Motor Control Practice

Four separate calculations, three different percentages, and one distinction — table current or nameplate current — that decides more marks than anything else in the topic.

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What it is, and why electricians need it

A motor circuit is four separate sizing calculations, each with its own percentage, and the first thing to get right is which current you feed into them. Conductors, switches and the branch-circuit short-circuit and ground-fault protective device are all sized from the table full-load current. The separate overload device alone is sized from the motor's nameplate full-load amperes.

That distinction exists for a reason. The tables give typical values for a motor of a given horsepower and voltage, which is what you want for sizing the wiring infrastructure: it stays correct when the motor is replaced with another of the same rating. The overload device, by contrast, is protecting this particular motor's windings, so it uses this particular motor's nameplate current.

The second thing to understand is why the branch device is sized so generously. A motor started across the line draws six or more times its running current for a second or two. A device set near running current would trip on every start, so the table permits up to 175 percent for a time-delay fuse, 250 percent for an inverse time breaker, 300 percent for a nontime-delay fuse and 800 percent for an instantaneous trip breaker. The consequence is that the branch device is blind to overload — it cannot see a 130 percent overcurrent that will destroy the windings over twenty minutes. Hence the separate overload relay, set near 115 to 125 percent of nameplate. Two devices, two different jobs, neither substituting for the other.

The control side is mostly about one circuit: three-wire start-stop. The sealing contact in parallel with the momentary start button keeps the coil energized after the button springs open, and because the circuit depends on continuous coil power, a supply interruption drops the motor out and it will not restart by itself. That low-voltage protection is the whole reason three-wire control is standard.

Formulas and principles

  • Conductors: 125% of TABLE full-load current

    A single continuous-duty motor.

    Several motors: 125% of the largest plus 100% of the others.

  • Overload: 125% or 115% of NAMEPLATE full-load amperes

    125% with a marked service factor of 1.15 or greater, or a marked temperature rise of 40 °C or less. Otherwise 115%.

    The only motor calculation that uses nameplate current.

  • Branch device: 175% TD fuse, 250% inverse time breaker, 300% nontime-delay fuse, 800% instantaneous trip

    Maximum, as a percentage of table full-load current, for squirrel-cage motors.

    Design B energy-efficient motors raise the instantaneous-trip column to 1100%.

  • Disconnecting means: 115% of full-load current

    Minimum ampere rating, for motor circuits of 1,000 volts or less.

  • Feeder for several motors: 125% × largest + Σ others + non-motor load

    Non-motor load at 100% if noncontinuous, 125% if continuous.

Full-load currents you will use most

Three-phase squirrel-cage induction motors. These are the table values for sizing conductors and branch devices — not nameplate values.

Three-phase motor full-load current by horsepower and voltage
HP208 V230 V460 V
14.64.22.1
1-1/26.66.03.0
27.56.83.4
310.69.64.8
516.715.27.6
7-1/224.22211
1030.82814
1546.24221
2059.45427
2574.86834
30888040
4011410452
5014313065
7521119296
100273248124

Single-phase motors draw considerably more. At 230 volts: 1/2 hp 4.9 A · 1-1/2 hp 10 A · 2 hp 12 A · 3 hp 17 A · 5 hp 28 A · 7-1/2 hp 40 A · 10 hp 50 A. Note that a 5 hp single-phase motor at 230 V draws 28 amperes against 15.2 for the three-phase equivalent.

Source: NEC 2017 Table 430.250 and Table 430.248, from Public.Resource.Org's publication of incorporated standards. Values are given as facts for working problems; no NEC table is reproduced.

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Worked examples

Scenario: A 10-horsepower 460-volt three-phase motor, continuous duty, nameplate 13.5 amperes with a service factor of 1.15, protected by an inverse time breaker. Size the conductors, the overload, the branch device and the disconnect.

  1. Table full-load current for 10 hp at 460 V is 14 amperes. Use this for everything except the overload.
  2. Conductors: 125 percent of table current = 14 × 1.25 = 17.5 amperes minimum ampacity. 12 AWG copper at 75 °C, good for 25 A, serves.
  3. Overload: use NAMEPLATE current. Service factor is 1.15 or greater, so 125 percent: 13.5 × 1.25 = 16.875 amperes maximum trip setting.
  4. Branch device: inverse time breaker at 250 percent of table current = 14 × 2.50 = 35 amperes, which is a standard rating.
  5. Disconnect: 115 percent of table current = 14 × 1.15 = 16.1 amperes minimum rating.
  6. Note three different percentages and two different currents in one problem.

Answer: 17.5 A conductors, 16.875 A overload, 35 A breaker, 16.1 A disconnect minimum

Scenario: A feeder supplies two three-phase motors with table full-load currents of 28 and 14 amperes. Find the minimum feeder conductor ampacity.

  1. Identify the largest motor: 28 amperes.
  2. Largest at 125 percent: 28 × 1.25 = 35 amperes.
  3. All other motors at 100 percent: 14 amperes.
  4. Minimum feeder ampacity = 35 + 14 = 49 amperes.
  5. 8 AWG copper at 75 °C, good for 50 A, serves. Note the feeder overcurrent device is sized by a separate rule, not simply 49 A rounded up.

Answer: 49 amperes

Common mistakes

  • Using nameplate current to size conductors or the branch device. Those come from the table. Only the separate overload device uses nameplate current. This is the most frequently tested distinction in the topic.
  • Mixing up the three percentages. 125 percent for conductors, 115 percent for the disconnecting means, and the Table 430.52 figures for the branch device. Three different numbers for three different purposes.
  • Expecting the branch device to protect against overload. It cannot. Sized at 175 to 300 percent of full-load current to survive starting inrush, it is blind to the sustained 130 percent overcurrent that destroys windings. That is the overload relay's job.
  • Applying 125 percent to every motor on a feeder. Only the largest motor gets the 125 percent factor. The rest are taken at 100 percent of their full-load currents.
  • Responding to a tripping overload by enlarging the device. An overload that keeps tripping is telling you something — a lost phase, a high-resistance connection, a mechanical problem, or a genuinely overloaded machine. Badly unbalanced line currents with one phase near zero is the signature of single-phasing.
  • Reversing a three-phase motor by rotating all three leads. That preserves the phase sequence and changes nothing. Interchange any two leads.
  • Assuming "in sight from" means visible at any distance. It means visible AND not more than 50 feet distant. Both conditions must be satisfied.

Related practice topics

Topics that come up alongside this one on the exam.

Questions about this topic

When do I use table current and when do I use nameplate current?

Conductor ampacity, switch ratings and the branch-circuit short-circuit and ground-fault protective device all come from the full-load current tables. The separate overload device is the one exception — it is sized from the motor's nameplate full-load amperes. This single distinction is the most heavily tested point in the whole topic and the most common source of lost marks.

Why are the branch-circuit device percentages so high?

Because a motor started across the line draws six or more times its running current for a second or two, and a device sized close to the running current would trip every time. The table allows up to 175 percent for a time-delay fuse, 250 percent for an inverse time breaker, 300 percent for a nontime-delay fuse and 800 percent for an instantaneous trip breaker. That headroom is why the branch device cannot protect against overload, and why a separate overload device is required.

What does the overload device protect against that the branch device does not?

A sustained modest overcurrent. Because the branch device is set at 175 to 300 percent of full-load current to ride through starting, it is effectively blind to a 130 percent overload that will cook the windings over many minutes. The overload relay, set near 115 to 125 percent of nameplate current, catches exactly that. Neither device replaces the other.

How do I size feeder conductors for several motors?

Take 125 percent of the highest-rated motor's full-load current, then add the full-load currents of all the other motors at 100 percent. Any non-motor load on the same feeder is added at 100 percent if noncontinuous and 125 percent if continuous. Applying 125 percent to every motor would substantially oversize the feeder.

What percentage applies to the motor disconnecting means?

At least 115 percent of the motor full-load current, for circuits rated 1,000 volts or less. Note this is a third distinct percentage: 125 percent for conductors, 115 percent for the disconnect, and the Table 430.52 percentages for the branch-circuit device. Keeping the three straight is half the battle on motor questions.

How is a three-phase motor reversed?

Interchange any two of the three line conductors. That reverses the phase sequence, which reverses the direction of the rotating magnetic field. Rotating all three leads preserves the sequence and changes nothing. A reversing starter is simply two contactors wired so that one delivers the leads in swapped order.

What is the advantage of three-wire control over two-wire?

Low-voltage protection. In a three-wire circuit the coil is held in by an auxiliary contact that bypasses the momentary start button, so losing control power drops the contactor out and it cannot re-energize on its own when power returns. A two-wire circuit using a maintained switch would restart the machine unexpectedly, which is why three-wire control is standard where an unexpected restart would be hazardous.

Where must the motor disconnect be located?

In sight from the controller, and also in sight from the motor and the driven machinery, subject to permitted exceptions. "In sight from" is a defined term meaning visible and not more than 50 feet distant — both conditions must be met, so a disconnect 20 feet away around a corner does not qualify. Where an exception permits a remote disconnect, the one at the controller must be capable of being locked open.

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