Three numbers, two currents, one motor

When to use this: specifying a motor circuit, checking an existing installation, or working out why a motor trips its breaker on start.

A motor circuit is protected twice, by two devices doing two different jobs, sized from two different currents. Merging them is where nearly all confusion in Article 430 comes from.

Worked through: 10 hp, 460 V, three phase

So: a 35 A breaker ahead of 12 AWG conductors. To anyone who has met 240.4(D) — which caps 12 AWG at 20 A — that reads as a serious violation. It is exactly what the code requires.

The breaker is not protecting the conductors from overload. The overload relay at 16.5 A does that. A motor draws six to eight times its running current for a second or two on start, so a device sized to the conductor ampacity would open every time the motor started. 430.52 therefore lets the short-circuit device be large enough to ignore inrush, and 430.32 requires a separate device sitting close to the real running current.

Fit a 20 A breaker here “to protect the wire” and you get a motor that will not start. That is the failure this arrangement exists to prevent.

Table vs nameplate

NEC 430.6(A)(1) requires the table value for conductors and the short-circuit device even where the nameplate disagrees. The tables are deliberately conservative, and using them means a motor swap does not invalidate the circuit design.

430.32 requires the nameplate for the overload, because that device is protecting this particular machine rather than a generic one. This is the only place in the whole article where the nameplate is used, and reversing the two is the most common mistake here.

Table 430.52 — maximum device rating

As a percentage of full-load current. These are ceilings, not targets.

430.52(B) requires the lowest rating that will carry the starting current, so these percentages are where you stop, not where you start. A dual-element time-delay fuse protects the same motor far more closely than a breaker — 175% against 250% — because it tolerates inrush by design rather than by being oversized.

The same 10 hp motor on each device

Full-load current, Table 430.250

Three-phase induction motors, amperes. This is the number every calculation starts from.

Rounding goes the other way

430.52(C)(1) Exception 1 permits the next standard size up where the calculated value is not a standard rating. Under 240.4 you normally round down to protect the conductor; here you round up, because the device is not protecting the conductor from overload in the first place.

A 5 hp 460 V motor at 7.6 A × 250% = 19 A, which is not a standard rating, so a 20 A breaker is permitted. A 10 hp at 14 A × 250% = 35 A, which is standard, so the exception never arises and 35 A is the answer.

If it still will not start

430.52(C)(1) Exception 2 raises the ceilings — 400% for an inverse time breaker or non-time-delay fuse, 225% for a dual-element fuse, 1300% for an instantaneous trip breaker. The condition is that the standard value has demonstrably failed to carry the starting current. It is a remedy, not a default: jumping straight to it leaves the circuit with far weaker short-circuit protection than it should have.

Feeders supplying several motors

430.62: the feeder device may not exceed the largest branch-circuit device in the group, plus the full-load currents of all the other motors. Not the sum of the branch devices, which would be enormous.

Three motors on one feeder — a 10 hp with a 35 A breaker, plus a 5 hp at 7.6 A and a 3 hp at 4.8 A — give 35 + 7.6 + 4.8 = 47.4 A, so a 50 A feeder device. Adding the three branch devices would have suggested 35 + 20 + 15 = 70 A. Conductors come from 430.24: 125% of the largest motor plus the full-load currents of the rest.

Things that change the answer

• Service factor decides the overload allowance. 1.15 or higher gets 125%; below that with a rise above 40 °C gets 115%. Read the plate rather than assuming — it is a real difference in headroom.
• A VFD changes everything. The drive provides the overload function and limits inrush, so 430.130 governs rather than 430.52. Do not apply this page to a drive-fed motor without checking the drive documentation.
• Conductor derating still applies on top. Ambient temperature, conduit fill and terminal ratings all reduce ampacity — our wire size calculator handles those, and a long run will push the size up for voltage drop regardless of what 430.22 asks for.
• The ground is sized on the breaker. Which for a motor circuit means a large device ahead of small conductors — and 250.122(A) then caps the equipment grounding conductor at the size of the phase conductors. Our ground wire size calculator handles that cap.

How this calculator is checked

Technical reviewer

Salamot Hok

Electrician · 10+ years of installation work in Bangladesh and the wider South Asian region

He reads the result the way an installer would: are the defaults values people actually meet, does the warning fire where you would stop and think, and is the answer something you could buy and fit? The code figures themselves come from the published standards cited below, not from him — that boundary is set out on his profile.

• The maths lives in a pure function with its own test suite, asserted against worked examples from published references and standards. A calculator does not ship until those tests pass.
• 2 sources cited by name and linked, so any figure on the page can be traced back to the document it came from.
• Last reviewed 17 August 2026. Review dates are advanced only when the page is actually re-read, never to look fresh.
• Unusable input returns no answer. Where the inputs do not describe a real design, the calculator says so and withholds the number rather than printing a plausible-looking wrong one.

The full process is written up in the methodology and editorial policy. Results are engineering guidance, not a code sign-off — see the disclaimer. If a result looks wrong, tell us; corrections are answered before anything else.