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    400 Amp Service

    I need to wire a service at a large home being renovated. Plan calls for a 400 amp service. I get confused by 400 amp services because it seems to be the only one where we call a service by the rating but then we typically refer to it as 320 continuous. When I have installed 400/320s in the past it was because a 200 was too small and a 400/320 was way more than needed but it worked well utilizing two 200 amp main panels coming off of one meter socket.

    I'm trying to nail down all the loads in order to do a service size load calculation. Many things are up in the air. They will have a couple of tesla car chargers and maybe a large amount of electric radiant heat, maybe even some snow melt. Nailing all these things down may take a while but in the mean time I want to be ready with the theory. So in order to get a better understanding of things I have some questions as follows:

    Is a 100 amp service really an 80 amp continuous?
    Is a 200 amp service really a 160 continuous?
    Is a 400 amp service really a 320 continuous? Why is it only the 400/320 that seems to point this out on the meter socket spec.?

    #2
    Originally posted by mkgrady View Post
    I need to wire a service at a large home being renovated. Plan calls for a 400 amp service. I get confused by 400 amp services because it seems to be the only one where we call a service by the rating but then we typically refer to it as 320 continuous. When I have installed 400/320s in the past it was because a 200 was too small and a 400/320 was way more than needed but it worked well utilizing two 200 amp main panels coming off of one meter socket.

    I'm trying to nail down all the loads in order to do a service size load calculation. Many things are up in the air. They will have a couple of tesla car chargers and maybe a large amount of electric radiant heat, maybe even some snow melt. Nailing all these things down may take a while but in the mean time I want to be ready with the theory. So in order to get a better understanding of things I have some questions as follows:

    Is a 100 amp service really an 80 amp continuous?
    Is a 200 amp service really a 160 continuous?
    Is a 400 amp service really a 320 continuous? Why is it only the 400/320 that seems to point this out on the meter socket spec.?
    Yep, you’ve got it. Because it’s typically figured as calculated continuous load times 120% which takes you to the total size your seeing.
    Don’t confuse with a true 400 amp service which would require CT metering as plug in meters stop at 320A continuous.

    Comment


      #3
      Originally posted by mopowr steve View Post

      Yep, you’ve got it. Because it’s typically figured as calculated continuous load times 120% which takes you to the total size your seeing.
      Don’t confuse with a true 400 amp service which would require CT metering as plug in meters stop at 320A continuous.
      Thanks. Without knowing the actual load would one assume a specified 400 amp service would normally be a 320 continuous? Bear in mind that this is for a residence. I have never seen a residence with a service that was large enough to need a CT cabinet. Not sure they have room for that anyway.

      Comment


        #4
        Your guess is as good as mine.

        Comment


          #5
          A few more comments. First lets just back up and recall that the continuous load derating applies to conductors (service, feeder and branch) and OCPD's. Things like transformers and panelboards can be used at 100% of their rating (although you have to get creative to feed them). Meter sockets dont have a continuous derating in the NEC, but there must be something in the product standards as class 320 sockets are always clearly labeled 320C/400NC. If you look closely at 200 amp sockets, you will see they most (all?) of them also have a continuous and non-continuous rating, 200C/240NC, it just doesnt seem to be universally marked liked class 320's.

          In regards to actual service size, note that considering the next size up rule, and 230.90(A) exception #3, many services are not actually 200,400, 600,800, etc.

          I would say that when a service size is stated, the "non-continuous size" is given. In fact C/NC shouldnt even be a qualifier since the C/NC is part of the load calc process. You just size a service to the load calc and thats it. A class 320 service would be a 400 amp service not a 320 (assuming 400 amp of conductors). I dont know why everyone wants to cal lit a 320. No one calls a 200 amp service with 3/0 copper or 250 AL a 160 amp service because that is the max continuous load you could apply.

          Ethan Brush - East West Electric. NY, WA. MA

          "You can't generalize"

          Comment


            #6
            Originally posted by electrofelon View Post
            A few more comments. First lets just back up and recall that the continuous load derating applies to conductors (service, feeder and branch) and OCPD's. Things like transformers and panelboards can be used at 100% of their rating (although you have to get creative to feed them). Meter sockets dont have a continuous derating in the NEC, but there must be something in the product standards as class 320 sockets are always clearly labeled 320C/400NC. If you look closely at 200 amp sockets, you will see they most (all?) of them also have a continuous and non-continuous rating, 200C/240NC, it just doesnt seem to be universally marked liked class 320's.

            In regards to actual service size, note that considering the next size up rule, and 230.90(A) exception #3, many services are not actually 200,400, 600,800, etc.

            I would say that when a service size is stated, the "non-continuous size" is given. In fact C/NC shouldnt even be a qualifier since the C/NC is part of the load calc process. You just size a service to the load calc and thats it. A class 320 service would be a 400 amp service not a 320 (assuming 400 amp of conductors). I dont know why everyone wants to cal lit a 320. No one calls a 200 amp service with 3/0 copper or 250 AL a 160 amp service because that is the max continuous load you could apply.
            Thanks EF. That makes a lot of sense.

            Comment


              #7
              Originally posted by mkgrady View Post
              Is a 100 amp service really an 80 amp continuous?
              Is a 200 amp service really a 160 continuous?
              Is a 400 amp service really a 320 continuous?
              Yes,yes, and yes.

              Why is it only the 400/320 that seems to point this out on the meter socket spec.?
              Probably because that's the only configuration that typically supplies two panels (i.e., main breakers) instead of one.
              Master Electrician
              Electrical Contractor
              Richmond, VA

              Comment


                #8
                Originally posted by electrofelon View Post
                If you look closely at 200 amp sockets, you will see they most (all?) of them also have a continuous and non-continuous rating, 200C/240NC, it just doesnt seem to be universally marked liked class 320's.
                Wouldn't it be 200C/250NC?

                Cheers, Wayne
                Last edited by wwhitney; 10-11-19, 12:00 AM.

                Comment


                  #9
                  Originally posted by wwhitney View Post
                  Wouldn't it be 200C/250NC?

                  Cheers, Wayne
                  Yes I cant do math apparently.

                  I found this:

                  Single-position meter sockets have an AC voltage rating of Current Ratings 300 or 600 volts per Underwriter’s Laboratory (UL) specification 414. UL 414 also identifies two operational current ratings for single postion meter sockets, a continuous current rating and a maximum current rating. Both ratings are in amperes, amps for short. As the name implies, the continuous current rating is the amount of current that the meter socket can handle continuously without damage.The maximum current ratings is the maximum current the socket can handle without damage for a short time. UL 414 allows the maximum current rating for a meter socket designed for a single-phase, three-wire service to be no greater than 125% of the continuous current rating.

                  But note, the way I read that it doesnt have to be capable of 125% of the continuous rating, just no greater. On the web I have seen some that say 240C, not sure if that is correct or just a typo.
                  Ethan Brush - East West Electric. NY, WA. MA

                  "You can't generalize"

                  Comment

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