Motor Circuit Sizing per NEC 430: A Worked Example From FLA to Breaker

Motor circuit sizing per NEC 430 trips people up because it breaks the pattern used everywhere else in the code: the breaker does not protect the conductors against overload, the overload relay does, and the breaker is allowed to be dramatically larger than the conductor ampacity. Once that division of labor clicks, Article 430 is a checklist. This post walks the checklist and then sizes a real circuit end to end.

Rule one: use the table FLA, not the nameplate, for sizing

NEC 430.6(A)(1) says conductor and branch-circuit protection sizing uses the full-load current from Tables 430.247 through 430.250, not the current stamped on the motor nameplate. The nameplate current is used for one thing: the overload protection in 430.32. This split exists so the wire and breaker do not change every time a motor is swapped for one with a slightly different nameplate.

For reference, from Table 430.250 (three-phase): a 10 hp motor at 460 V is 14 A, at 230 V it is 28 A. From Table 430.248 (single-phase): a 5 hp motor at 230 V is 28 A. Always pull the value for your actual voltage and phase, because guessing from memory is how 208 V motors end up wired like 230 V motors.

Rule two: conductors at 125 percent

NEC 430.22 sizes branch-circuit conductors for a single continuous-duty motor at 125 percent of the table FLA. That is the whole rule for the common case. The motor runs at some load up to its service factor all day, so the conductors get the same 125 percent treatment a continuous load gets elsewhere in the code.

Rule three: the branch-circuit protection is for short circuit and ground fault only

NEC 430.52 sizes the branch-circuit short-circuit and ground-fault protective device from another table, as a percentage of the table FLA. For an inverse-time circuit breaker the table value is 250 percent. Two reliefs apply: 430.52(C)(1) Exception 1 lets you round up to the next standard size in 240.6 when the calculation does not land on one, and Exception 2 allows up to 400 percent for an inverse-time breaker rated 100 A or less when the motor cannot start on the calculated size. The breaker is sized to ride through inrush; the running protection lives in the overload relay.

Rule four: overloads from the nameplate

NEC 430.32(A)(1) sets separate overload protection for motors over 1 hp at 125 percent of the nameplate current for motors with a service factor of 1.15 or more or a temperature rise of 40C or less, and 115 percent for everything else. In practice this is the heater element or the electronic overload dial setting in the starter, set from the nameplate of the motor actually installed.

The worked example: 10 hp, 460 V, three-phase

Squirrel-cage induction motor, continuous duty, 1.15 service factor, nameplate FLA 13.2 A, fed from a 480 V MCC with an inverse-time breaker, THWN-2 copper in conduit.

Step 1, table FLA. Table 430.250 gives 14 A for 10 hp at 460 V. The 13.2 A nameplate goes in the pocket for step 4.

Step 2, conductors. 14 A x 1.25 = 17.5 A. From Table 310.16 in the 75C column (typical termination rating), 14 AWG copper at 20 A covers it where 14 AWG is permitted, but most commercial specs floor branch circuits at 12 AWG, and 12 AWG copper at 25 A is the practical answer. Long runs to a remote pump or fan deserve a voltage drop check on top of ampacity; at 460 V it is rarely the governing factor, but the check costs nothing.

Step 3, breaker. 14 A x 2.50 = 35 A, which is a standard size in 240.6, so a 35 A inverse-time breaker is the calculated maximum. A 12 AWG conductor on a 35 A breaker looks wrong to anyone trained outside Article 430, and it is completely code-compliant: the overload relay protects the conductor against overload, the breaker only has to clear faults. If the motor has a hard-starting load and trips the 35, Exception 2 allows stepping up toward 400 percent (56 A calculated, so up to a 50 A standard size and a 60 A would exceed it) while the conductors stay at 12 AWG.

Step 4, overloads. Service factor 1.15, so 430.32(A)(1) allows 125 percent of nameplate: 13.2 A x 1.25 = 16.5 A. Set the electronic overload at 16.5 A, or select the heater the starter manufacturer’s table maps to a 13.2 A nameplate, which already bakes in the percentage.

Step 5, the disconnect. NEC 430.110 wants the disconnecting means rated at least 115 percent of the table FLA: 14 A x 1.15 = 16.1 A, so a 30 A non-fused disconnect at the motor is fine, and is what gets installed anyway.

Where this goes wrong in the field

The recurring mistakes are predictable. Sizing conductors from the nameplate instead of the table, which usually lands one size small on older motors. Sizing the breaker at 125 percent like a feeder, which then trips on inrush, gets replaced with the right 250 percent size by someone who knows Article 430, and generates a confused phone call. Setting overloads from the table FLA instead of the nameplate, which over-protects a lightly-nameplated motor and nuisance-trips. And forgetting that multi-motor feeders have their own rules in 430.24 (largest motor at 125 percent plus the sum of the rest), which is its own post.

For a quick lookup of table FLA values, the free motor FLA calculator on this site covers the standard single-phase and three-phase tables and runs the 125 percent conductor math for you. Verify against the actual code tables on anything going on a permit set, as always.