Gas Furnace Grounding


Old 05-05-14, 05:59 PM
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Gas Furnace Grounding

I had a new furnace installed last year by an HVAC co. I noticed today that in the power box INSIDE the unit where the house wire connects with the furnace wires their is a green ground wire from furnace not attached to anything. That power box inside the unit is connected with flex and locknut to the switch on the side. My electrical system uses the metal conduit as the EGC.

I have continuity between the conduit, boxes, furnace housing and gas pipes. Its bonded together. However, I am not sure how this effects the controls of the furnace.

The installation instructions for the furnace doesn't specifically say , just says ground as instructed by local code etc.

Should this be connected somewhere? Or am I already sufficiently grounded?
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Old 05-05-14, 06:21 PM
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TAke your meter and read between the green wire and the steel of the furnace (or steel conduit) I suspect the green wire is connected to the steel chassis of the furnace and is there to connect a ground to if a non-metallic wiring method is used.

Short answer: It is likely already connected to ground.
Old 05-05-14, 07:00 PM
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Thanks for the tip Tolyn... just checked and indeed there is continuity between that green wire and the junction box on the side of the unit.

Technically it should be bonded to the box a second time I assume like you might do with a ground from a 4 wire dryer and a metal egc. Maybe not?

Unrelated... another thing I don't understand, why is the furnace supply wires allowed to be #16? Why don't they burn up?
Old 05-05-14, 07:15 PM
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why is the furnace supply wires allowed to be #16? Why don't they burn up?
The wiring has a high heat rating and it is enclosed in a metal cabinet. The furnace undergoes UL testing and that wiring is approved as a part of the furnace.

You'll many times see smaller wiring used inside a unit. Some examples I can think of are furnaces, ovens, ranges, water heaters A/C compressor units.
Old 05-06-14, 05:11 AM
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If the gas piping already has a bonding jumper (typically a fat ground wire clamped to one pipe and connected to a similar wire going between breaker panel and ground rod) then the furnace is already adequately grounded provided that the gas piping all the way to the bonding jumper is rigid.

If the only connection to the power supply is via flexible conduit, a bare ground wire or metal strip must go through the conduit outside of any paper lining. For spiral metal conduit properly fastened to metal boxes, etc. the ground wire or strip inside does not have to come into the boxes at each end and be wire nutted to other ground wires.

Sixteen gauge wire can handle something like ten amperes, and the furnace probably does not draw even that much power. However 16 gauge branch circuits, even if equipped with 10 amp panel breakers, are not permitted by the National Electric Code.
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