Voltage Between Adjacent Devices

Occasionally, an electrical installation practice is discovered that is dangerous to both electrical maintenance personnel and the facility where it is located. I attend various electrical inspector meetings and often answer questions about the National Electrical Code (NEC) and listing requirements for electrical products. Periodically, someone asks, “I am using a two-pole, 20-amp, 277-volt rated switch and want to switch a 277-volt light and a 120-volt fan with the same device. Is this acceptable?”

Let’s look at the various issues with an installation of this type. First of all, there is the issue of being capable of safely de-energizing the switch and the branch circuits so that safe maintenance can be performed on the circuit. With a 277-volt branch circuit supplying one pole of the two-pole switch and a 120-volt circuit supplying the other pole of the switch, the two circuits would not constitute a multiwire branch circuit based on the definition of “multiwire branch circuit” in Article 100. The definition states it is, “A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.”

The 120-volt circuit and the 277-volt circuit comply with the first part of the definition by having two ungrounded conductors with a voltage between them. The circuits do not, however, comply with the second half of the definition because there are two separate and different grounded conductors without an equal voltage between the grounded conductor and each ungrounded conductors of each circuit.

Since the two circuits do not comprise a multiwire branch circuit, compliance with 210.4, covering multiwire branch circuits, is unnecessary. Some would turn to Section 210.7(B) because connecting two or more branch circuits to devices or equipment on the same yoke would require simultaneous disconnecting of the ungrounded conductors supplying those devices at the point at which the branch circuits originate.

This would make sense because anyone working on the switch mentioned above should be protected by being able to disconnect the two “hots” simultaneously. However, upon further investigation, while 210.7(B) applies to devices or electrical equipment on a common yoke, the title of 210.7 shows that this section applies only to receptacles and not switches.

The danger certainly is present for anyone de-energizing only one circuit of the two connected to the switch and, when realizing that one of the circuits is 277 volts, that danger level is multiplied greatly by the very real possibility of a much higher voltage being present on the switch if only the 120-volt fan circuit has been disconnected at the breaker. The word “receptacles” was deleted in the title of 210.7 for the 2008 NEC, so it will apply to all devices. Both the 120-volt and the 277-volt branch circuit overcurrent protective devices would have to be simultaneously disconnected to comply with 210.7(B), which is not possible, since the circuits originate from different panelboards.

The next section of the NEC that will be accessed for this type of situation is 404.8(B), covering voltage between adjacent devices. This section ensures a snap switch is not grouped or “ganged” together in enclosures with other snap switches, receptacles or similar devices, unless the devices are arranged so that the voltage between adjacent devices does not exceed 300 volts, or unless identified, securely installed barriers are inserted between the adjacent devices. The purpose is to keep the potential between two devices to a reasonable level or barrier them so someone cannot easily insert fingers or tools between the devices. Although the basic concept is similar, Section 404.8(B), covering a maximum of 300 volts between adjacent devices, applies only to multiple devices and would not apply to a single double-pole switch connected to both 120-volt and 277-volt circuits. To partially address the issue, new Section 404.8(C) has been added for the 2008 NEC, and it states, “Multipole Snap Switches. A multipole, general use snap switch shall not be permitted to be fed from more than a single circuit unless it is listed and marked as a two-circuit or three-circuit switch, or unless its voltage rating is not less than the nominal line-to-line voltage of the system supplying the circuits.”

So where is the NEC violation for this installation? The two-pole switch connected to 120 volts for the exhaust fan and 277 volts to the ceiling luminaire is a violation of Section 110.3(B), since the multipole, general-use switch must be listed and marked as ‘‘2-circuit’’ or ‘‘3-circuit.” The installation also would be in violation of 210.7(B) in the 2008 NEC, as simultaneous disconnection of both circuits would be necessary.  EC

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at mark.c.ode@us.ul.com.





About the Author

Mark C. Ode

Fire/Life Safety Columnist 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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