12.3 Motor & Generator Wiring, Protection & Disconnects

12.3 Motor & Generator Wiring, Protection & Disconnects

Motors and generators are graded heavily on the RME exam because they combine several layers of protection into a single piece of equipment -- get the layering wrong and an installation either leaves a motor unprotected or nuisance-trips it every time it starts. This section walks through conductor sizing, the three-layer protection scheme, disconnect requirements, and the parallel set of rules that apply to generators.

Motor Branch-Circuit Conductor Sizing

Motor branch-circuit conductors are not sized off the motor's nameplate current the way most other loads are. Because a motor is treated as a continuous-duty load with a predictable starting inrush that must not nuisance-trip the wiring, conductors are sized at 125% of the motor's full-load current (FLC) as found in the applicable FLC table for the motor's voltage, phase, and type, not the nameplate current stamped on the motor itself, except in specific limited circumstances.

Worked example: A 5 hp, 230V, single-phase motor has a tabulated FLC of 28A. The minimum branch-circuit conductor ampacity is:

28A x 1.25 = 35A

That means the RME must select a conductor with an ampacity rating of at least 35A at the applicable termination temperature -- a common choice is 10 AWG copper, rated 35A at 75 degrees C, sized right at the calculated minimum. If the same motor were instead a larger frame with a higher FLC, the same 125% multiplier still applies; only the base FLC value changes.

The Three Layers of Motor Overcurrent Protection

A motor circuit is protected in three distinct, independent layers -- this layering is one of the most frequently tested concepts on the exam because each layer has a different job and a different sizing method.

The branch-circuit protection is deliberately set loose, at a high percentage, because it only needs to catch a catastrophic short or ground fault -- it is not meant to trip on ordinary starting current. The overload protection is set tight, close to FLC, because its entire job is to catch a sustained overload that the branch-circuit device would never see. Confusing the two, for example expecting a branch-circuit breaker to protect against a slowly overheating motor, is a classic exam trap.

Motor Disconnecting Means

Every motor must have a disconnecting means that either:

• Is located within sight of both the motor and its controller, generally meaning visible and not more than about 15 m (50 ft) away, or
• Is capable of being locked in the open position if it cannot be located within sight, so the motor cannot be re-energized while someone is working on it.

The disconnect must be rated for the motor's horsepower, or carry an ampere rating not less than about 115% of the motor's FLC, and must be readily accessible for emergency shutdown. A common field problem is a motor and its disconnect ending up on opposite sides of a wall or partition, with the disconnect out of sightline -- this fails the within-sight requirement even if the physical distance is well under 50 ft, because sight, not just distance, is the test.

Generator Conductor Sizing and Overcurrent Protection

Generator circuit conductors are sized from the generator's nameplate current rating, not from a horsepower table. Two cases matter:

1. Generator without inherent overload protection -- conductors and overcurrent protection are generally sized with an allowance above the nameplate current rating, with a commonly applied multiplier of 115%, similar in spirit to the motor branch-circuit approach.
2. Generator with inherent (built-in, non-adjustable) overcurrent protection -- where the manufacturer has built in overload protection that cannot be bypassed or reset above its set point, that protection can be recognized as meeting the overcurrent protection requirement, and the conductor ampacity can be based on that protective device's rating rather than requiring the separate 115% allowance.

Grounding and Bonding of Generators

This is where generator wiring intersects with the bonding concepts from earlier in this chapter, and it hinges on one question: does the transfer scheme switch the neutral?

• Transfer switch that switches the neutral (all current-carrying conductors, including the neutral, are transferred) -- the standby generator becomes a separately derived system. It needs its own system bonding jumper, bonding the generator's neutral to its frame or equipment ground at the source, and, where required, its own connection to a grounding electrode, just like a new service.
• Transfer switch that does not switch the neutral (only the ungrounded conductors are transferred, and the neutral stays solidly interconnected to the existing service neutral at all times) -- the generator is a non-separately-derived system. No new neutral-to-ground bond is made at the generator; its frame is bonded to the equipment grounding conductor, and the installation relies on the building's existing grounding electrode system.
• Portable, cord-and-plug-connected generators supplying loads only through receptacles mounted on the generator itself are typically treated as non-separately-derived power sources per their listing, with the frame serving as the equipment ground reference -- no separate grounding electrode is required for that stand-alone use. If that same portable unit is instead connected to feed a building's wiring through a transfer switch, the separately-derived-system determination above applies based on whether the switch transfers the neutral.

RME Field Scenarios

Sizing a 5 hp motor circuit. Pull the FLC from the table, not the nameplate, multiply by 1.25 for the branch-circuit conductor, size the overload device near 115-125% of FLC depending on the motor's service factor and temperature-rise marking, and size the branch-circuit short-circuit and ground-fault device from the appropriate percentage table for the device type installed.

Wiring a standby generator transfer scheme. Before pulling conductors, determine whether the automatic transfer switch is a neutral-switching (4-pole) or non-switching (3-pole) type -- this single decision determines whether the generator needs its own system bonding jumper and grounding electrode connection, or must instead rely on the existing service ground with no new neutral bond at the generator.

Verifying a motor disconnect is within sight. Walk the sightline from the disconnect to the motor exactly as an operator would need to see it in an emergency; if a wall, machine, or corner blocks the view, the disconnect fails the within-sight test regardless of the measured distance, and it must either be relocated or provided with a lockable-open means.