Motor circuit protection

What to try

1. 1Pick Starting. The motor draws 168 amps for a few seconds, and the 70 amp breaker stays on.
2. 2Set Breaker size to 35 A and pick Starting again. A breaker sized like an ordinary circuit trips before the motor gets up to speed.
3. 3Pick Short circuit. The magnet in the breaker trips it in one cycle, and the overload relay is far too slow to help.

NEC 2026 sections it covers

Table 430.248

This table gives the motor's amps, and the wire and breaker are sized from it.

430.6(A)(1)

The wire and breaker are sized from the table, not the nameplate.

430.6(A)(2)

The overload relay is sized from the nameplate.

430.32(A)(1)

With a service factor of 1.15, the relay may be set at 125% of nameplate amps.

430.52(B)

The breaker must carry the starting current.

Table 430.52(C)(1)

An inverse time breaker may be up to 250% of the table current.

Table 240.6(A)

70 A is a standard breaker size.

430.22

The wire carries at least 125% of the table current.

Table 310.16

The table of allowed amps for each wire size.

Table 250.122

A 70 A breaker gets an 8 AWG copper ground wire.

240.4(G)

Motor circuits follow their own rules for breaker size.

Table 430.251(A)

The highest starting current allowed for a single-phase motor of each size.

430.102(A)

The disconnect is in sight of the starter.

430.102(B)(2)

A disconnect in sight of the motor can also serve as the motor disconnect.

Section numbers only. Read the full text in your code book. Temperatures and times in the visual are teaching models.

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