Motor calculations, and the one distinction that decides them

A motor question hands you two different currents. Using the wrong one for the wrong step is how most people lose these points, and the wrong answer always looks reasonable.

Two currents, and they are not interchangeable

Every motor problem involves two numbers that both describe current, and the code uses them for different jobs.

The first comes off a table in Article 430, selected by horsepower, voltage and phase. Call it the table current. The second is printed on the motor itself. Call it the nameplate current.

They are usually close. They are almost never equal. And the code is specific about which one belongs to which step (NEC 430.6).

• Table current sizes the conductors, the switches and the short-circuit protection.
• Nameplate current sizes the overload protection, and nothing else in the common calculation.

That is the whole distinction. An exam question that gives you both numbers is testing whether you know it, and the distractor answers are built from the other current.

Why the code works this way

It looks arbitrary until you think about what each device is protecting against.

Overload protection is watching the motor. It cares what that specific motor actually draws, so it reads the nameplate.

Short-circuit and ground-fault protection is watching the circuit, and it has to survive inrush every time the motor starts. Inrush is a property of the motor class, not of one unit, so the code standardizes it with a table.

Once you see that, you stop having to memorize which is which. Protecting the motor means nameplate. Protecting the wire means table.

The sequence

1. Look up the full-load current in the Article 430 table using horsepower, voltage and phase. Not the nameplate (NEC 430.6(A)(1)).
2. Size the branch-circuit conductors at not less than 125 percent of that table current (NEC 430.22), then apply ambient and conduit-fill corrections from Article 310.
3. Size the overload protection from the nameplate current (NEC 430.32).
4. Size the branch-circuit short-circuit and ground-fault device from the percentage in the branch-device table, applied to the table current, then round up to the next standard rating (NEC 430.52).
5. For a group, size the feeder conductor from the several-motors rule (NEC 430.24).
6. Size the feeder protective device from the largest branch device plus the other motors full-load currents.

Steps two and four both start from the table current and can be done in either order. Step three starts somewhere else entirely. Candidates who work straight down the list without noticing the source change get step three wrong every time.

The 125 percent is not the continuous-load rule

You already know a 125 percent rule from Article 210, the one for continuous loads. The motor rule looks identical and is not the same rule.

The Article 210 rule turns on whether the load runs for three hours or more. The motor rule turns on whether the application is continuous duty, which is a classification of the driven machine, not a stopwatch reading.

Any motor application is treated as continuous duty unless the thing it drives cannot run continuously under load. A conveyor that runs all shift and a hoist that runs in bursts are classified differently, and the hoist is sized from the duty-cycle table instead.

Applying the continuous-load reasoning to a motor gets the right number for the wrong reason on most questions, and the wrong number on the ones that test duty cycle.

Rounding goes the other direction here

On a general conductor, 240.4(B) lets you round the protective device up to the next standard size only if three conditions are met, and one of them caps it at 800 amperes.

On a motor branch circuit, rounding up is the normal path. The percentage from the branch-device table rarely lands on a standard rating, and the code expects you to take the next one (NEC 430.52).

Two rules that both say next higher standard size, pulling in different directions, in two articles a candidate uses in the same problem. Know which article you are standing in.

What to practice

Not the table values. You will have the book.

Practice the habit of writing both currents at the top of your scratch paper and labeling them before you start. Table current on the left, nameplate on the right. Then every step points at one of them.

That one habit is worth more on this section than any amount of memorization, because the arithmetic is easy and the sourcing is where the points go.

What this page cites

• NEC 430.6 Which current is used for which purpose. Read this before anything else in the article.
• NEC 430.22 Branch-circuit conductors for a single motor.
• NEC 430.24 Conductors supplying several motors or a motor and other loads.
• NEC 430.32 Overload protection for continuous-duty motors.
• NEC 430.52 Branch-circuit short-circuit and ground-fault protection.
• NEC 240.6(A) Standard ampere ratings, for the rounding step.