How to size a motor control center: feeder ampacity, NEMA starter sizes and bus rating (NEC 430.24, CEC 28-108)

A motor control center (MCC) is sized from its motor list, not from the sum of horsepower or the transformer above it. NEC 430.24 sets the feeder at 125% of the largest motor’s table full-load current plus 100% of every other motor; NEC 430.62(A) caps the feeder device at the largest branch device plus the other motor currents; and MCC bus is generally not offered below 600 A horizontal and 300 A vertical, with the assembly built to UL 845. For a 12-motor, 522.5 hp lineup at 480 V with a 125 hp air compressor as the largest load, the arithmetic gives 677 A of running current, a 716 A feeder, an 800 A feeder breaker and an 800 A horizontal bus, rising to 1200 A once 25% spare is reserved. The same motors on a Canadian 600 V system draw 542 A and need a 573 A feeder under CEC Rule 28-108. The transformer that feeds the lineup is sized by the same current-to-kVA rules the transformer sizing calculator runs; the steps below show where each number comes from.

What does NEC 430.24 require for an MCC feeder?

An MCC is a group of motor branch circuits fed from one bus, so its feeder is a multi-motor feeder. The rules sit in Article 430 (clause numbers per the 2023 NEC; the cited sources use earlier editions with the same rules, so confirm against the edition your AHJ enforces):

• NEC 430.6(A)(1) — ampacity and short-circuit device sizing use the full-load current (FLC) from Table 430.250 for three-phase motors, not the nameplate. Overloads under 430.32 use the nameplate, at no more than 115% for a motor without integral protection.
• NEC 430.22 — a single motor’s branch conductors carry at least 125% of its FLC.
• NEC 430.24 — conductors supplying several motors carry at least 125% of the largest motor’s FLC, plus the FLC of the other motors, plus the load of any non-motor equipment on the feeder.
• NEC 430.52 and Table 430.52 — each branch short-circuit and ground-fault device is limited to a percentage of FLC: 250% for an inverse-time breaker and 175% for a dual-element fuse on a squirrel-cage motor. 430.52(C)(1) Exception No. 1 allows the next higher standard rating from NEC 240.6(A).
• NEC 430.62(A) — the feeder device may not exceed the largest branch device rating plus the FLC of the other motors. The next-size-up exception does not apply here, so the result is rounded down.
• NEC 430.94 to 430.99 — MCC-specific rules: overcurrent protection under Article 240 (430.94), bonding of sections (430.96), busbar arrangement (430.97), marking of the short-circuit rating (430.98) and documentation of the available fault current with its calculation date (430.99).

Feeder ampacity = 1.25 × FLC(largest) + Σ FLC(others) + other loads

Feeder device ≤ branch device(largest) + Σ FLC(others)

The two formulas pull in opposite directions. The 125% term protects the feeder conductors from overheating; the device limit lets the feeder breaker ride through the largest motor’s start while the others run. For the protection of the transformer upstream, see sizing transformer overcurrent protection.

Worked example: 12-motor MCC at 480 V

A process building takes a 480 V, three-phase, three-wire MCC with the motor list below. All motors are squirrel-cage, continuous duty, across-the-line started; branch protection is an inverse-time breaker; conductors are copper with 75 °C terminations. Currents are the 460 V column of NEC Table 430.250 (460 V motors on a 480 V system).

1. Sum the table currents. 156 + 96 + 96 + 65 + 65 + 52 + 40 + 34 + 27 + 21 + 14 + 11 = 677 A, for 125 + 75 + 75 + 50 + 50 + 40 + 30 + 25 + 20 + 15 + 10 + 7.5 = 522.5 hp.
2. Feeder ampacity (430.24). 1.25 × 156 = 195 A for the 125 hp compressor; the other eleven motors carry 677 − 156 = 521 A; 195 + 521 = 716 A.
3. Largest branch device (430.52). 2.5 × 156 = 390 A, raised under 430.52(C)(1) Exception No. 1 to the 240.6(A) standard rating of 400 A.
4. Feeder device limit (430.62(A)). 400 + 521 = 921 A. No 900 A standard rating exists in 240.6(A), and rounding up is not permitted, so the feeder breaker is 800 A. 800 A ≥ 716 A, so it also carries the feeder load.
5. Feeder conductors. From the 75 °C copper column of NEC Table 310.16, 500 kcmil = 380 A; two sets give 2 × 380 = 760 A ≥ 716 A.
6. Horizontal bus. The bus must carry at least the 716 A feeder load and sit at or above the 600 A minimum horizontal rating generally offered, so the next catalogue rating at or above 716 A applies: 800 A. With 25% reserved for future buckets, 1.25 × 716 = 895 A, which moves the bus to 1200 A.
7. Vertical bus per section. A section holding the two 75 hp pumps and one 50 hp fan carries 96 + 96 + 65 = 257 A ≤ 300 A, inside the 300 A minimum vertical rating generally offered. The 125 hp compressor (156 A) plus the 40 hp (52 A) and 30 hp (40 A) units load another section to 248 A.
8. Transformer. Running load 677 A at 480 V, converted to kVA: √3 × 480 × 677 ÷ 1000 = 562.8 kVA; at the 716 A feeder current, √3 × 480 × 716 ÷ 1000 = 595.3 kVA. Both exceed 500 kVA, so a 750 kVA transformer applies, whose secondary current is 750 000 ÷ (√3 × 480) = 902.1 A.

S (kVA) = √3 × V × I ÷ 1000

The transformer step is where this calculation meets the transformer sizing calculator: enter the 562.8 kVA running load, the 480 V secondary and the growth margin, and compare against the method in how to size a transformer. A 750 kVA unit is also a standard pad-mounted size when the MCC is fed from outdoors.

Results for the motor list

Branch values per 430.22 (125% conductors) and Table 430.52 (250% inverse-time breaker, next standard rating up); starter sizes are the smallest NEMA size whose 460 V horsepower limit covers the motor.

Only the largest motor takes the 125% factor at the feeder. Applying 125% to every motor gives 1.25 × 677 = 846 A, an 18% overstatement that can push the bus from 800 A to 1200 A for no code reason.

What NEMA starter size and bucket does each motor need?

NEMA ICS 2 starter sizes are defined by continuous current and a maximum horsepower at each voltage for across-the-line starting. The MCC bucket height follows the starter size and disconnect frame, and is taken from the manufacturer’s unit-height table.

Two cautions. First, horsepower, not current, is the selection basis: a 50 hp motor at 65 A fits a Size 3 (90 A, 50 hp), but a 60 hp motor at 77 A does not, even though 77 A < 90 A. Second, manufacturers’ own Size 5 listings vary; at least one manufacturer’s Size 5 cut sheet lists a single three-phase maximum of 150 hp rather than 200 hp, so the starter data sheet governs. Variable-frequency drives replace the NEMA size with the drive’s output current rating, and their harmonic currents belong in the study covered in harmonic distortion and IEEE 519.

Wiring class and type are specified with the bucket list: Class I has no factory interwiring between units, Class II includes factory-wired controls and interlocks; Type A has no terminal blocks, Type B puts control terminals in each unit, and Type C runs all wiring to master terminal boards at the top or bottom of the lineup.

Reference: three-phase motor full-load currents

Values from NEC Table 430.250, permitted for system voltages of 440 to 480 V (460 V column) and 550 to 600 V (575 V column). The 575 V figures shown match the CEC Table 44 value checked (52 A for 50 hp); confirm the remaining rows against the edition of Table 44 in force.

What short-circuit rating does the MCC need?

NEC 430.98 requires the MCC marking to include its short-circuit rating, and NEC 430.99 requires the available fault current at the MCC to be documented, with the date of the calculation, for those who inspect, install or maintain it. A screening value comes from the transformer plus the motors’ own contribution, which a common method based on IEEE C37.13 takes as about 4 times the connected motor FLC on 240 V to 1000 V systems.

I_sc ≈ FLA(transformer) ÷ Z(pu) + 4 × Σ FLC(motors)

1. Transformer contribution (infinite-source assumption, this article’s screening method). 902.1 ÷ 0.0575 = 15 689 A, for a 750 kVA unit with an assumed 5.75% nameplate impedance.
2. Motor contribution. 4 × 677 = 2708 A.
3. Total at the MCC bus. 15 689 + 2708 = 18 397 A, about 18.4 kA.

The MCC short-circuit current rating and every bucket’s combination rating must be at or above that figure; a 42 kA nameplate rating, for example, leaves margin for a lower-impedance replacement transformer. A detailed study replaces the infinite-source assumption with the utility impedance and cable impedance; see available fault current and SCCR and, for the trip-curve side, selective coordination.

Where Canada differs: CEC Rules 28-108 and 28-204

The Canadian Electrical Code reaches the same structure with different rule numbers and, on most industrial sites, a different voltage. Rule 28-106 gives the 125% single-motor conductor rule; Rule 28-108 sizes conductors supplying a group of two or more motors at 125% of the largest motor’s FLC plus the FLC of the others, and permits sizing for the largest group running at one time where interlocks prevent simultaneous operation. Rule 28-200 limits branch devices to Table 29, and Rule 28-204 caps the feeder device at the value permitted for the motor with the highest device rating plus the FLC of all other motors running at the same time. Where a demand factor is applied, the feeder device may not exceed the feeder ampacity except as Rule 14-104 and Table 13 allow. The assembly is built to CSA C22.2 No. 254 rather than UL 845, and overcurrent devices must be rated for the available fault current at their terminals under Rule 14-012.

Second example: the same lineup at 600 V

The same twelve motors as 575 V units on a 600 V system, with 575 V currents (CEC Table 44 values as noted above):

1. Sum. 125 + 77 + 77 + 52 + 52 + 41 + 32 + 27 + 22 + 17 + 11 + 9 = 542 A.
2. Feeder ampacity (Rule 28-108). 1.25 × 125 = 156.25 A; 542 − 125 = 417 A; 156.25 + 417 = 573.3 A.
3. Largest branch device (Table 29, inverse-time breaker 250%). 2.5 × 125 = 312.5 A; this example takes 300 A, the conservative reading of Table 13 (confirm under Rule 28-200 whether the next higher rating is permitted on your project).
4. Feeder device limit (Rule 28-204). 300 + 417 = 717 A, so the feeder device is the next Table 13 rating not exceeding 717 A.
5. Bus and transformer. 573.3 A fits a 600 A horizontal bus with no spare; 1.25 × 573.3 = 717 A with 25% spare needs 800 A. Running load √3 × 600 × 542 ÷ 1000 = 563.3 kVA, again a 750 kVA transformer, now at 750 000 ÷ (√3 × 600) = 721.7 A.

The same 522.5 hp lineup needs 573.3 ÷ 716 = 0.80 of the 480 V feeder current: one bus size lower, smaller feeder conductors and smaller buckets. That is the practical case for 600 V distribution covered in choosing a facility distribution voltage.

Common mistakes

• Using nameplate amps for the feeder. NEC 430.6(A)(1) requires Table 430.250 currents; a hypothetical 142 A nameplate on the 125 hp compressor instead of the 156 A table value understates the 125% term by 1.25 × 14 = 17.5 A. Plan review checks the motor schedule against the table.
• Applying 125% to every motor. 1.25 × 677 = 846 A instead of the 716 A that 430.24 requires, enough to push an 800 A bus to 1200 A. Only the largest motor takes the factor.
• Rounding the feeder device up. 921 A does not become a 1000 A breaker; 430.62(A) sets a ceiling and the next-size-up exception of 430.52(C)(1) is branch-only, so the correct device is 800 A.
• Picking starters by current alone. A 60 hp motor at 77 A does not fit a Size 3 starter rated 90 A, because Size 3 is limited to 50 hp at 460 V; it needs Size 4.
• Leaving the fault current undated or uncalculated. An MCC marked 10 kA under 430.98 on an 18.4 kA bus violates 110.10; 430.99 requires the dated calculation that would have caught it.

What to specify

• Motor schedule with hp, voltage, duty and starting method, and the Table 430.250 or Table 44 FLC for each, from the process load list.
• Feeder ampacity per 430.24 or CEC Rule 28-108, and the feeder device rating per 430.62(A) or Rule 28-204, from the calculation above.
• Horizontal and vertical bus ratings, at or above the feeder ampacity plus the spare percentage, with the 600 A horizontal / 300 A vertical minimums generally offered, and the assembly built to UL 845 or CSA C22.2 No. 254, from the bucket layout.
• Short-circuit rating of the assembly and of each combination unit, at or above the available fault current documented under 430.99, from the fault study or the screening figure.
• Starter size or drive rating per unit, NEMA class and wiring type (Class I or II; Type A, B or C), from the motor schedule and control philosophy.
• Supply transformer kVA and %Z, from the running kVA in the transformer sizing calculator and the fault-current target.

Where Entogo fits

The MCC numbers above are the inputs to a lineup order. The Entogo low-voltage switchgear and motor control center combines a power-distribution section with drawout starter and VFD units for systems up to 600 V and 3150 A, with short-circuit ratings set to the available fault current; the modular low-voltage switchgear and low-voltage switchboard cover the distribution upstream. The switchgear and MCC assemblies are designed and built to UL 1558 and UL 845, and the switchboard to UL 891; UL (cULus) / CSA certifiable on request. For the switchboard-versus-switchgear choice at the main, see switchboard vs. switchgear.

Entogo is a Toronto-headquartered supplier backed by its own source factory, with engineering support for MCC and transformer orders. Carry the running kVA from the transformer sizing calculator into the transformer configurator or the three-phase dry-type distribution transformer range, and see industrial EPC for plant packages.

An MCC is sized from its motor list in five moves: read the table currents, add 25% of the largest to the sum, cap the feeder device at the largest branch device plus the rest and round down, pick the bus at or above the feeder current plus spare, and document the fault current the lineup must withstand. At 480 V, 522.5 hp lands at 716 A and an 800 A bus; at 600 V in Canada, the same motors need 573 A under Rule 28-108.