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Thread: 400/320 amp service

  1. #1

    Default 400/320 amp service

    Hi everyone and thank you for your valuable time spent on this site
    I have absorbed a lot of knowledge from you.

    I would like to know a little more about 400/320 amp service. I do understand that 320 is the continuous rating or 80% of 400 amp. But as far as feeder wire if I go in the table 310.15(B)(6) would I be looking under 350 amps or 400 amps or would that be related to my calculated load. For the panels I would use 2 - 200 amp MB panels and use 2/0 cu wire from the metar can (320 type) into each panel then I would not be absolutely sure about size of the GEC. Would I run #2 or 1/0 to the one panel and then from that panel to the other panel size #4 according to 250.66. I would really appreciate your help. I talked to some people about this but nobody seem to be absolutelly confident about it.

  2. #2
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    A multi panel service can be confusing. First, service equipment ratings should be for peak loading. A 320/400A meter base should be called a 400A meter base. Any continuous loads calculated during a service calculation will be multiplied by 1.25, so you are calculating a peak load.

    You can use a 320/400 meter base for a service from 201 to 400 amps. Pick the size you want, and install the appropriate conductors. I'd go 400 unless you're way short of needing that, but you don't have to. It does protect the equipment though from people who don't do load calculations. Stupid electricians may just assume its a 400A service even though you may have only wired it for 300A.

    The GEC is the trickiest part. You need to look at all your service wires -- the size feeding the meter, and those parallel 2/0's coming out, and see which is the largest. If you feed the meter with 350 MCM wire or less, then a #2 cu GEC will be fine. If you use 400 MCM, you'll need a 1/0 GEC. Parallel 2/0's calculate out to a #2 cu also. If you go with 400 MCM, make sure you can find a water pipe clamp that will take a 1/0 wire -- that took me a while but I did find one at Home Depot. Last time I looked there, they didn't carry them again. But just about all clamps will take a #10 to a #2.

    All you have to do for your GEC is to run it from one panel to your ground electrode system. If all you have is ground rods, then run a #6 from a panel to the rods. You'll need to bond your water pipes with a #2 or 1/0, and its best to take that from the same panel (but I don't think you have to). NEC 250.142 allows the neutral to be a dual purpose neutral and grounding conductor. Because of this, you don't need to run a second GEC to the other panel, but that is specifically permitted should you wish to do that. You just have to make sure your neutral is sized as large or larger than the 250.66 size.
    Mark
    Kent, WA

  3. #3

    Default 400/320 service

    Thank you very much suemarkp

    This is the first answer that actually makes a lot of sense. So I did calculated the load and came up with about 210 amp.To calculate this I used the optional calculation method. This does not include hot tub or the detached garage that will be build in the future. I do not know if the hot tub should be included into calcs. So I should be allright running 350 mcm cu on my feeder side and then from double lugs on the metter go with 2/0 cu into each 200 amp panel. One thing that bothers me a little is that this meter is about 350 ft from the pole with the transformer. Should I be concerned about voltage drop or not.

  4. #4
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    You'll need to do the hot tub now or later (calculations are different for initial -vs- added loads). But that should add no more than 25 amps to your service requirement (40% of 60 amps). You add the garage too, but if it has no fixed equipment, its load is 0.

    Yes, you need to be concerned about voltage drop. Anything over 150 feet would prompt me to calculate the voltage drop and try to keep it to 3% or less. I'd also consider aluminum wire, since copper seems to be turning into gold and you need a huge amount of wire. If we say your service is 250 amps worst case, and we want the voltage drop less than 7.2V, then ohms law says the wire resistance must be 0.03 ohms or less. This would be the end-to-end resistance or about the equivalent of 700 feet of wire for your install (350' there and 350' back). Here are some DC resistance values from a table in the back fo the NEC for 1000' of wire (multiply by about .7 to get the resistance for 700 feet):

    300 MCM copper = .0446 ohms/1000'
    350 MCM copper = .0367 ohms/1000'
    500 MCM aluminum = .0424 ohms/1000'
    600 MCM aluminum = .0353 ohms/1000'

    I think that 350 MCM copper or 500 MCM aluminum will be fine. 300 MCM copper may even be OK.

    If you plan to limit yourself to around 250 or 300 amps, you could help ensure that by installing a 200A and 100A panel side by side instead of two 200A panels. However, I'd check your meterbase before doing this. Some of the lug types require that both wires in the lug be the same size (I think I found a note on a website for my Milbank meter base indicating this). You may have a difficult time getting a 2/0 copper into a 100A panel lug (check the size ratings of the main breaker lugs of a candidate 100A panel). You could change the meterbase lugs or hope that a 1/0 and 2/0 in the same lug will be OK (and I think most 100A panels will take a 1/0 conductor). There are other solutions too, like using a gutter and split bolting 2/0 and #2 copper taps off of a 300 MCM wire from the meter.
    Mark
    Kent, WA

  5. #5

    Default 400/320 amp service

    Hi suemarkp

    I really appreciate your advice and time spent on this forum.
    I had calculated the voltage drop with 250 amp and 350 ft(x2) of 350 MCM copper and got 6.3 volts, which is about 2.6%. The meter base that I will be using is also Milbank but I had to buy lugs separately and they are double lugs that will accept #6-250mcm wire. One thing that confuses me from your last post is your suggestion of using 200 and a 100 amp panel instead of 2 200 amp. Are you suggesting it because of the cost savings or load reason. Will I be still O.K. with 2-200amp panels and 350 mcm cu at the line side. The reason I would like to keep it this way is that 200 qo panel gives me 40 circuit places.
    P.S. This will be an underground service


    Thanks

  6. #6
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    The reason is to protect your service wires from overload. If you only have one panel, the main breaker must match your service size. When you have two panels, the main disconnects can add up to whatever you want as long as the calculated load doesn't exceed the wire ampacity and the sum of the main breakers is greater than or equal to the calculated load. If you're calculating 250 to 300 amps, you can clue in some future electrician that you don't have a full 200 amps available in your panels.

    This is really just a choice. You know what your service is and won't exceed it. Some future home owner may not, but its their fault (or their lousy electrician's fault) if the burn the house down when they put 400A on your panels. You're also in a good position since your service is so long you will still have 350 amps of capacity, but the voltage drop will be more noticable by then that its probably not too much of a risk to go with dual 200A panels.

    The final thing to consider is you can't always get the full capacity out of your panels if the panels match your service size. I put in a full 400A service with two 200A panels. However, when you calculate the load on each panel, its kind of like starting over with a 200A service. Some of the penalties you get in the full 400A service are applied to each panel separately (like the minimum load before a demand factor is applied). So I'd calculate what you plan to run on each panel and see if it will work on a 100A and a 200A. If not, then you may need two 200A panels. If you really need 80 slots, it would be cheapest to put in two full size panels, but you could also run a subpanel off the 200A panel to get more slots.
    Mark
    Kent, WA

  7. #7

    Default 400/320 amp service

    Thanks again

    I am preety clear as far as what to do now. One thing that we did not cover is if I would want to make this a true 400 amp service I would have to run 400 MCM cu wire and then my two 200 amp breakers would be an adequate protection, or would it be? I know that the cost will be prohibitive but I'd like to know for the future.

  8. #8
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    Yes, you'd need 400MCM copper to have a true 400A service and it would be well protected by the two 200A breakers. But because your service run is so long, it wouldn't be a great 400A service because of the voltage drop with that size wire. But the code places no limits on voltage, so the only issue for 400A service compliance is the rating of the service wires.

    If you jump to a 400 MCM service, then your GEC and water bonds will need to be 1/0 copper too.

    It is usually fairly difficult to draw 400A in a residence. But if you have some rather big loads that could be running on either panel (but rarely both), then your dual 200 panels on a 350 MCM service line make perfect sense. As long as a load calculation is done PER PANEL, I doubt you'll be able to draw more than 350 total amps when both panels are fully operating as the service calculations assume, even if the service calculations indicate each panel is serving 199.9 amps.
    Mark
    Kent, WA

  9. #9

    Default 40/320 amp service

    Thanks suemarkp

    I was away for couple of days and I did not have a chance to thank you right away

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