How to Wire a Three-Phase Motor Starter

1. Start with the nameplate, size everything on paper first

Pull the nameplate data: HP, voltage, FLA, service factor, temperature rise. Then pull the FLC from NEC Table 430.250 for that HP and voltage — not the nameplate amps. Per 430.6(A)(1), the table value sizes conductors and short-circuit protection; nameplate FLA sizes only the overloads. Running example: a 10 HP, 460V three-phase motor — Table 430.250 says 14 amps.

2. Size the branch-circuit conductors per 430.22

Conductors to a single continuous-duty motor must be at least 125% of the Table 430.250 FLC. For the 10 HP example: 14A × 1.25 = 17.5A, so 12 AWG THHN copper (25A at 75°C per Table 310.16) handles it. Derate for conduit fill and ambient as usual, and size the EGC from Table 250.122.

If the breaker ends up bigger than the conductor ampacity — on motor circuits it usually does — that's fine. 240.4(G) exempts motor circuits from the small-conductor rule; the overload relay protects the wire from overload, not the breaker.

3. Size the short-circuit and ground-fault protection per 430.52

The breaker or fuses ahead of the starter protect against faults, not overloads, so Table 430.52(C)(1) lets them run big: up to 250% of FLC for an inverse-time breaker, 175% for dual-element time-delay fuses. For the 10 HP motor: 14 × 2.5 = 35A inverse-time breaker. If the math misses a standard size, Exception No. 1 lets you round up to the next standard rating. These are maximums — go smaller if the motor still starts reliably.

While you're at it, confirm the disconnecting means: 430.102 requires one for the controller and, generally, one within sight of the motor.

4. Land the power wiring — line and load

Locked out and verified dead: supply conductors land on the line side, L1, L2, L3. Load side runs to the motor, T1, T2, T3, connected per the diagram on the motor nameplate — a 9-lead dual-voltage motor connected wrong will run, badly, then fail.

Torque every lug to the manufacturer's labeled value with a torque tool — 110.14(D) requires it. Land the EGC on the ground bar or lug, never a painted surface.

5. Select and set the overload relay per 430.32

Overloads work off nameplate FLA, full stop. Per 430.32(A)(1), a continuous-duty motor over 1 HP gets overload protection at no more than 125% of nameplate FLA if the service factor is 1.15 or greater or the marked temperature rise is 40°C or less; everything else gets 115%. On an adjustable relay, dial in the nameplate FLA — the trip curve builds in the multiplier. On a melting-alloy type, pick heaters from the manufacturer's table.

If it trips on a legitimate start, 430.32(C) allows stepping up to a maximum of 140% (SF 1.15+/40°C rise) or 130% (all others) — a troubleshooting allowance, not a starting point. Set trip class per the load: Class 10 for pumps, Class 20 as the default, Class 30 for high-inertia loads, per the manufacturer.

6. Establish control power

On 480V systems you'll almost always use a control power transformer — 480V primary, 120V secondary — with primary and secondary fusing, control conductors protected per 430.72. Put all switching in the ungrounded control leg so a ground fault can't start the motor.

Verify the coil voltage stamped on the contactor matches the control source before wiring. A 120V coil across 480V lasts about half a second; a 480V coil on 120V just hums and chatters.

7. Wire the 3-wire control circuit with seal-in

The classic ladder: control hot (X1) feeds the NC stop button, stop feeds the NO start button, start feeds the coil, and the coil returns through the overload relay's NC trip contact (95-96 on IEC relays) to X2. Press start, the coil pulls in — but only while your finger's on the button.

Now the seal-in: wire the starter's NO auxiliary contact (terminals 2 and 3 on a NEMA starter; 13-14 IEC) in parallel with the start button — and only the start button. When the contactor closes, the aux holds the coil in after you release start. Press stop and it all drops out. That's low-voltage protection: after a power failure the seal-in has dropped, so the motor can't restart on its own — the whole reason 3-wire control exists, and why the seal-in must never bridge across the stop button.

8. Check out, energize, and bump for rotation

Before energizing, ring out the control circuit against the ladder diagram: stop breaks it, start makes it, seal-in is across start only. Megger the motor and load conductors if there's any doubt.

Energize and bump the motor — a quick start-stop, just enough to see the shaft. Confirm rotation matches the driven equipment; a pump running backward moves some water and lies to you. To reverse: lock out, verify dead, swap any two load-side leads. Then run under load and clamp all three legs — current should be balanced and at or below nameplate FLA. Log the readings as your baseline.

What's the difference between 2-wire and 3-wire motor control?

Two-wire control uses a maintained contact like a thermostat or float switch, so the motor restarts automatically when power returns. Three-wire control uses momentary start-stop buttons with a seal-in contact, so the starter drops out on power loss and stays off until someone presses start. Use 3-wire anywhere an automatic restart could endanger people or equipment.

What does the seal-in (holding) contact actually do?

It's a normally open auxiliary contact wired in parallel with the momentary start button. When the contactor pulls in, that contact closes and keeps the coil energized after the button is released. Pressing stop or losing power breaks the circuit and the starter drops out.

How do I set the overload relay on a motor starter?

Set it from the motor nameplate FLA, not the breaker size or the FLC table. NEC 430.32(A)(1) allows a maximum of 125% of nameplate FLA for motors with a service factor of 1.15 or more or a temperature rise of 40°C or less, and 115% for all others. Most adjustable relays are dialed to the nameplate amps; the trip curve builds in the margin.

Why is the breaker on a motor circuit so much bigger than the wire's ampacity?

The breaker or fuses only protect against short circuits and ground faults, so NEC Table 430.52(C)(1) allows up to 250% of full-load current for an inverse-time breaker to ride through motor inrush. The overload relay handles sustained overloads, and NEC 240.4(G) exempts motor circuits from the normal small-conductor limits.

How do I reverse the rotation of a three-phase motor?

De-energize, lock out, verify absence of voltage, then swap any two load-side leads — T1 and T3 at the starter is the common convention. Reversing the phase sequence reverses rotation. Always bump-test again after the swap.

What wire size does a 10 HP, 460-volt three-phase motor need?

Table 430.250 lists 14 amps for a 10 HP motor at 460 volts, and NEC 430.22 requires conductors at 125% of that, or 17.5 amps. That's 12 AWG copper THHN in a typical raceway run, subject to derating, with up to a 35-amp inverse-time breaker ahead of it.