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Thread: Resistance Heat (Auxiliary Heat Strips) Circuit Sizing

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    Resistance Heat (Auxiliary Heat Strips) Circuit Sizing

    I have a question regarding sizing of conductor ampacity and overcurrent protection for an electric furnace that is part of a heat pump system.

    1) I am under the impression that this fixed heating equipment is considered a continuous load, and therefore the overcurrent protection device can be rated at 100%-125% of the conductor ampacity, correct?

    Here is the nameplate of the unit in question:
    Name:  heater_nameplate.jpg
Views: 152
Size:  135.4 KB

    The system is fed from the main panel by a 60 amp breaker with 6 AWG NM-B. There are two 60 amp breakers installed at the unit (sealed area, not allowed to open as a home inspector to look into it).

    This doesn't seem right... It would seem that since it has a single circuit supply, it has undersized conductors and OCPD? No?

    I feel like I'm missing something obvious.

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    If just Circuit No. 1 is connected and not No. 2 as well, then it might be OK. When you said there are two 60A breakers on the furnace, did you mean two 2-pole or two single pole?

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    Quote Originally Posted by CuriousHomeInspector View Post
    I have a question regarding sizing of conductor ampacity and overcurrent protection for an electric furnace that is part of a heat pump system.

    1) I am under the impression that this fixed heating equipment is considered a continuous load, and therefore the overcurrent protection device can be rated at 100%-125% of the conductor ampacity, correct?

    Here is the nameplate of the unit in question:
    Name:  heater_nameplate.jpg
Views: 152
Size:  135.4 KB

    The system is fed from the main panel by a 60 amp breaker with 6 AWG NM-B. There are two 60 amp breakers installed at the unit (sealed area, not allowed to open as a home inspector to look into it).

    This doesn't seem right... It would seem that since it has a single circuit supply, it has undersized conductors and OCPD? No?

    I feel like I'm missing something obvious.
    HVAC and motors are different in how they are sized as compared to normal OCPD and conductor sizing. The motors/compressors have internal overload protection so the circuit only has to protect from a short circuit. The conductors are sized according to the MCA on the name plate. The 125% is already figured in. So the conductors must be at least the MCA rating but can be larger. The breaker or OCPD can be up to the Max OCPD listed, it can be smaller but you risk the chance of the breaker tripping on in-rush. So usually the Max size is installed.

    With that said, if what you are reporting is true and you haven't missed anything, the installation is not correct. Depending on whether there is a single circuit ran that covers both the individual circuits or one circuit for each, the numbers on the plate don't match what you reported.

    If single circuit you would need minimum size conductors for either 83.5A or 95.4A, depending on whether it is 208V or 240V and Max breaker of 90 or 100A
    If two circuits, you would need minimum size conductors for either 51.7 or 59A and Max breaker of 60A for one circuit
    For the other, you need minimum size for 31.6 or 36.5 with Max breaker of 35 or 40A depending on the voltage

    If you have only a single circuit ran with #6NM then it might be undersized since NM can only be rated using the 60 deg column and that would only be 55A. It would depend on what circuit that was for and the voltage
    If you aim at nothing, you will hit it every time!

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    OP, You looked in the electrical panel, how did things look at the cb feeding the furnace?

    Is is there a chance that second Heat bank is not used? That would be my hopeful guess.
    Tom
    TBLO

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    Quote Originally Posted by ptonsparky View Post
    OP, You looked in the electrical panel, how did things look at the cb feeding the furnace?

    Is is there a chance that second Heat bank is not used? That would be my hopeful guess.

    There is a chance the second heat bank is not being used. That is my hopeful guess as well. If both heat banks were being used and are being fed off of a single 60 amp circuit, I don't see how it wouldn't trip the breaker while I was testing it in emergency heat mode.

    As stated above, I don't disassemble sealed HVAC assemblies, so I don't know how/what is wired inside the unit.

    I wish HVAC contractors would actually mark what heat kits etc are installed on the systems they install like they are supposed to... Unfortunately I've NEVER seen one marked/indicated like it is supposed to be.....

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    Quote Originally Posted by Little Bill View Post
    HVAC and motors are different in how they are sized as compared to normal OCPD and conductor sizing. The motors/compressors have internal overload protection so the circuit only has to protect from a short circuit. The conductors are sized according to the MCA on the name plate. The 125% is already figured in. So the conductors must be at least the MCA rating but can be larger. The breaker or OCPD can be up to the Max OCPD listed, it can be smaller but you risk the chance of the breaker tripping on in-rush. So usually the Max size is installed.

    With that said, if what you are reporting is true and you haven't missed anything, the installation is not correct. Depending on whether there is a single circuit ran that covers both the individual circuits or one circuit for each, the numbers on the plate don't match what you reported.

    If single circuit you would need minimum size conductors for either 83.5A or 95.4A, depending on whether it is 208V or 240V and Max breaker of 90 or 100A
    If two circuits, you would need minimum size conductors for either 51.7 or 59A and Max breaker of 60A for one circuit
    For the other, you need minimum size for 31.6 or 36.5 with Max breaker of 35 or 40A depending on the voltage

    If you have only a single circuit ran with #6NM then it might be undersized since NM can only be rated using the 60 deg column and that would only be 55A. It would depend on what circuit that was for and the voltage

    Thank you. With the name plate ratings I always read it as "this has already been figured out by someone else smarter than me, so I won't question it," even though I do understand the way it is calculated and the rules are different from a normal branch circuit... If I understand correctly that is why looking at the nameplate the amperage and minimum circuit ampacity they have figured follows the 125%/80% rule (depending on direction).

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    The hvac guys are supposed to mark the unit as to what size is required. You are correct that if the unit was installed this way for years then I assume that it would trip the breaker if both circuits were energized.

    It sounds like the unit is setup for one or 2 circuits and they opted for one circuit.
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    If that circuit is for circuit #1 then IMO the #6 nm cable is not compliant.
    They say I shot a man named Gray and took his wife to Italy
    She inherited a million bucks and when she died it came to me
    I can't help it if I'm lucky



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    This is likely

    Quote Originally Posted by ptonsparky View Post
    OP, You looked in the electrical panel, how did things look at the cb feeding the furnace?

    Is is there a chance that second Heat bank is not used? That would be my hopeful guess.
    This is often times what you will find. The entire wattage of the heat array will not be utilized, line voltage will not be landed on the spare elements, and the control circuit that deals with the unused elements will also be disabled, typically. This usually means that only that amount of supplemental heat was required for the load.
    This is also assuming that the system worked well across all conditions encountered, and there were no issues with it being short of capacity during peak load conditions. On board breakers were common and useful on that generation of gear.
    Microwave Radiation Dangers should be openly discussed

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    OP. Sq ft of house?
    Tom
    TBLO

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