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Is A Fused Disconnect Required For A Gas Furnace In A Home?

December 22, 2005

Q: When wiring a gas-fired central heating furnace in a dwelling unit, I usually provide a toggle switch at the furnace to satisfy the requirement for a disconnecting means within sight of the blower motor. However, some jurisdictions require a fused disconnect and do not allow a toggle switch. Is a fused disconnect required by the National Electrical Code?

A: The disconnecting means for the motor must comply with Parts III of Article 422 and Part IX of Article 430. For appliances with a motor larger than 1/8 horsepower, a disconnect switch must be located within sight of the appliance or must be capable of being locked in the open position. This requirement appears in 422.31(B).

However, a revision in 420.102(B) now requires a disconnecting means within sight of the motor. The Exception to 430.102(B) does not apply to this installation.

Part (C) of 430.109 recognizes a toggle switch as a motor disconnect under limited conditions. To use a toggle (snap) switch, the motor cannot exceed 2 horsepower and the voltage cannot exceed 300.

These are the requirements: a general use snap switch suitable must have an ampere rating that is at least twice the full load current rating of the motor; a general use snap switch suitable for use only on AC (not a general use AC/DC snap switch) may serve as a motor disconnect provided that the motor full-load current does not exceed 80 percent of the Ampere rating at the switch. Similar rules dealing with the use of toggle switches are found in 404.14(A) and (B).

Part (A) of 422.11 covers branch circuit overcurrent protection. This part says that, “If a protective device rating is marked on an appliance, the branch-circuit overcurrent device rating shall not exceed the protective device rating marked on the appliance.”

Does the branch circuit overcurrent protective device rating in the panelboard exceed the value marked on the appliance? If the overcurrent protective device in the panelboard is rated 20A and the nameplate on the furnace calls for a 15A overcurrent protective device, this could be the reason for the inspector’s request for a fused disconnect at the furnace rather than a toggle switch.

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