60 Amp Electric Furnace

AverageJoseph

My cousin is installing a electric furnace which calls for 60 amp breaker to suppily it. The suppiler he bought the furnace from told him to use #6 copper so he bought 6-2 NM-B. will this do?

jwhite

that depends on if the furnace needs a neutral or not.

What do the wiring instructions say you need?

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AverageJoseph

I does not need a neutral. The hook up just has a 60 amp double pole breaker and a ground lug inside the furnace.

jwhite

6-2 w/g

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AverageJoseph

6-2 with ground NM-B is what he bought.

JohnJ0906

I disagree. #6 copper romex is only rated for 55 amps.
I would use #6 copper SEU cable. Rated 65 amps.

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John from Baltimore
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bigMikeB

Seems I remember reading in the last code book something about sizing the wire for 125% of the load.

JohnJ0906

424.3(B)

An electric furnace is considered a continuous load.

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jwhite

Who calculated the load at over 55 amps?????

The mfg said 60 amp breaker.

Since 55 amps is not a common size fuse or breaker. A number six romex can be put on a 60 amp breaker.

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AverageJoseph

So the furnace might pull 55 amps so they just call for a 60 so it can carry the load with out tripping?

another question is I was told by an electrician that a breaker is only good for 80% of the rating. so a 100 amp breaker is actually going to trip just over 80 amps. Is this true?

NateHanson

I think what you're recalling is that it's good practice to load a circuit to 80% capacity (ie, put ten 100 watt lights on a 15A breaker - 12A is 80% of 15A).

Jwhite, I don't understand the 55A explanation. If #6 romex has a rated ampacity of 55A, isn't it required that it's protected by an over current device no higher than 55A? I wouldn't think that the common availability of 60A breakers would be much concern to the NFPA. (But then, as I've proven in the past, I don't know a whole lot about this stuff.)

JohnJ0906

Sorry, I still disagree. This is a contiuous load. Unless I calculate the load x 125%, and come up 55 amps or under, I will pull wire for the 60.

Who said it wasn't over 55. We dont have all the nameplate info.

This is a common red-tag around here.

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jwhite

Joseph, what does the nameplate say about the amperage?

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Speedy Petey

(B) Devices Rated 800 Amperes or Less The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.

240.6 Standard Ampere Ratings
(A) Fuses and Fixed-Trip Circuit Breakers The standard ampere ratings for fuses and inverse time circuit breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes. Additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601. The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.

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