Sizing a Motor Starter: Contactor, Overload, and the IEC vs. NEMA Question

A motor starter is two decisions, not one. The contactor has to make and break the motor current for the life you expect. The overload relay has to trip before the windings cook, and not before. Here is how the sizing works, in the order you should do it.

Start with the right amps

The NEC gives you two full-load current numbers, and it tells you which one to use for what. Section 430.6 requires the values in Tables 430.248 through 430.250 for sizing branch-circuit conductors, disconnects, and short-circuit protection. The nameplate current is what you use for overload protection. Size the wire off the table. Set the overload off the nameplate.

Section 430.32 sets the ceiling on that overload setting: 125 percent of nameplate full-load current for motors marked with a service factor of 1.15 or more, or a temperature rise of 40 C or less, and 115 percent for all other motors. Those are maximums, not targets. The 2026 edition of NFPA 70 is the current one.

The contactor: AC-3 or AC-4

IEC contactors are rated by utilization category under IEC 60947-4-1, and the current edition is IEC 60947-4-1:2023, Edition 5.0, published November 2023, which replaced the 2018 edition. The two categories that matter for squirrel-cage motors are AC-3 and AC-4.

AC-3 covers a motor that is started and then switched off while running. On closing, the contactor makes an inrush of roughly five to seven times rated current; on opening, it breaks only the normal running current. Pumps, fans, conveyors, compressors, and mixers are AC-3 duty.

AC-4 covers plugging and inching (jogging). The contactor makes a high current and then has to break that same high current while the motor is stalled or reversing, which is far harder on the contacts. Manufacturer life curves show AC-4 operation gives materially shorter contact life than AC-3 at the same current. If the machine jogs all shift, size the contactor off its AC-4 rating, not the AC-3 number on the front of the catalog.

The UL side of the same standard is UL 60947-4-1, Edition 4, dated May 31, 2022, which covers the same contactors and starters for installation under the NEC. Older devices are often still listed to UL 508, and both listings are in the field.

The overload relay: trip class

A trip class tells you how long the relay lets an overload ride before it trips. Per IEC 60947-4-1, the class number is the maximum trip time in seconds at 7.2 times the setting current, and the standard also fixes tripping and non-tripping times at 1.5 and 7.2 times setting. In the US, the same idea is commonly stated at 600 percent of full-load current: Class 10 trips in 10 seconds or less, Class 20 in 20 seconds or less, Class 30 in 30 seconds or less.

Which class you pick depends on how long the motor takes to come up to speed. Class 10 suits modern T-frame motors, which have less thermal mass than older designs and need faster protection. Class 20 is the common choice for general-purpose motors. Class 30 is for high-inertia loads such as large fans and crushers where a normal start would nuisance-trip a Class 10 device. Class 10A is a faster variant of Class 10 defined in the IEC standard and offered on some relays. Per Schneider's published trip-class table, Classes 10A and 10 both trip between 2 and 10 seconds at 7.2 times the setting current from a cold start; the difference shows at 1.5 times setting, where a Class 10A relay trips within 2 minutes and a Class 10 relay within 4. Check the published trip curve of the relay you buy.

Phase loss is the other job of the overload. A bimetallic relay with a differential trip mechanism accelerates tripping when one phase drops out; the Sprecher + Schuh CT7N, for example, is described as tripping in about 45 seconds at 115 percent under single-phase conditions. Electronic overloads handle this in firmware and typically add a wider 5:1 current adjustment range plus selectable trip class; the CEP7 basic model selects Class 10 or 20 by dip switch and the advanced model dials Class 10 through 30.

Type 1 vs. Type 2: what happens after the fault

Overload relays do not clear short circuits. The fuse or breaker ahead of the starter does. IEC 60947-4-1 defines two levels of coordination between that device and the starter, and they promise very different things.

Type 1: after a short circuit, the starter must not endanger people or the installation, but it may not be usable without repair or replacement of parts. Contact welding, burning, and heater burnout are all permitted.

Type 2: after a short circuit, the starter must not endanger people or the installation and must be suitable for further use. Light contact burning or tack welding is allowed only if the contacts are easily separated.

Type 2 is only real if you use the specific fuse or breaker the manufacturer tested with that contactor and overload. Coordination tables are published per contactor frame with maximum fuse ratings and fault current levels. Sprecher + Schuh states its CA7 contactors have been designed and tested for both Type 1 and Type 2 coordination.

When a NEMA starter is worth it

NEMA starters are sized by frame number under NEMA ICS 2, sizes 00 through 9, and the current edition is NEMA ICS 2-2000, reaffirmed in 2005 and again in 2020. NEMA publishes a separate guide, ICS 2.4-2020, that compares NEMA and IEC contactors and overload relays of equivalent electrical rating. Manufacturer tables put the range at roughly 9 A and 2 hp at 460 V for a Size 00 up to 2,250 A and 1,600 hp for a Size 9.

The practical difference: a NEMA size is a broad envelope, and a NEMA starter is expected to handle any motor within that envelope at the listed horsepower. Even so, NEMA manufacturers publish reduced horsepower ratings for plugging and jogging service, so the envelope is not unlimited. An IEC contactor is rated closer to the specific application (AC-3 or AC-4, at the actual amps), so the correct IEC frame for a given motor is usually smaller and takes less panel space, at the cost of doing the selection work. NEMA starters are generally larger and cost more for a given horsepower.

Choose NEMA when the spec calls for it, when the panel already has NEMA frames and you want interchangeable parts across the plant and would rather cross-reference your existing starter than re-engineer it, or when the duty is undefined and you want margin. Choose IEC when you know the duty, need the panel space, and will follow the coordination tables.

Sprecher + Schuh in the current line

Stateside is an authorized Sprecher + Schuh distributor. The CA7 IEC contactor line runs from 9 A to 97 A and covers motors to 75 hp at 460 V, with a CAN7 variant labeled to NEMA sizes 00 through 3 for shops that specify by NEMA size. The CA9 line picks up from 116 A to 2,650 A and up to 1,150 hp. For overloads, the CT7N bimetallic relay direct-mounts to CA7 contactors with adjustment ranges from 0.10 A up to 97 A, and the CEP7 solid-state relay mounts direct or separate on the CA7 with Class 10, 20, or 10-to-30 selectable tripping.

Whatever brand you land on, the sequence is the same: table amps for the wire, nameplate amps for the overload, AC-3 or AC-4 for the contactor, trip class for the start time, and the manufacturer's coordination table for the fuse.