Generators under Article 445

Two weeks after the transformer was energized, a summer storm knocked out power to the Harlow industrial park for nine hours. Delgado Fabrication lost a day's production, and Hector called Ray the next morning asking for a standby generator. Ray gave the job to Dana, with Priya Natarajan pricing the equipment. A generator job crosses several articles: Article 445 for the generator itself, Article 702 for an optional standby system, Article 700 or 701 if the system is required by law, Article 702 and 250.30 or 250.35 for grounding, and the transfer equipment rules. Exams focus on a handful of calculations and decisions within that group.

This lesson covers generator ratings and currents, conductor sizing from the generator terminals under 445.13, generator overcurrent protection and disconnects, the three classes of standby system and how to tell them apart, transfer equipment, how optional standby systems are sized under 702.4, and grounding of the generator system.

Generator ratings

A generator nameplate lists kilowatts, kilovolt-amperes, power factor, voltage, phase and current. Generators are usually rated at a power factor of 0.8, so a 100-kilowatt generator is a 125-kVA machine. The current on the nameplate is the full-load current at rated kVA. When the nameplate is missing from a question, you can calculate the current from the kilowatt rating if you know the power factor, or from the kVA rating directly.

Exam questions sometimes give only the kilowatt rating at unity power factor, especially for small generators, and expect you to divide by the voltage and the square root of 3 without a power factor. If the question states a power factor, use it; if it gives kVA, use the kVA.

Conductors from the generator terminals

Section 445.13 sets the minimum ampacity of the phase conductors from the generator terminals to the first overcurrent device. In general terms, they must have an ampacity of not less than 115% of the nameplate current rating of the generator. The reason is that generators can deliver modestly more than rated current for a time, and the conductors from the terminals are not protected by a device ahead of them. Neutral conductors may be sized for the maximum unbalanced load under 220.61, and the section includes an exception for generator designs that prevent overloading, where 100% is permitted.

Generator protection and the disconnect

Section 445.12 requires generators to be protected from overload by inherent design, circuit breakers, fuses, protective relays or other acceptable means. Most packaged generators come with a main output breaker mounted on the unit, sized by the manufacturer, which serves as the first overcurrent device and often as the disconnect. Section 445.18 requires a disconnect for generators, lockable in the open position, by which the generator and its protective devices can be disconnected from the circuits supplied, with stated exceptions. Since the 2020 edition, an emergency shutdown device has also been required for certain one- and two-family dwelling generators, located outside the dwelling; exams on commercial work focus on the disconnect itself.

Three kinds of standby system

Chapter 7 separates standby power into three articles according to who requires it and what it serves. Exam questions often describe a load and ask which article applies, so the distinctions matter.

Delgado's generator is optional standby: no code requires it, and its purpose is to keep production running. That puts it under Article 702, which has the lightest requirements. The building's existing emergency lighting is battery-powered unit equipment under Article 700 and is not part of this project.

Emergency systems have the strictest rules: wiring kept entirely independent of other wiring except at transfer equipment and a few other points, selective coordination of overcurrent devices, testing and maintenance records, and specific signage. Legally required standby systems fall between. Optional standby systems mainly require proper transfer equipment, capacity, signage and grounding.

Transfer equipment

A transfer switch moves the load from the utility to the generator and back. It must be listed for the purpose and must prevent the two sources from being connected together, which would backfeed the utility lines and endanger line workers. Sections 702.5 and 700.5 require transfer equipment suitable for the use, and automatic transfer switches for emergency systems must be listed for emergency use. Transfer switches can be open-transition, breaking one source before making the other, or closed-transition, briefly paralleling the sources under utility-approved conditions.

For an optional standby system, the transfer can be manual or automatic, and that choice changes how the generator must be sized.

Sizing an optional standby generator: 702.4

Section 702.4 requires the optional standby system to have adequate capacity for the load, and it distinguishes two cases. With manual transfer, the generator must have enough capacity for all equipment intended to be operated at one time, and the user chooses which loads to run. With automatic transfer, the generator must be sized for the full load that will be transferred, calculated under Article 220 or another approved method, unless an energy management system automatically limits the load to the generator's capacity. That rule is why homes with automatic standby generators often have load shedding modules on air conditioners and electric ranges.

Ray and Hector chose to transfer only the compressor, the dust collector, one saw and the office panel. The motor portion calculates at 1.25 × 34 + 21 + 14 = 77.5 amperes, or 1.732 × 480 × 77.5 = 64.4 kVA, and with the 48 kVA office panel the total is 112.4 kVA, under the generator's 125 kVA rating. Dana documented the choice in a load calculation for the permit, which is what Gloria Haynes will review.

Motor starting on a generator

Motors draw locked-rotor current when they start, and a generator is a much weaker source than the utility. A large motor starting on a generator can cause a voltage dip deep enough to drop out contactors or stall other motors. Generator sizing for motor loads therefore considers starting kVA, found from the motor's code letter in Table 430.7(B), not just running kVA. The code itself does not specify a starting calculation, but manufacturers' sizing software does, and exam questions occasionally ask you to estimate starting kVA from a code letter and horsepower.

Grounding the generator system

How a generator is grounded depends on whether it is a separately derived system. If the transfer switch switches the neutral, the generator's neutral is not solidly connected to the utility neutral, and the generator is a separately derived system that must be grounded under 250.30 with its own system bonding jumper at the generator. If the transfer switch does not switch the neutral, the generator's neutral is solidly connected to the service neutral, the generator is not separately derived, and its neutral must not be bonded to ground at the generator, because that would create a parallel path for neutral current. Section 250.35 covers the bonding of the generator in each case. On exams, the deciding question is whether the neutral is switched.

import math
kva, volts = 125, 480
i = kva * 1000 / (math.sqrt(3) * volts)
print("Generator rated current:", round(i, 1), "A")
print("Output conductors (115%):", round(1.15 * i, 1), "A")

motor_kva = math.sqrt(3) * 480 * 133 / 1000
total = motor_kva + 48
print("Transferred load:", round(motor_kva, 1), "+ 48 =", round(total, 1), "kVA versus", kva, "kVA")
print("Compressor starting kVA at 6.0 kVA/hp:", 6.0 * 25)

Generator rated current: 150.4 A
Output conductors (115%): 172.9 A
Transferred load: 110.6 + 48 = 158.6 kVA versus 125 kVA
Compressor starting kVA at 6.0 kVA/hp: 150.0

Portable generators and interlocks

Small businesses and homes often use portable generators connected through a power inlet and a manual transfer switch, or through an interlock kit on the main panel that mechanically prevents the main breaker and the generator breaker from being on at the same time. A listed interlock is a form of transfer equipment, and it is acceptable when it is listed for the specific panel. A homemade double-male cord plugged into a receptacle, by contrast, is never acceptable: it energizes exposed prongs and can backfeed the utility. Section 702.12 covers outdoor generator sets and power inlets, including the requirement for a disconnect in sight of a permanently installed outdoor generator, which can be the generator's own breaker when it qualifies.

The generator's neutral handling matters here too. Most portable generators have the neutral bonded to the frame, which is correct when the generator is used alone with cords, but when it is connected through a transfer switch that does not switch the neutral, the bond must be removed or a switched-neutral transfer switch used, following the generator manufacturer's instructions.