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Thread: Electric furnace- breaker size

  1. #1
    Join Date
    Nov 2003
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    6

    Electric furnace- breaker size

    I replaced an electric furnace that had two feeders, one 240 volt 20 amp for the motor and control and one 240 volt 60 amp for the resistance heaters. This is a heat pump system but the compressor is on a third circuit.

    The replacement unit installed takes a single feeder and only has 5 kw of resistance heat. I used the 20 amp feeder to feed the electronic air cleaner only and used the 60 amp feeder to feed the furnace (including the motor, control and 5 kw resistance grid). I replaced the 60 amp breaker with a 40 amp breaker. The resistance grid takes approximately 23 amps and the motor around 3 amps (running load). The manual says that the ampacity of the feed must be at least 29 amps and to fuse it at 30 amps.

    If I had installed 7.5 kw heaters, it would have to be fused at 45 amps and if 10 kw, it says to fuse at 60 amps. Any higher than 10 kw requires multiple feeders (branches).

    Question: Is is OK, legal and safe to leave the unit fused at 40 amps or do I have to go back in and replace the breaker with a 30 amp unit. The branch circuit wire is sized for 60 amps.

  2. #2
    Join Date
    Feb 2003
    Location
    Burbank IL
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    982

    Re: Electric furnace- breaker size

    If the instructions say 30amp max over current, then you would have to do so.

    Other wise I think your OK
    Russ Burbank IL

  3. #3
    Join Date
    Feb 2003
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    Burbank IL
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    982

    Re: Electric furnace- breaker size

    5000 / 240 = 20.833333 + 3 =23.833333 x 1.25 = 29.79
    Russ Burbank IL

  4. #4
    Join Date
    Jun 2003
    Location
    Hawthorne, NY
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    2,397

    Re: Electric furnace- breaker size

    Was the motor and controls in the new unit factory wired with the resistance elements for supply from a single 30A feeder or were they separate and this is how you decided to wire them?

  5. #5
    Join Date
    Nov 2003
    Posts
    6

    Re: Electric furnace- breaker size

    Originally posted by hbiss:
    Was the motor and controls in the new unit factory wired with the resistance elements for supply from a single 30A feeder or were they separate and this is how you decided to wire them?
    This is how it was designed. I tried splitting the two circuits and the two were merged enough that the fan motor was being backfed from the Heater feed. It would have required another relay and I didn't feel that it was worth a redesign.

  6. #6
    Join Date
    Feb 2003
    Location
    Massachusetts
    Posts
    48,184

    Re: Electric furnace- breaker size

    Originally posted by jgriffin02:
    The manual says that the ampacity of the feed must be at least 29 amps and to fuse it at 30 amps.
    110.3(B) Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
    I would cover my rear side and use the over current protection that the manufacturer recommended.

    This has nothing to do with the branch circuit conductors, it has to do with how the unit was tested and listed by UL or other testing lab.

    In other words what size wires and components inside the unit are now protected by a 40 amp over current device?

    Originally posted by jgriffin02:
    I tried splitting the two circuits and the two were merged enough that the fan motor was being backfed from the Heater feed. It would have required another relay and I didn't feel that it was worth a redesign
    And I would not try to modify the unit into two circuits.

    Install per instructions!

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