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100 amp sub panel from 200 amp service

fromPok | Posted in General Questions on

Experts, What size cable should I run to a 100 amp sub panel from  200 amp main panel 4/3 or ??

seems that cable is expensive.. in that case ..can I do a 60 amp breaker from main panal to 60 amp sub panel? 6/3 cable?

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  1. Expert Member
    BILL WICHERS | | #1

    You just size from the ampacity charts in the code book. 100A needs 3 gauge, which you’ll probably have to run in conduit. You might be able to find 2/3 (2 gauge, three conductor with ground) cable, but that’s a bit of an oddball. This all assumes copper wire.

    You can use 6/3 NM cable for a 50A circuit, not 60A, because NM cable has to be sized from the 60 degree column in the ampacity chart. You can use 6 gauge conductors in conduit for a 60A circuit though.

    And yes, wire has gotten expensive over the past several years! You still need to size the di panel based on the loads it needs to support though, and then install wire capable of handling the demands of the subpanel. You can’t undersized wire just because it’s expensive. What do you want to run with that subpanel? What size loads?


    1. fromPok | | #4

      Bill, Good morning & Thank you. Here is my setup. 200 amp service panel in garage. the sub panel is for other end of house to avoid long home runs. separation between panels would be 50 ft linear lenght, figure 60-75 ft through walls to be safe The sub panel would feed 5 bed rooms, 2 bathrooms, laundry. That’s why I thought 60amp might be enough.

      Can you point a link at amp & wire size chart?

      Finally, i have a heat pump from entertech which is in basement. the instructions from the manufacturer are below. There are recommending 60 amp circuit with 6/3 wire. I am thinking of feeding this from
      main panel. Are you suggesting, I must run this in conduit?

      Thank you

      1. Expert Member
        BILL WICHERS | | #5

        60A is probably big enough for that subpanel, 50A too, and with 50A you can use 6/3 with ground NM cable if you want to avoid using conduit. If you want 60A, you'd need to step up to 4/3 with ground.

        NM cable is required to be sized from the 60 degree column of the ampacity chart. If you run THHN conductors in conduit, then you can size from the 75 degree column. If you want to stick with flexible cable and not conduit, you could try using 6 gauge SER cable, but you'd need to make sure to use copper conductor SER -- aluminum is more common.

        An example of the ampacity charts is here:

        These charts are from the National Electrical Code book.

        Your link requires a login so I can't access it.


        1. fromPok | | #6

          Bill . Thank you. as always.. your insights are super helpful. Here is the PDF from the link to avoid login. I am using 6 ton YT-072

          1. Expert Member
            BILL WICHERS | | #7

            You could probably get buy running that on a 50A circuit (I'm assuming one of the single phase options here), but it really wants a 60A circuit if it's going to be running for longer periods of time and on voltage code 10 or 11. With voltage code 00 or 01 your're fine with a 50A circuit.

            You're not supposed to exceed 80% of the rating of the breaker with a load that may run continuously for periods of 3 hours or more. That means a 50A circuit is good for 40A max of continuously operated load.


          2. Patrick_OSullivan | | #8

            > but it really wants a 60A circuit if it's going to be running for longer periods of time and on voltage code 10 or 11.

            Let's just take voltage code 10 since the minimum circuit ampacity is at 50A even. In that case, isn't 50A including an upsizing for the duty cycle since the FLA (fully loaded amps) is listed at 42.6A? "Bigger is always better" aside... there's no code reason to go beyond 50A if I am reading this correctly.

            Honestly I'm more curious about what needs a 6 ton heat pump more than anything else.

          3. fromPok | | #9

            Bill - Thank you.

            Can I use this for 60 AMP breaker?

            Also for the 100 AMP sub panel.

            In general SER cable seems cheaper than NM-B or THW type cable.

            Thanks again.

          4. Expert Member
            BILL WICHERS | | #10

            Reply to post #8:

            If you class this as a "continuous load", then a 50A circuit is limited to 40A, due to the "80% rule" that I mentioned earlier. This doesn't seem to already be factored into the numbers listed in the table for voltage codes 11 and below. This would mean that if the full load amps (FLA) is over 40A, then you'd need to go up to a 60A circuit, which is allowable based on the maximum breaker ampacities listed in the table. If the voltage code FLA listing is 40A or below, then you're OK with a 50A circuit, which is the case for voltage codes 00 and 01.

            HVAC loads are usually considered to be continuous loads. I would especially class heat pumps as continuous loads, because they tend to run longer operating cycles compared to "regular" A/C units. We all know heat pumps don't like to cycle, they like to maintain steady state temperatures.

            Note that since all the voltage codes are dual listed for both 208V and 230V operation, all of the single phase options are likely to actually use under 40A when operating on the higher 230V supply, which would be the case with standard 240V residential electrical service. If this is the case (which can't be known for certain with the information in the table), then a 50A circuit would be good for any of those single phase options.


          5. Patrick_OSullivan | | #11

            Bill (#10),

            My point overall was it was my understanding that when determining breaker size, if the manufacturer stipulates a minimum circuit ampacity, you size the wire based on that directly, rather than factoring in something like duty cycle.

            Maybe that viewpoint is wrong, but I've certainly seen it held by otherwise reasonable/informed folks. Curious for your thoughts.

  2. walta100 | | #2

    One of the biggest variables one needs to consider is how long will this wire be. If the panels are side by side, you can you a smaller wire than if the second panel is 600 feet away in your barn.

    Also, you can use a smaller copper wire than if you chouse aluminum.


    1. Expert Member
      BILL WICHERS | | #3

      True, but you still need to use the mimimum size wire listed for the ampacity of the circuit in the ampacity chart in the code book. You can use as much bigger as you want, especially in the case of volt drop concerns.

      If the run is really long, such as to a pole barn located away from the main house, I'd consider using aluminum cable in underground conduit. This tends to save some money compared to using copper cabling. If the cable run is just within the home, such as from one side of the basement to the other, I would try to stick with copper wire.


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