Mismatched: Should Transfer Switches and Generators Match?

Photo credit: Michael Johnston

Let’s assume a 120/208-volt (V), three-phase service was installed on a building a few years ago and the new owner of the building needs a standby generator for use during power outages. The electrical contractor installs a 120/240V single-phase, standby generator with a 120/208V, single-phase transfer switch. A 100-ampere (A), 120/208V, single-phase panelboard also is installed to supply normal and backup power through the transfer switch to selected equipment using two phases of the 208V system and a neutral from the three-phase service. Does this installation comply with the 2017 National Electrical Code (NEC)?

Is the electrical equipment being used within its voltage rating, and what types of loads are being supplied? Will the electrical equipment supplied by normal power at 120/208V, single-phase still operate properly within the 120/240V, single-phase rating of the generator during backup mode? There are multiple locations one must go within the NEC to answer these questions, and the answers may be different depending upon the type of load being supplied.

Section 110.4 covers the general requirement for voltage and states: “Throughout this Code, the voltage considered shall be that at which the circuit operates. The voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected.”

The 100A, 120/208V, single-phase panelboard and the 120/208V, single-phase transfer switch are acceptable to be supplied by the normal 120/208V, single-phase source.

Is there a problem, however, with the 120/240V, single-phase generator supplying the backup power to these two pieces of electrical equipment without regard to the loads connected to the circuits in the panelboard? In this case, where the panelboard and the transfer switch are rated at 120/208V and the generator is supplying a feeder circuit to the transfer switch at 120/240V, the answer is no, this equipment is not rated for the 240V of the generator.

In addition, the control circuits within the transfer switch, such as the control transformer and other similar control devices, may fail due to the extra voltage with higher resulting current through the devices. I would question whether the transfer switch control system would operate adequately at 120/240V with the transfer switch rated at 208V and the controls at 120/208V. There may be transfer switch manufacturers who build their systems to be able to withstand the extra voltage and more current; however, the installer should ask those pertinent questions of the supplier before buying and installing the equipment. The transfer switch warranty may also be compromised where a failure occurs within the control circuits, so verify in writing that the manufacturer will honor the warranty with this mismatched voltage.

I spoke with two manufacturers and received two different answers on this issue without addressing either the generator and transfer switch or the downstream loads. Where the downstream loads are rated 208V, supplying these loads with 120/240V, single-phase power from the generator will also be a problem with the requirements in 110.4 as stated before, such as the voltage rating of electrical equipment shall not be less than the nominal voltage of a circuit to which it is connected. The single-phase voltage from the generator is 240V, and the equipment is rated at 208V, which, again, would be an NEC violation.

If the loads are resistive, the equipment is rated at 240V, and the supply is 208V, the resistive loads would be approximately 75 percent of the rated wattage. Where the loads are resistive, the equipment is rated at 208V, and the supply is 240V, the output would exceed the equipment rating, and the equipment may very well be very hot, such as a hair dryer or combination hair dryer and blower.

If the loads are inductive, such as motor loads and similar loads, the motors and inductive loads would draw more current and may run much hotter than originally designed. Where there are mixtures of resistive and inductive loads, these issues may be exaggerated. At the very least, the mismatch of the 120/240V, single-phase generator to a 120/208V, single-phase transfer switch to a 120/208V, single-phase panelboard and to 208V-rated loads is not a good idea.

Ensuring all electrical installations are well-designed and properly installed is the responsibility of every electrical contractor. Installing a mismatched system may not be in the best interest of the facility owner, may cause problems for future work when it doesn’t operate properly, and may lead to warranty issues.

About the Author

Mark C. Ode

Fire/Life Safety, Residential and Code Contributor

Mark C. Ode is a lead engineering associate for Energy & Power Technologies at Underwriters Laboratories Inc. and can be reached at 919.949.2576 and Mark.C.Ode@ul.com.

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