Parallel Conductors Practice Questions

C. 1/0 AWG

The floor is higher than most candidates guess, and guessing low is the cheapest point to lose in this topic. It applies to each ungrounded conductor, each grounded conductor and each neutral conductor. Below that size the section names a short list of specific applications, and nothing outside that list qualifies.

NEC 2026 310.10(G)(1)

A. Conductors supplying control power to indicating instruments, contactors, relays, solenoids and similar control devices

That case and circuits at frequencies of 360 hertz and higher are the two the exception names, and both come with three conditions. The conductors have to be in the same raceway or cable, each one has to carry the entire shared load current on its own, and the protection has to keep any single conductor from being overloaded if a partner disconnects. All three, or the exception does not apply.

NEC 2026 310.10(G)(1)

D. Grounded neutral conductors of 2 AWG and 1 AWG, under engineering supervision, for existing installations

It is written to existing work rather than to new work, and the informational note explains why it is there: it relieves overheating in neutrals carrying a heavy content of triplen harmonic currents. Engineering supervision is a condition and not a formality. An item that offers this exception for a new installation is offering something the words do not cover.

NEC 2026 310.10(G)(1)

B. Five

Same length, same conductor material, same size in circular mil area, same insulation type, and terminated in the same manner. Four of the five are about resistance, since resistance is what divides the current between two paths joined at both ends. The fifth is about what the conductor is allowed to reach once the resistance has done its work.

NEC 2026 310.10(G)(2)

A. Each ungrounded conductor, the grounded conductor, the neutral, the equipment grounding conductors, the equipment bonding jumpers and the supply-side bonding jumpers

The comparison runs inside one group and never between groups, so the set on A phase owes nothing to the set on B phase. That is spelled out in the subsection immediately after, which says conductors of one paralleled set are not required to have the same physical characteristics as those of another. An item offering cross-phase matching is offering the inversion of the rule.

NEC 2026 310.10(G)(2), 310.10(G)(3)

C. No, since conductors of one paralleled set are not required to have the same physical characteristics as those of another set

The code says this outright and it is the half of the rule people forget. Each phase is compared with itself and the sets are free to differ from one another. Whether that installation is a good idea is a different question from whether the matching conditions bar it.

NEC 2026 310.10(G)(3)

B. That they contain the same number of conductors and have the same electrical characteristics

The condition lands on the enclosures rather than on the conductors, and it is the clause that stops one set running in steel while its partner runs in a nonmetallic raceway. Steel adds inductive reactance that a nonmetallic raceway does not, so the two paths would no longer share current evenly. Trade size and routing are not what the rule names.

NEC 2026 310.10(G)(3)

D. The raceways do not have the same electrical characteristics, so the sets will not divide current evenly

Equal length and equal size do not save an installation whose two paths sit in raceways with different impedance. The overcurrent device ahead of the run sees the total, so it will never detect the split. This is the case the separate raceway condition was written for.

NEC 2026 310.10(G)(3)

C. They apply, since parallel conductors comply with the ambient correction and conductor count adjustment rules like any others

The subsection says so in one line and it is worth reading, because paralleling into a single raceway can hand back the ampacity you paralleled to obtain. 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.

NEC 2026 310.10(G)(4), 310.15(C)(1)

B. One sized from the equipment grounding conductor table on the rating of the single overcurrent device ahead of the whole run

Not a share of that rating, and not divided by the number of raceways. A ground fault inside one raceway returns through that raceway's own grounding conductor, 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.

NEC 2026 250.122(H)(1)(b)

A. 3 AWG

Read the table on the 400 ampere device and take that size four times over, once in each raceway. The 1 AWG answer is the aluminum entry on the same row and is the trap for anyone reading across instead of down. Every wrong answer people produce on this item is a fraction of the right one or the wrong material column.

NEC 2026 250.122(H)(1)(b), Table 250.122

D. 3/0 AWG

The aluminum column on the 800 ampere row gives the answer, and it goes in every raceway rather than being split among them. The 1/0 answer is the copper entry from that same row, which is what you land on by reading the wrong column. Nothing about the number of raceways enters the sizing.

NEC 2026 250.122(H)(1)(b), Table 250.122

B. A single wire-type conductor sized from the equipment grounding conductor table on the feeder or branch circuit device

With everything in one enclosure there is only one fault path to build, so one conductor does the work. The multiple raceway rule sits in the next paragraph and reads very differently, which is why an item always tells you how many raceways there are. Read that number before you read anything else in the stem.

NEC 2026 250.122(H)(1)(a)

A. No larger than the largest ungrounded conductor in that raceway

The subsection carries the ceiling in its own numbered item, right underneath the sizing rule. It matters on a large parallel run where the table value can climb above the phase conductor in one pipe. Without it you would install a grounding conductor bigger than the conductor it is protecting, which the code has never asked for.

NEC 2026 250.122(H)(1)(b)

C. Each raceway holds a complete set, one conductor of every phase plus the neutral

The requirement to keep the conductors of a circuit together is applied separately to each portion of the paralleled installation, which is what produces the complete set in each pipe. Put every A phase conductor into one steel raceway and the magnetic fields stop canceling, and the raceway heats. The wrong answer here looks tidier than the right one, which is exactly why it is offered.

NEC 2026 300.5(B)(1)

D. That it connect to all conductors of the paralleled set, grounded and ungrounded as applicable

Connecting to one conductor of a set would force that conductor to carry the whole tap load while its partners carry none. Connections, taps and extensions all fall under the same sentence. The 25 foot figure belongs to a tap rule in Article 240 and is offered here because it sounds familiar.

NEC 2026 300.5(B)(1)

A. One raceway per phase, run in close proximity, as isolated phase, neutral and grounded conductor installations

The relief exists because a nonmetallic raceway has no magnetic core to heat, so sorting by phase does not produce the induced heating the general rule guards against. Close proximity is a condition rather than a suggestion, since separation would reintroduce the impedance imbalance. The conductors still have to satisfy the rule on inductive heating in metal enclosures where they land.

NEC 2026 300.5(B)(1)

C. Because current divides between paths by impedance, and no device protecting the set as one can detect an unequal split

Two paths joined at both ends see the same voltage, so the current divides in inverse proportion to impedance whatever anyone intended. The overcurrent device is sized for the pair and sees the total, which has not changed, so it stays content while one conductor runs above its share. The code removes the causes of the split rather than trying to detect it.

NEC 2026 310.10(G)(2)

B. 70 percent

Each set contributes three ungrounded conductors and a counted neutral, so there are eight current-carrying conductors and eight falls in the band that runs from seven through nine. Paralleling divides the current but not the heat, since every conductor in the raceway still has its own share to get rid of. Counting a parallel set as one conductor per phase is the shortcut that lands you in the wrong band.

NEC 2026 310.15(C)(1), Table 310.15(C)(1)

D. 400 amperes

The table gives 200 amperes for that conductor in the 75 degree column, and two of them joined at both ends are treated as one conductor of the combined size. Two times 200 is 400. The 200 ampere answer is one conductor rather than the pair, which is the mistake that shows up when someone forgets what paralleling was for.

NEC 2026 310.10(G), Table 310.16

C. The bonding jumper table, either as an individual jumper per raceway or as a single jumper sized on the sum of the circular mil areas of the largest ungrounded conductors from each set

Two methods, both keyed to the same table, and an item can hand you the geometry for one and the answer for the other. The individual jumper is selected on the largest ungrounded supply conductor in its own raceway. The single jumper is selected on the sum across all the raceways, which is why the two answers differ.

NEC 2026 250.102(C)(2), Table 250.102(C)(1)

D. It is installed in each raceway, connected in parallel, and none of them is smaller than 1/0 AWG

The rule sizes each one from the bonding jumper table and then puts a hard floor of 1/0 AWG underneath the result. The floor is what the item is really testing, since the table alone can hand you something smaller on a modest service. This is a service rule and it sits in Article 250 rather than in the paralleling section.

NEC 2026 250.24(D)(2)

A. In accordance with the bonding jumper section, 250.102

The paralleling section sends equipment grounding conductors to one place and bonding jumpers to another, and they are different sections with different tables behind them. Getting the two confused is easy, since both conductors are green and both are in the same pipe. Read which conductor the stem is naming before you open anything.

NEC 2026 310.10(G)(6), 250.102

B. They may terminate in a single terminal identified for connection of more than one conductor

This is the only exception to the one conductor per terminal rule, and the word identified is the condition. An ordinary bar is not identified for more than one conductor even if two will physically fit. The exception exists because those grounded conductors are already electrically one conductor at both ends.

NEC 2026 408.41

D. An adjustment factor question, dressed as a paralleling question

Paralleling multiplies the conductor count in whatever raceway you put it in, so the count is the point and the paralleling is the costume. Work the count before you open the table, the same as with any adjustment problem. Whether the neutrals are counted turns on the system and on the kind of load, which is a separate decision to make first.

NEC 2026 310.15(C)(1)