, and how a large run gets into a building nobody could bend a single conductor into.

What is the smallest conductor size allowed in parallel?

1/0 AWG. NEC 310.10(G)(1) permits aluminum, copper-clad aluminum or copper circuit conductors to be connected in parallel only in sizes 1/0 AWG and larger. Two exceptions sit under it: sizes smaller than 1/0 AWG for control power to indicating instruments, contactors, relays, solenoids and similar control devices, or for frequencies of 360 Hz and higher, each subject to three stated conditions; and, under engineering supervision, 2 AWG and 1 AWG grounded neutral conductors for existing installations.

Do cables in parallel need to be the same length?

Yes. NEC 310.10(G)(2) requires the conductors making up each ungrounded conductor, grounded conductor, neutral, equipment grounding conductor, equipment bonding jumper or supply-side bonding jumper to be the same length, of the same conductor material, the same size in circular mil area, of the same insulation type, and terminated in the same manner. Length is resistance and resistance is what divides the current between the paths. An unequal set puts one conductor above its share indefinitely under a device that sees only the total and will never operate.

Can you parallel different sizes of conductor?

Not inside one set. Every conductor making up one phase, or the neutral, matches the others in that same group on circular mil area along with the other four conditions in NEC 310.10(G)(2). The comparison runs inside the group only, and NEC 310.10(G)(3)(b) says the conductors of one paralleled set are not required to have the same physical characteristics as those of another set. Where the sets run in separate cables or raceways, though, those cables or raceways have to have the same number of conductors and the same electrical characteristics.

How is the equipment grounding conductor sized in parallel raceways?

On the rating of the single overcurrent device ahead of the whole run, never on a share of it. NEC 310.10(G)(5) sends parallel equipment grounding conductors to 250.122 for sizing, and NEC 250.122(H) puts a wire-type conductor in each raceway, connected in parallel with the others and sized on that one device rating. A fault inside one raceway returns through that raceway's own conductor at whatever the system can deliver, rather than at one raceway's fraction of the load. Every wrong answer on a parallel item is a fraction.

Are neutrals paralleled too?

Yes. NEC 310.10(G)(1) names the grounded conductor and the neutral conductor alongside each ungrounded conductor, so the 1/0 AWG floor and the identity conditions apply to them the same way. Equipment bonding jumpers and supply-side bonding jumpers installed in parallel raceways are sized and installed under 250.102 instead (NEC 310.10(G)(6)). The one relief is the exception permitting 2 AWG and 1 AWG grounded neutral conductors in parallel, under engineering supervision, for existing installations.

What this page cites

• NEC 310.10(G) Conductors in parallel. The permission, the 1/0 AWG floor in (G)(1), the matching conditions in (G)(2), and the separate cables or raceways rule in (G)(3). Read the applications listed below the floor in your own book. source
• NEC 310.15(C)(1) Adjustment for more than three current-carrying conductors, which a parallel run in one raceway triggers immediately. The bands are in Table 310.15(C)(1) in your own book. source
• NEC 250.122(H) Conductors in Parallel. Equipment grounding conductors for paralleled circuits, in raceways and in multiconductor cables. This is 250.122(F) in a 2023 book. NFPA code-making panel 5 second revision 8360 relettered the section for the 2026 edition and its revised text runs 250.122(A) through 250.122(I). source
• NEC 250.102(C)(2) Supply-side bonding jumpers where ungrounded supply conductors run in parallel. Two sizing methods, both keyed to Table 250.102(C)(1). source
• NEC 300.3(B) Conductors of the same circuit run together, applied separately to each portion of a paralleled installation. source
• 16 TAC 73.100 Texas adopted the 2026 edition effective September 1, 2026. source