Feeders for a group of motors, and the allowance candidates hand out more than once

Where the difficulty sits

Feeder problems are where the motor area gets hard. The area holds 43 questions, of which two sit at the easiest difficulty level and nine at the hardest. Five of those nine involve a feeder. If you are triaging revision time and you want the questions that separate a pass from a near miss, this is the section they live in.

The reason is structural. A single motor problem has one current, one multiplier and one answer. A feeder problem has a group, a rule that treats one member of the group differently from the rest, a second rule that treats the protective device differently again, and a rounding direction that reverses between the two. Every one of those joints is a place to put a distractor, and the bank uses all of them.

Three of the feeder questions also mix voltages inside the group, which adds a fifth joint. That one is covered further down, and it is the single most expensive habit on this topic.

Two sums that look alike

The conductor and the device protecting it are sized by different methods, which is unusual enough in the code that the exam leans on it. NEC 430.24 builds the conductor ampacity out of currents. NEC 430.62 builds the ceiling on the feeder protective device out of one device rating plus the currents of everything else.

One worked example from the bank makes the difference concrete. Three motors draw 44, 28 and 16 amperes from the table, and the largest branch-circuit device in the group is the 70 ampere device protecting the 44 ampere motor. The feeder device ceiling is 70 plus 28 plus 16, which is 114 amperes. The 44 ampere motor is represented by its device and counted once. The wrong option adds its full-load current on top of the device that already stands for it, and that option reads as careful work.

The allowance is granted once

The 125 percent in the conductor sum is the same 125 percent that sizes branch-circuit conductors for a single motor under 430.22. The feeder rule applies it one time, to the group, on the worst case machine.

The explanation you hear on site is that the margin covers the largest motor starting. Treat that explanation as the source of the error. Inrush runs six to eight times full-load current, so a quarter more would cover no motor starting anywhere. The 125 percent is a continuous duty margin borrowed from the single-motor rule and handed out once as a diversity allowance. The one motor at a time reasoning belongs to the feeder overcurrent rule at 430.62, which really does have to pass one motor's inrush, and keeping those two ideas in separate rooms is most of this topic.

Highest rated means largest current

NEC 430.17 settles which motor is the highest rated one, and the test is rated full-load current taken from the Article 430 tables. On a group that runs at one voltage and phase, that is the same machine as the biggest horsepower and the shortcut costs nothing. On a mixed group it can be a different machine entirely.

The bank question that carries this is worth working. Motor A is 20 horsepower on 240 volt three phase with a table current of 54 amperes. Motor B is 25 horsepower on 480 volt three phase with a table current of 34 amperes. Motor B has the larger horsepower and Motor A draws the larger current, because Motor B runs at twice the voltage. The 125 percent goes to Motor A: 54 times 1.25 is 67.5, plus 34 at full load, for 101.5 amperes. The option built on horsepower is there for everyone who sorted the list the quick way.

So sort the group by amperes before you pick the motor that gets the multiplier. A question that mixes voltages inside one group has mixed them deliberately, and the same habit protects you on the grouped disconnect, where 430.110(C) sums the full-load currents of everything that can run at once and then applies 115 percent to the total.

The non-motor side of the sum

Continuous and noncontinuous is a distinction that lives on the non-motor part of the feeder. The motors are already handled by the first two terms, and applying the continuous load rule to them adds the same margin twice.

A worked case from the bank: two continuous duty motors at 30 and 18 amperes, a 12 ampere continuous lighting load and a 10 ampere noncontinuous receptacle load. The largest motor takes 125 percent, giving 37.5. The second motor comes in at 18. The continuous lighting takes 125 percent under the ordinary feeder rule, giving 15. The noncontinuous receptacle load stays at 10. The total is 80.5 amperes.

Two different 125 percents appear in that calculation and they come from different rules. One is the motor diversity allowance at 430.24 and the other is the continuous load rule that applies to lighting on a commercial feeder. Neither one reaches across into the other's territory, and the distractor set contains the answer for a candidate who let them merge. If the continuous load definition is what you are shaky on, the three-hour test and what it changes is covered under load calculations.

The feeder device rounds down

430.62(A) sets a maximum. Where the calculated figure misses a standard ampere rating, the answer is the next size down.

The permission to round up belongs to 430.52(C)(1) on the branch circuit, and it stays there. Carrying it forward to the feeder is the classic way to land one standard size above the ceiling the code allows, and it is a wrong answer that took more work than the right one. On the 114 ampere example above, the answer is the standard rating at or below 114 amperes.

This is also the rounding direction that gets reversed in the field. Nobody has ever been called back for a feeder device that was one size small, so the error survives inspection and sits there until somebody checks the calculation on a drawing. The single-motor sequence is where the opposite direction is drilled, which is exactly why it transfers so easily and so wrongly.

Exceptions, and packaged equipment

430.24 carries four exceptions, and three of them turn on what kind of load is in the group.

• A motor in the group running short-time, intermittent, periodic or varying duty goes to the duty-cycle rule at 430.22(E), where the percentages act on the nameplate current.
• Motor-operated fixed electric space-heating equipment goes to 424.5, where that equipment and any motor with it are treated as a continuous load.
• Where the circuitry is interlocked so that selected motors or loads are unable to run together, you may size on the combination that can actually run at once and produces the highest total.
• A conductor temperature provision for Design BE and CE motors, new with the 2026 edition, which is where those designations enter Article 430.

Packaged equipment sits outside all four. A rooftop unit, a chiller or any other multimotor or combination-load assembly arrives with a minimum circuit ampacity marked on it under 430.7(D), and 430.25 takes the conductor ampacity off that marking. You do not open the cabinet and add up the internal motors, because the manufacturer has already accounted for what runs together. Equipment that is not factory wired and whose individual motor nameplates are visible is the case that sends you back to 430.24. That marked minimum is the same number HVAC questions are built around, and knowing 430.25 is a separate section stops you hunting for it among the exceptions under time pressure.

What to do before test day

Feeder questions reward a written method more than any other part of this area, because there are enough moving parts that mental arithmetic loses track of which term it is on.

1. List the motors with their table currents in a column, largest current at the top. Sorting is the step that defends against a mixed-voltage group.
2. Mark which sum the question wants before you add anything. Conductors read 430.24 and the device reads 430.62, and the two lists share only their middle terms.
3. Keep the non-motor loads in a second column with their own multipliers, so the continuous load rule stays on its own side.
4. Round last, in the direction the section you are standing in allows, and write which section that was beside the figure.

Tab 430.17, 430.24, 430.25 and 430.62. Four tabs cover every feeder question in the area, and 430.17 is the one most candidates have never opened, which is why the mixed-voltage trap keeps working. For the same sums with the figures worked through end to end, the question sets carries the feeder sizing page, which is where the worked arithmetic lives. Then run the thirty free questions timed, because this topic punishes the slow more than the unsure.

Questions people ask

Does every motor on a feeder get 125 percent?

One of them does. NEC 430.24 takes 125 percent of the full-load current of the highest rated motor and 100 percent of the full-load currents of all the others. Applying the multiplier across the group produces a larger number that looks like caution and is wrong, and that option appears in the answer set on almost every feeder question we have written. The margin comes from the single-motor conductor rule at 430.22 and the feeder rule grants it once, as a diversity allowance for the group, so a tie between two equal motors still yields only one multiplier.

What does highest rated motor mean on a mixed feeder?

The motor with the largest full-load current from the Article 430 tables, which NEC 430.17 states in a section of its own away from the sum. Where every motor in the group runs at the same voltage and phase, that is also the biggest horsepower. Where voltages differ it often is not, because a motor at twice the voltage draws roughly half the current for the same horsepower. Sort the group by amperes before you choose. Questions that list motors in horsepower order are encouraging the shortcut, and the answer built on horsepower will be among the options.

Why does the feeder overcurrent device round down?

Because NEC 430.62(A) states a maximum. The sum of the largest branch-circuit device plus the full-load currents of the other motors is a ceiling on what the feeder device may be, so where it misses a standard ampere rating you take the next size down. The permission to round up lives at 430.52(C)(1) and applies to the branch circuit. Two rules in one problem talk about standard ratings and point opposite ways, so note which article and which part you are standing in before you round anything at all.

How are continuous loads handled on a motor feeder?

On the non-motor side only. The motors are covered by the first two terms of the 430.24 sum, so the continuous and noncontinuous split applies to the lighting, receptacle and other non-motor load sharing the feeder. Continuous non-motor load takes 125 percent under the ordinary feeder rule and noncontinuous load stays at 100 percent. Two different 125 percents can therefore appear in one calculation, one from the motor diversity allowance and one from the continuous load rule, and a candidate who merges them adds the same margin to loads that never earned it.

How do I size the feeder for a rooftop unit or a chiller?

From the marking on the equipment. NEC 430.7(D) requires multimotor and combination-load equipment to carry a minimum circuit ampacity, and 430.25 takes the conductor ampacity from that marking, so the several-motors sum at 430.24 stays closed. The manufacturer has already combined the compressor, the fans and any electric heat in the same cabinet and applied the multipliers. The case that sends you back to 430.24 is equipment that left the factory unwired with its individual motor nameplates visible, where there is no assembly rating to rely on.

Does one disconnect serving several motors use the feeder rules?

No, and that is a separate trap. NEC 430.110(C) covers a disconnecting means for a combination load. You sum the full-load currents of everything that can run at the same time and then apply 115 percent to the whole group. There is no largest motor allowance and no 125 percent anywhere in it, because the disconnect has to carry everything that may be running when somebody pulls the handle. A bank question puts three motors at 20, 14 and 9 amperes through it: 43 amperes summed, times 1.15, giving 49.45 amperes.