320A Service

[Mr. Spinosa]

I am putting together an estimate for a house that is going to have a 320A service. - It's been a looooooonggg time since I have done one of these. I am using 310.15(B)(7) of the 2014 NEC and to me it looks like I need to use 400 kcmil aluminum or 300 kcmil copper. (I plan on downsizing the grounded conductor.) I took 83% of the service rating and used that as my guide, however I am not completely sure this is correct. Anyone care to enlighten me some more on this? I hope I have included enough information with my question. Thanks in advance.

 

[luckylerado]

I am accustomed to seeing 2 200amp breakers in the meter enclosure for a 320A residential service drop. The feeders would need to be sized for that. Each to their own panel board. Technically neither of those feeders are

"... supplying the entire load associated with an individual dwelling unit in a two family or multifamily dwelling..."

as it states in 310.15(B)(7) 1 and 2

In 2011 Table 310.15(B)(7) would have said 2/0 Cu 4/0 Al

 

[Dennis Alwon]

I agree with lucky. The cost of a 400 amp panel is extraordinary compared to the cost of 2 -- 200 amp panels. Each of these panels would need to be sized to 310.15(B)(16) not 310.15(B)(7) as neither one carries the entire load of the dwelling.

The 320 map service is really a 400 amp services as the meter is rated 320 amps continuous but 400 amps non-continuous. We call this a 400 amp service especially because we use 2- 200 amp panels and the meter is rated for that use

 

[Greg Swartz]

Just a question before the question

Just a question before the question

I understand and agree with everything Lucky and Dennis stated.

Quite often, on a project of this nature, an overhead riser would be required, and the electrician would be required to install the conduit and feeder to a weather head, in which various local utilities / POCOs will then tie into that point.
The wire on the line side of the meter (for the utility / POCO tie in) would then need to be rated at 83% of the full 400A (which is 332A, if we use the 2 - 200A panel option, which as stated is very common) which will need to be 400KCmil CU.

All of this, so that the utility / POCO can hook up to your 400 CU with a 2/0 (or less) AL.

 

[augie47]

I agree with the others but if your install is in Washington (based on your profile) you might wait for one of the guys familiar with WA Code. Sometimes it's a bit "different".

 

[kwired]

Do you have one service disconnect or more then one? More then one leaves you with a lot of options in some cases.

320 amp meter socket only suggests you likely have more then a 200 amp main, but doesn't limit you to any one specific setting, and you can in some cases have multiple mains that the sum of the ratings could exceed 400 amps and it would still be code compliant as long as the load calculations don't exceed the common supply conductor ampacity or the rating of the 320 meter socket.

If you only have a single main then you are permitted to use reduced conductor sizes allowed in 310.15(B)(7), but again the 320 socket only makes us assume you need more then 200 amps but not necessarily have to have 400 amps overcurrent device and conductors either, though it is common to set up 400 amps capacity when you have a 320 meter socket even if you don't really need all 400 amps of capacity.

One example - you could supply three 200 amp main breaker panels from a 320 meter socket - each main would require a 200 amp conductor to supply it. But say your total calculated load is only 375 amps (less then 320 is continuous), you could connect these to the 320 meter socket with only 375 amps worth of conductor on the meter supply side, and that conductor can be reduced to the 83% permitted in 310.15(B)(7).


Confused yet?