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Push It to the Limit: Intentionally overloading an electrical system

By Mark C. Ode | Feb 15, 2024
Cover of the National Electrical Code book
Is it acceptable in the National Electrical Code to overload branch circuit conductors, feeder conductors, service conductors or conductors from emergency, legally required standby or optional standby systems?

Is it acceptable in the National Electrical Code to overload branch circuit conductors, feeder conductors, service conductors or conductors from emergency, legally required standby or optional standby systems?

Circuits and motors

This question is raised fairly often, depending on circumstances for residential, commercial and industrial installations, as well as other applications. A circuit overload is common where the overload is momentary without increasing the resulting heat on the insulation covering the conductors or on the circuit’s conductors. 

For example, an electric motor will briefly overload the circuit during startup. However, the overload protective device protecting the motor will trip off if the overload lasts too long. The overload protective device protects the conductors and other parts of the motor circuit ahead of the motor. Overload protection is moved out of the circuit breaker or fuse, either into the motor or adjacent to it, using the breaker or fuse to provide only short circuit and ground fault protection.

Inverse time circuit breakers are commonly used for feeders and branch circuits. Inverse tripping is a characteristic of circuit breakers that allows a certain amount of delay in the breaker’s tripping action. As the magnitude of the current increases through the circuit breaker, it opens faster, protecting the circuit against overloads, short circuits and ground faults. Properly sized conductors and overcurrent protective devices will provide overload protection for the circuit.

In the 2023 NEC and previous editions, 230.90 requires each ungrounded service conductor connecting to the main service equipment to have a circuit breaker or fuse installed in series with each ungrounded service conductor to provide overload protection. The service overcurrent device does not protect the service conductors from a short circuit or a ground fault on the line side of the service disconnecting means.

Most people don’t realize that when transformers are first energized, a transient current of up to 10–15 times larger than the rated transformer current can flow for several cycles. Without going into minute details, the transient current happens when the primary winding of the transformer is connected at the zero crossing of the primary voltage (which, for a pure inductance, would be the current maximum in the AC cycle). The iron core of the transformer saturates, thus causing the startup current plus any for the load on the secondary side of the transformer. 

To help control any startup current for the transformer, it should be energized before any secondary load currents are energized. For large transformers with low winding resistance and high inductance, these inrush currents can last for several seconds until the transient has died away. Technically, these could be considered to be overloads as far as the transformer is concerned.

Emergency and standby systems

I was recently contacted by an electrical inspector who had concerns about overloading emergency systems, legally required standby systems and optional standby systems. For emergency systems and legally required standby systems, 700.4 and 701.4 state, “the system must have adequate capacity to safely carry, at any one time, the entire load in accordance with Parts I through IV of Article 220 or by another approved method. The system capacity must be sufficient for any rapid load changes and transient power and energy requirements associated with any expected loads.” 

Load management

Selective load management can be used for load pickup and shedding to ensure that optional standby loads can be shed first, and legally required standby loads shed next to prioritize emergency and legally required standby loads (over optional standby loads). 

Emergency loads and legally required standby loads are not optional, apply to system installation essential for safety to human life and are legally required by municipal, state, federal or other codes or by a governmental agency having jurisdiction. Power sources for the emergency systems and legally required standby systems must not be overloaded.

The optional standby systems covered by Article 702 consist of those permanently installed in entirety, including prime movers, and those arranged for a connection to a premises wiring system from any portable alternate power supply. If the load connection is manual or nonautomatic, an optional standby system must have adequate capacity and rating for the supply of all equipment intended to be operated at one time. 

The user of the optional standby system shall be permitted to select the load connected to the system. For an optional standby system, if the connection of a load is automatic, the standby source must be capable of supplying the full load that is automatically connected or an energy management system is required. Intentionally overloading any optional standby source, be it a generator, a battery supply system or any other source, is not acceptable.

About The Author

ODE is a retired lead engineering instructor at Underwriters Laboratories and is owner of Southwest Electrical Training and Consulting. Contact him at 919.949.2576 and [email protected]

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