Motor circuits and controls

Why it matters

Motors run pumps, fans, compressors, conveyors and almost every machine in a commercial or industrial building. They are also unusual loads: they draw several times their running current when they start, and they can overheat slowly if they are overloaded or a phase is lost. Article 430 handles this by splitting protection into separate jobs, each sized differently. Add the control circuit that starts and stops the motor, drawn as a ladder diagram, and you have a topic every electrician uses and every exam tests.

Awareness level only. Motor circuits involve high voltages, stored energy and moving machinery. Working on them requires employer training, lockout/tagout procedures and supervision by a qualified person. This lesson is paper and calculator practice.

Two different currents

• Full-load current (FLC) from the code tables (Article 430's tables by horsepower and voltage) is used to size conductors, the short-circuit and ground-fault protection and disconnects. This keeps the design independent of which brand of motor is installed later.
• Full-load amps (FLA) from the motor nameplate is used to size overload protection, because overloads must match the actual motor.

Mixing these up is the most common motor mistake.

The protection layers

1. Branch-circuit conductors: generally at least 125% of the motor's table FLC (430.22).
2. Short-circuit and ground-fault protection (fuses or an inverse-time breaker): sized from a percentage of the FLC in Table 430.52. It must be large enough to let the motor start, because inrush current is high.
3. Overload protection (the heaters or electronic overload in the starter): protects the motor and conductors from sustained overcurrent. It is sized from the nameplate FLA.
4. Controller (the starter or contactor) and disconnecting means, sized and located per Article 430.

Overload sizing (430.32)

For a continuous-duty motor over 1 horsepower, the general rule is:

• 125% of nameplate FLA for motors with a service factor of 1.15 or more, or a marked temperature rise of 40 °C or less.
• 115% of nameplate FLA for all other motors.

If that setting does not let the motor start or carry its load, 430.32(C) allows a higher setting up to a limit. Check your edition for those values.

Worked example: a motor nameplate shows FLA 28 A, service factor 1.15.

service factor 1.15 → 125 %
overload = 28 × 1.25 = 35 A

The same motor with service factor 1.0 and temperature rise 50 °C: 28 × 1.15 = 32.2 A.

Reading a ladder diagram

A ladder diagram draws control circuits as horizontal rungs between two vertical rails (the control power supply, often 120 V or 24 V from a control transformer). Each rung reads left to right: contacts (switches) on the left, an output (a coil or lamp) on the right. The output energises when there is a closed path through the contacts.

The classic three-wire start/stop circuit:

L1 ──[ STOP (NC) ]──┬──[ START (NO) ]──┬──( M coil )──[ OL (NC) ]── L2
                    │                  │
                    └────[ M (NO) ]────┘

• STOP is normally closed (NC): pressing it opens the rung.
• START is normally open (NO): pressing it closes the path and energises coil M (the motor starter).
• The M auxiliary contact in parallel with START closes when M energises. This is the seal-in (or holding) contact: when you release START, current flows through it and the motor keeps running.
• The overload contact (OL), normally closed, opens if the overload trips, dropping out the coil.

In logic terms, the coil is on when:

M = STOP not pressed AND OL not tripped AND (START pressed OR M already on)

This also gives low-voltage protection: if power fails, M drops out, the seal-in opens, and the motor does not restart by itself when power returns. Someone has to press START again. That is a safety feature for machinery.

Common mistakes

• Sizing overloads from the table FLC or conductors from the nameplate.
• Forgetting the 125% on motor branch-circuit conductors.
• Sizing the short-circuit protection like an overload (the motor would never start).
• Wiring STOP as normally open, or putting the seal-in contact in series instead of in parallel with START.

Quick recap

Table FLC sizes conductors and short-circuit protection; nameplate FLA sizes overloads: 125% with service factor ≥ 1.15 or temperature rise ≤ 40 °C, otherwise 115%. In a three-wire control circuit, the coil runs when STOP and OL are closed and either START or the seal-in contact is closed. Look up Article 430 in your adopted edition (NFPA 70; checked on 2026-10-23).

Try it

Open the Do tab. The ladder-rung simulator is written; you add the overload percentage rule.