Tap Rules Practice Questions

B. A conductor whose overcurrent protection ahead of it is rated higher than the conductor's own ampacity

The word describes the protection and not the joint, which is why a mechanical connection to a bigger wire is not by itself a tap. Everywhere else conductors are protected in accordance with their ampacities, and the tap permissions are named departures from that. So a stem handing you a small conductor behind a large device is not describing a mistake, it is asking whether you know a permission exists and what it costs.

NEC 2026 240.4

A. At the point where the conductor receives its supply, except as the subdivisions of that section specify

Location in circuit is the parent rule and the tap permissions all live underneath it as named departures. Reading it first is what tells you that a tap is an exception being claimed rather than an ordinary installation. The 10 foot figure belongs to one subdivision and using it as a general rule is the mistake the second wrong answer collects.

NEC 2026 240.21

C. No, that permission is withdrawn for tap conductors at the front of the feeder tap section

One sentence at the head of the feeder tap section throws the rounding permission out, and it is true of every subdivision underneath. So an option that sizes a tap and then rounds the device up is wrong before you work out which subdivision applies, which is worth more than any rule of thumb about lengths. The transformer secondary section carries the same sentence for the same reason.

NEC 2026 240.21(B)

D. 240.21, Location in Circuit

The subdivision is a short list of pointers, and Location in Circuit is the one that opens the feeder tap and transformer secondary families. The rest of the list reaches household ranges and cooking appliances, fixture wire, two busway sections and single motor taps. Knowing the pointer exists is what keeps you from working a tap question out of the general protection rule and producing a conductor far too large.

NEC 2026 240.4(E)

A. A tap conductor may not supply another conductor except through an overcurrent device meeting the general protection rule

This sentence sits at the head of the section and reaches every subdivision below it, so a stem that runs one tap into a second unprotected run has already failed on structure and the lengths in it do not matter. It pairs with the withdrawal of the rounding permission as the two eliminations that come from the parent rather than from a pattern across the children. Both are worth striking answers on before you name the subdivision.

NEC 2026 240.21

B. There is no length limit

This is the item in the family most likely to turn up, because an option saying there is no limit reads like the one somebody invented to fill a slot. It is correct, and what the subdivision charges instead is location and termination: protection from physical damage, a single circuit breaker or single set of fuses holding the load to the conductor ampacity, that device part of or immediately adjacent to the disconnecting means, and the disconnect at a readily accessible location outside or inside at the nearest point of entrance.

NEC 2026 240.21(B)(5)

D. From the ampacity rules in Article 310, after any correction and adjustment

A tap rule sets a floor and the conductor's real ampacity is what has to clear it, so the lookup is the ordinary one and the derating you would do anywhere else still happens. Several subdivisions say this outright by requiring the terminating device to hold the load to the ampacity permitted by that article. Reaching for the standard device ratings instead is the same error as rounding, which this family does not allow.

NEC 2026 310.14, 240.21(B)

C. The transformer secondary conductor subdivisions of the location rule

Ask what the conductor is connected to where it starts. A feeder puts you in the feeder tap subdivisions and a transformer secondary puts you in the secondary conductor subdivisions, unless that transformer is itself sitting on a tap. Article 450 protects the transformer and is referenced from here, but it does not answer the conductors on its own.

NEC 2026 240.21(C)

C. Not less than the combined calculated loads on the circuits supplied, and not less than the rating of the equipment containing an overcurrent device supplied by them or the rating of the device at their termination

The 10 foot rule does not price itself against the feeder device at all, which is the difference people run together. It looks downstream instead, at the load and at what the conductors land in, and both halves have to be met. The one tenth figure belongs to the same subdivision but only to field installations where the tap conductors leave the enclosure or vault in which the tap is made.

NEC 2026 240.21(B)(1)

D. 60 amperes

One tenth of 600 is 60 amperes, and that condition applies alongside the load and equipment conditions rather than instead of them, so the larger of the results governs. The 200 ampere figure is one third of the feeder device, which belongs to the 25 foot subdivision and not to this one. Importing an ampacity condition from a neighboring subdivision is the single most common way to lose one of these items.

NEC 2026 240.21(B)(1)

A. The tap conductors do not extend beyond the enclosure of the switchboard, switchgear, panelboard, disconnecting means or control devices they supply

The 10 foot rule keeps the conductors inside the equipment they serve and, except at the point of connection to the feeder, inside a raceway running from the tap to that equipment. The minimum size, the no-splice rule and the height above the floor all belong to the high bay manufacturing subdivision, which buys a much longer run by charging conditions on the building. Options in this family are routinely built by lifting a clause from the subdivision next door.

NEC 2026 240.21(B)(1)

B. 133.3 amperes

The 25 foot subdivision prices the tap at one third of the device protecting the feeder conductors, and one third of 400 is 133.3 amperes. The 40 ampere figure is one tenth, which belongs to a field condition in the 10 foot subdivision. Nothing here is measured against the load, which is what separates this rule from the 10 foot one.

NEC 2026 240.21(B)(2)

D. 1/0 AWG

The requirement is 133.3 amperes, and in the 75 degree C copper column 1 AWG carries 130 amperes, which is short, while 1/0 AWG carries 150. Rounding the conductor down to 1 AWG because it is close is the same instinct as rounding the device up, and this family allows neither. 2/0 AWG works but is not the smallest size that does.

NEC 2026 240.21(B)(2), Table 310.16

C. A single circuit breaker or a single set of fuses limiting the load to the ampacity of the tap conductors, which may then supply any number of overcurrent devices on its load side

One device, sized to hold the load to what the tap conductors can carry, is the price of the permission, and the sentence about additional devices on its load side is there to say the single device is a gate and not a limit on the distribution beyond it. A main breaker of any rating misses the sizing half. The clause about what a tap terminates in is the one candidates stop reading before they reach.

NEC 2026 240.21(B)(2)

B. Those of a single-phase transformer with a 2-wire secondary, or a three-phase delta-delta transformer with a 3-wire secondary

Two configurations are carved out and everything else needs protection of its own or a subdivision that permits something else. The reason is that only in those two does the primary current track the secondary current closely enough for the upstream device to mean anything. The same carve-out appears twice in Article 240, once in the general protection rule and once at the head of the secondary conductor subdivisions.

NEC 2026 240.21(C)(1), 240.4

A. It may not exceed the secondary conductor ampacity multiplied by the secondary-to-primary voltage ratio

The device is on the primary and the conductors being credited are on the secondary, so the rating has to be translated across the transformer by the voltage ratio before the comparison means anything. The rule also requires the protection to comply with the transformer protection article, so both conditions run at once. Comparing a primary device directly with a secondary ampacity is the error the first wrong answer collects.

NEC 2026 240.21(C)(1), 450.5

A. No, that permission is withdrawn for transformer secondary conductors as well

The sentence appears twice, once at the head of the feeder tap subdivisions and once at the head of the transformer secondary subdivisions, so both families lose the rounding permission. A candidate who read it in one place and assumed it belonged only there will answer this one wrong with some confidence. Both sections state it before any subdivision, which means it applies to all of them by construction.

NEC 2026 240.21(C)

B. 200 amperes

The primary conductors are priced at one third of the feeder device, which is 100 amperes, and the secondary conductors at the primary-to-secondary voltage ratio times that same third. The ratio here is 480 over 240, which is 2, so the secondary needs 200 amperes. Answering 100 stops at the primary condition and never crosses the transformer.

NEC 2026 240.21(B)(3)

C. One primary conductor plus one secondary conductor, excluding any portion of the primary that is protected at its ampacity

This is the one subdivision in the family that measures across the transformer, and the exclusion at the end is what lets a long protected primary run reach a remote transformer without spending the allowance. The secondary conductors of a transformer fed this way are answered by this same subdivision rather than by the ordinary secondary rules, which the secondary section says outright. A transformer in a stem does not automatically move you into the secondary family.

NEC 2026 240.21(B)(3)

D. The tap conductors are 6 AWG copper or 4 AWG aluminum or larger, are continuous with no splices, and the tap is made not less than 30 feet from the floor

This subdivision buys its distance with conditions on the building and the installation rather than with copper, and its ampacity condition is the same one third the shorter indoor rule uses. The building is over 35 feet high at the walls, only qualified persons service the systems, and the run is not over 25 feet horizontally or 100 feet in total. That is why the rule of thumb about longer runs needing fatter conductors breaks here.

NEC 2026 240.21(B)(4)

B. 120 amperes

The condition takes one tenth of the primary device rating, which is 30 amperes, and multiplies it by the primary-to-secondary voltage ratio, which is 480 over 120, or 4. That gives 120 amperes. Stopping at the tenth and never crossing the transformer produces the 30 ampere answer, which is the miss this condition is written to catch.

NEC 2026 240.21(C)

D. 100 amperes

This subdivision prices the secondary at the primary-to-secondary voltage ratio times one third of the primary device rating. One third of 150 is 50, the ratio is 2, and the product is 100 amperes. Answering 50 stops before the ratio, which is the same slip that costs people the transformer tap item.

NEC 2026 240.21(C)(6)

A. Qualified persons servicing the systems, an ampacity not less than the secondary current rating of the transformer, the sum of the device ratings not exceeding that ampacity, all the devices grouped, and protection from physical damage

This is the only subdivision in the family that lets the load terminate in more than one device, and it pays for that with the transformer's own secondary rating as the ampacity floor plus a cap on the sum of the device ratings. It is limited to industrial installations supplying switchgear or switchboards, and only where maintenance and supervision ensure qualified service. The other three options are the ampacity conditions of neighboring subdivisions.

NEC 2026 240.21(C)

C. What the conductors are connected to at their supply end, since the unlimited length permission exists on the feeder side and again on the transformer secondary side

Both families carry an outside rule with no length limit and nearly identical conditions, so the word outside on its own does not tell you which one you are in. Ask what the conductor is connected to where it starts, then read that subdivision's conditions rather than the ones you remember. Length, material and construction type never decide which subdivision applies.

NEC 2026 240.21(B)(5), 240.21(C)

A. Grounding electrode conductors

The subdivisions run from branch-circuit conductors and feeder taps through transformer secondary conductors, service conductors, busway taps, motor circuit taps and conductors from generator terminals. A grounding electrode conductor is not a circuit conductor and is sized and protected under Article 250 instead. Knowing what is on the list is what keeps you from hunting for a tap rule that was never written.

NEC 2026 240.21