Put in the kVA and both voltages and read back the rated current on each side, the largest overcurrent device Table 450.5(B) allows, and the secondary conductor that goes with it. Table values are the 2026 edition as printed.
The nameplate rating. It is the same number on both sides.
Three-phase puts the square root of three in the denominator on both sides.
The line to line voltage. On a 208Y/120 secondary that is 208, not 120.
Which row of the table you are in. The two rows give different answers for the same transformer.
Only these two secondaries can be covered by the primary device, per 240.4(F) and 240.21(C)(1). A 208Y/120 or a 240/120 secondary is neither of them.
Fuses carry the extra standard ratings of 1, 3, 6 and 601 amperes, which matter on a small control transformer.
The column the secondary conductor is read in. Terminations on a circuit of 100 amperes or less sit on the 60 degree column unless the equipment is marked otherwise.
This page holds the table values so you get both currents and a device rating while you are still standing in front of the transformer. The transformer tool on the calculators page makes you type each percentage in from your own book, because finding the right row of Table 450.5(B) is the thing the exam is testing.
Use this one on the job. Use that one the week before the test, and read the transformer calculations study page for why the secondary conductors are a separate question from the transformer itself.
Two currents come first, and both come from the same kVA. Rated current is the volt-amperes divided by the voltage on that side, and on a three-phase transformer that voltage is multiplied by the square root of three before you divide. The same kVA sits on both sides, so on a step down transformer the secondary current is the larger of the two.
Protecting the transformer itself is 450.5. That is the 2026 number; the same rule was 450.3 in the 2023 book, and a tab reading 450.3 now opens on Reconditioned Equipment. Table 450.5(B) is the one most journeyman questions use, because it covers transformers of 1000 volts and less.
Which row you are in is set by the installation, not by the transformer. Primary only protection reads 125 percent where the rated primary current is 9 amperes or more, 167 percent where it is less than 9 amperes and 300 percent where it is less than 2 amperes. Protection on both sides reads 250 percent on the primary at any current, with the secondary at 125 percent for 9 amperes or more and 167 percent below that.
Footnote 1 is the next size up rule, and it sits on the 125 percent cells only. Where 125 percent of the rated current does not land on a standard rating in 240.6, the next higher standard rating is permitted. The 167, 250 and 300 percent cells carry no footnote, so those are ceilings: you take the largest standard rating at or below the number. Section 240.6(A) does permit devices with nonstandard ratings, which is how a very small control transformer gets a device at all.
The secondary conductors are a separate obligation with a separate section. Section 240.4(F) says a single-phase secondary other than two-wire, and a multiphase secondary other than delta-delta three-wire, is not considered protected by the primary device. A 208Y/120 secondary and a 240/120 secondary are both in that group, so the primary breaker does nothing for them.
What makes those conductors lawful is 240.21(C), which lists six cases with their own lengths and ampacity conditions. It also says, in its opening lines, that 240.4(B) is not permitted for transformer secondary conductors. That removes the next size up habit from the secondary side: the device at the end of the run cannot be larger than the ampacity of the conductor feeding it.
Two ratio numbers are worth carrying into the exam. For a run of 10 feet, condition (4) of 240.21(C)(2) asks for the primary-to-secondary voltage ratio times one tenth of the primary device where the conductors leave the enclosure the supply connection is made in. For a run of 25 feet, 240.21(C)(6) asks for that same ratio times one third of the primary device.
Where the secondary really is two-wire single-voltage, or delta-delta three-wire, 240.21(C)(1) lets the primary device cover it, provided the primary protection follows 450.5 and does not exceed the secondary conductor ampacity times the secondary-to-primary voltage ratio. Turn that around and the conductor has to carry the primary device times the primary-to-secondary ratio, which on a 480 to 120 control transformer is four times the primary device rating.
Then the ordinary conductor rules finish the job. Section 110.14(C) holds the conductor to the column that matches the terminations, 240.4(D) caps the device on 14, 12 and 10 AWG, and Table 250.122 gives the equipment grounding conductor read on the rating of the device ahead of the equipment.
One more obligation this page names and does not size. A transformer secondary is usually a separately derived system, so 250.30(A) applies: the system bonding jumper, the grounded conductor connection and the grounding electrode conductor all have their own sizing rules there. And if the transformer is installed as a motor control circuit transformer, 450.5(B) hands it to 430.72(C) instead.
A 75 kVA transformer, three-phase, 480 volts in and 208Y/120 out, protected on the primary only. Rated primary current is 75,000 divided by 480 times the square root of three, which is 90.2 amperes. The secondary is 75,000 divided by 208 times the square root of three, or 208.2 amperes.
The primary current is 9 amperes or more, so the primary only row of Table 450.5(B) reads 125 percent. That is 112.8 amperes, which is not a standard rating, so footnote 1 permits the next one up in Table 240.6(A): a device rated 125 amperes. Table 250.122 then gives 6 AWG copper for the equipment grounding conductor on a 125 ampere device.
The secondary is 208Y/120, so 240.4(F) keeps the primary device from covering the secondary conductors, and 240.21(C) is where they live. Read against the rated secondary current in the 75 degree column, 4/0 AWG copper at 230 amperes is the first size that carries 208.2 amperes, with 3/0 AWG at 200 amperes short of it. A 25 foot run under 240.21(C)(6) also has to carry 2.31 times one third of the 125 ampere primary device, which is 96.2 amperes, and the 4/0 clears that easily. Put those numbers in the calculator above and the same three answers come back.
The value used for your inputs is read from NEC 2026 Table 240.6(A), Table 250.122, Table 310.16 and Table 450.5(B), the way you would read it in the exam room, and the working above names the row it came from. The tables themselves belong to NFPA and are not printed on this page. Bring your own copy of the 2026 code, which is what the exam requires anyway, and check the row there.
Divide the volt-amperes by the voltage on the side you want. A 75 kVA transformer is 75,000 volt-amperes, so at 480 volts single-phase that is 156.3 amperes. On a three-phase transformer the voltage is multiplied by the square root of three first, which is why the same 75 kVA at 480 volts three-phase is 90.2 amperes.
With protection on the primary only, Table 450.5(B) allows 125 percent of the rated primary current when that current is 9 amperes or more. For a 75 kVA three-phase transformer at 480 volts that is 125 percent of 90.2 amperes, or 112.8 amperes, and footnote 1 permits the next standard rating in Table 240.6(A), which is 125 amperes. That is a ceiling rather than a required size.
Table 450.5(B) does not require one where the primary device is set at 125 percent or less of the rated primary current. The conductors are a separate question. Section 240.4(F) says a secondary that is not a two-wire single-phase secondary or a delta-delta three-wire secondary is not considered protected by the primary device, so those conductors need one of the six cases in 240.21(C).
They are the columns of the table. On primary only protection a rated primary current of 9 amperes or more is 125 percent, less than 9 amperes is 167 percent and less than 2 amperes is 300 percent. With protection on both sides the primary column is 250 percent at any current, and the secondary is 125 percent at 9 amperes or more and 167 percent below that.
On the 125 percent cells, yes. Footnote 1 of Table 450.5(B) permits the next higher standard rating in 240.6 where 125 percent of the rated current does not land on one. The 167, 250 and 300 percent cells carry no such footnote, so they are ceilings and you take the largest standard rating at or below the number. Section 240.4(B), the next size up rule, is not permitted for transformer secondary conductors at all, and 240.21(C) says so in its opening lines.
The rated secondary current is 208.2 amperes, and 4/0 AWG copper is 230 amperes in the 75 degree column of Table 310.16, so 4/0 is the first size that carries it. 3/0 AWG at 200 amperes is short. A 25 foot run under 240.21(C)(6) also has to carry the voltage ratio of 2.31 times one third of the 125 ampere primary device, which is 96.2 amperes, and 4/0 AWG clears that as well.
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