Sizing Motor Circuits (NEC Article 430)

Motor circuits do not follow the rules used for ordinary branch circuits. A motor draws several times its running current when it starts, so a breaker sized like a lighting circuit would trip every time. NEC Article 430 solves this by splitting protection into separate jobs, each with its own math. Once you know which current value goes with which step, a motor circuit is a short calculation. Getting it wrong can leave a motor unprotected, or a circuit that never starts.

What You Will Be Able to Do

• Explain why motor circuits have separate overload and short-circuit protection.
• Pick the right current value: table FLC or nameplate FLA.
• Size branch-circuit conductors for a single motor.
• Size a fuse or breaker with Table 430.52.
• Size the overload, disconnect and grounding conductor, and work out a feeder for several motors.

Section numbers match recent NEC editions. Your state or city may enforce a different edition, with local changes, so confirm the rule that applies. This guide covers general-purpose, continuous-duty, three-phase induction motors. Other motor types and duties have other rules.

How Motor Protection Is Divided

A complete motor circuit has separate parts, each with a separate job:

Because the fuse or breaker is much larger than the conductors could carry continuously, it does not protect them from overload. The overload relay does that job. So Section 240.4(D), which limits small conductors to 15, 20 and 30 amps, does not apply here. Section 240.4(G) sends you to Article 430 instead.

Table FLC or Nameplate FLA?

This is the most common error in motor work.

• FLC (full-load current) is the value in the NEC tables. Use it for conductors, short-circuit protection and disconnects. NEC 430.6(A)(1) requires this, so that a replaced motor with a slightly different nameplate does not outgrow the circuit.
• FLA (full-load amps) is the value on the motor nameplate. Use it only for overload protection, as NEC 430.6(A)(2) says.

Table 430.250 gives the current for three-phase motors. Here are some common values:

Single-phase motors use Table 430.248.

Step 1: Conductors

NEC 430.22 requires branch-circuit conductors for one continuous-duty motor to have an ampacity of at least 125 percent of the table FLC.

Conductor ampacity = table FLC x 1.25

Look up the wire in Table 310.16, using the column for the terminals. Most motor circuit terminals allow 75 degrees C, but check the marking. Apply any correction for heat or for several conductors in a raceway.

Step 2: Short-Circuit and Ground-Fault Protection

NEC 430.52 sets the maximum device rating as a percentage of table FLC, based on the type of device. For most three-phase motors:

Rounding. If the result does not match a standard size, you may use the next size up from NEC 240.6.

If the motor still will not start. The exceptions in 430.52 let you go higher if the motor cannot start with the first value. Those limits are usually 225 percent for a time-delay fuse, 400 percent for a nontime-delay fuse up to 600 amps, and 400 percent for an inverse-time breaker on a motor of 100 amps or less. Use these only when needed.

Step 3: Overload Protection

NEC 430.32 sets the highest overload rating for a continuous-duty motor over 1 HP:

If that setting will not allow the motor to start, NEC 430.32(C) may allow up to 140 or 130 percent, using the same motor categories. Treat that as a last resort, and find out why the motor needs it.

Step 4: The Disconnect

A motor disconnect must have a horsepower rating at least equal to the motor's, and an ampere rating of at least 115 percent of table FLC under NEC 430.110. It must clearly show whether it is open or closed.

NEC 430.102 requires a disconnect within sight of the controller, and one within sight of the motor and the driven machine. "Within sight" means visible and no more than 50 feet away. One device can do both jobs if it meets both rules. There are narrow exceptions where a controller disconnect can be locked open, but the rule of thumb is that the person at the motor should be able to see the disconnect. The device must be lockable in the open position.

Step 5: The Equipment Grounding Conductor

Size the equipment grounding conductor (EGC) from the rating of the fuse or breaker, using Table 250.122. For motor circuits, NEC 250.122(D) uses the short-circuit device rating. An EGC is never required to be larger than the circuit conductors.

Worked Example: One Motor

A 20 HP, 460-volt, three-phase motor has a nameplate FLA of 26 amps and a 1.15 service factor. Terminals are rated 75 degrees C, and you are using an inverse-time breaker.

1. Find the table FLC: 27 amps from Table 430.250.
2. Conductors: 27 x 1.25 = 33.75 amps. 10 AWG copper is good for 35 amps at 75 degrees C, so 10 AWG works.
3. Breaker: 27 x 2.50 = 67.5 amps. The next standard size is 70 amps.
4. Overload: 26 x 1.25 = 32.5 amps at most, using nameplate FLA.
5. Disconnect: at least 20 HP, and 27 x 1.15 = 31.05 amps. A 30-amp switch is too small, so use a 60-amp switch rated for at least 20 HP at 460 volts.
6. EGC: a 70-amp device falls in the 100-amp row of Table 250.122, which is 8 AWG copper. The EGC need not exceed the 10 AWG circuit conductors, so 10 AWG is enough.

Notice that a 70-amp breaker sits on 10 AWG wire, which looks wrong if you think about lighting circuits. It is correct here, because the overload relay protects the conductors from overload, and the breaker handles faults.

The Same Motor With Time-Delay Fuses

Dual-element fuses at 175 percent give 27 x 1.75 = 47.25 amps, so 50 amps is the next size. Fuses are often smaller than a breaker for the same motor, and they clear faults faster.

Worked Example: A Smaller Motor at 230 Volts

A 10 HP, 230-volt, three-phase motor has a table FLC of 28 amps.

1. Conductors: 28 x 1.25 = 35 amps, so 10 AWG copper at 75 degrees C.
2. Inverse-time breaker: 28 x 2.50 = 70 amps.
3. Dual-element fuses: 28 x 1.75 = 49 amps, so 50 amps.

Several Motors on One Feeder

When a feeder serves several motors, the rules change.

Feeder conductors, NEC 430.24: 125 percent of the largest motor's FLC, plus 100 percent of the others, plus any other loads.

Feeder protection, NEC 430.62: the largest branch-circuit device rating, plus the FLC of all the other motors. Unlike a branch circuit, you may not go up to the next size. If the result is not a standard size, go down to the next lower one.

Worked Example: Three Motors

A feeder supplies three 460-volt motors: 20 HP (27 A), 10 HP (14 A) and 5 HP (7.6 A). The 20 HP branch circuit has a 70-amp breaker.

1. Feeder conductors: (27 x 1.25) + 14 + 7.6 = 33.75 + 21.6 = 55.35 amps. 6 AWG copper is good for 65 amps at 75 degrees C.
2. Feeder breaker: 70 + 14 + 7.6 = 91.6 amps. The next size down is 90 amps.

Motors on Drives

A motor fed by a variable frequency drive follows a different path. Under NEC 430.122, the conductors on the input side are sized at 125 percent of the drive's rated input current. Use the drive's marked ratings, and the fuses or breakers the manufacturer names. If the drive provides listed overload protection, you may not need a separate overload relay. Read its listing.

Control Circuits

Control conductors have their own protection rules in NEC 430.72. Small control wires tapped from a motor circuit need protection, typically fuses at the control transformer. Follow the manufacturer's data for the transformer primary and secondary protection.

Common Mistakes

Check Your Understanding

1. A 15 HP, 460-volt motor has a nameplate FLA of 20 amps. What current do you use for the conductors?

The table FLC of 21 amps, so the conductors need at least 21 x 1.25 = 26.25 amps.

2. For the same motor, what is the largest inverse-time breaker in normal conditions?

21 x 2.50 = 52.5 amps, and the next standard size is 60 amps.

3. For the same motor, with a 1.0 service factor and no temperature rise marked, what is the maximum overload setting?

20 x 1.15 = 23 amps, using nameplate FLA.

4. Why is it acceptable to have a 60-amp breaker on 10 AWG conductors for that motor?

The overload relay protects the conductors from overload. The breaker only has to clear short circuits and ground faults, and Section 240.4(G) points to Article 430.

5. A motor feeder calculation gives 118 amps for the protective device. Which size do you use?

The next size down, 110 amps. Feeder protection does not round up.

Key Takeaways

• Table FLC sizes conductors, fault protection and disconnects. Nameplate FLA sets overloads.
• Conductors are 125 percent of table FLC.
• An inverse-time breaker is up to 250 percent, and a dual-element fuse is up to 175 percent.
• Overloads are limited to 125 or 115 percent of FLA, depending on the motor.
• A disconnect needs a horsepower rating, must be within sight, and must lock open.
• Feeder conductors use 125 percent of the largest motor plus the rest, and feeder protection rounds down.