Sizing overhead service entrance conductors

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Bantamben

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What is the proper way to size the the service entrance conductors for an overhead riser in a residential single family dwelling unit. I tried the search and I'm sure this has been covered but wasn't able to find a clear answer. I've only done a few panel upgrades and they have all been replacing existing 200 amp services, so I wasn't required to upgrade the wireing. There might be an opportunity though to upgrade a 200 amp service on a large home we just did some work at in the future to a 400 amp service. It has a overhead feed. I feel guilty that I've really never owned this calculation and gone through the code on it. I did see in one table minimum size 400kcmil, is that without doing a load calculation? It looks like there is two options? I believe the customer will want not the minimum but the space to be able to load the panel up more in the future.
Thanks in advance.
 
Although thru experience and repetitiveness we often build a 200 amp service here and a 400 there, the correct way is to first calculate your load. Choose the conductor from 310.16 {'08} or 310.15(B)(6) (if applicable) and protect the conductor accordingly.
Keep in mind, a service conductor terminated in a single overcurrent device must be sized in relation to that device. A conductor feeding two or more devices can be sized per calculated load.
 
Yeah that's kinda what I was thinking instead of just saying yeah its a 200 amp it needs 3/O or its s 400 it needs 500kcmil. I know this particular house seems to be under 200 amps load calc by just quickly going through the sq ft and calculations. But I really wanted to know the options. Like is there a basic calculation without a load calculation but will require larger conductors. And then if I went through all the load calculations I could have the option of sizing wires smaller and installing smaller OCPD. Seems most 400 amp panels have 2-200 mains sized for them.
 
So if I'm protecting the 2- 200 amp mains in a 400 if I was to go that size, then the calculated value would always be under the requirement for the 400 amps total OCPD. And like I mentioned customer would probably not use buy would want the ability to use that 400 amps in the future or at least know he got what he paid for.
 
Not sure how you all install 400 amp services. Around here we either use a 400 amp meter main- It has the meter and 2-200amp breakers in the panel that can be used to feed other panels. Some of these units also have extra circuits available in the panel area.

The other method is to use a 400 amp meter with 2-200 amp panels with feed thru lugs. Using this method I would install 3/0 to each panel. The riser could be based on 310.15(B)(7)-- 400kcm copper for 400 amps or you could parallel 3/0 copper.

If the customer is not using 200 amps at this time I would not think that there would be an issue using (B)(7) as that is based on the diversity of the load. I have done many 400 amp services and have yet to see one get anywhere close to 300 amps.
 
Yeah I think I would get the 400 with 2-200 breakers all in one with 30/40 etc.. When you size this if you are only going to use 1 set of feeders since its overhead what process do you go through.
 
Bantamben said:
Yeah I think I would get the 400 with 2-200 breakers all in one with 30/40 etc.. When you size this if you are only going to use 1 set of feeders since its overhead what process do you go through.
Click to expand...

What do you mean you would size it as if there were only one feeder? You would need to size it for the 400 amps IMO
 
I'm just saying sizing it for one main feeder not parralell a as the previous poster mentioned. I'm guessing I'd just size for the 400 so if I went by the table it would be 400 kcmil correct?
 
If you want to take time to provide the AHJ with a load calculations, the fact that you are feeding more than one service disconnect, would allow you to possibly reduce the service conductors to the line side of the meter. Perhaps less than allowed by 310.15(B)(6) but for most it's not worth the effort (to prove smaller conductors could be used)
 
Dennis Alwon said:
Not sure how you all install 400 amp services. Around here we either use a 400 amp meter main- It has the meter and 2-200amp breakers in the panel that can be used to feed other panels. Some of these units also have extra circuits available in the panel area. 310.15(b)(7) IS permitted for these feeders?

The other method is to use a 400 amp meter with 2-200 amp panels with feed thru lugs. Using this method I would install 3/0 to each panel. The riser could be based on 310.15(B)(7)-- 400kcm copper for 400 amps or you could parallel 3/0 copper. 310.15(b)(7) is NOT permitted for the feeder conductors?
Click to expand...

My questions in red.
 
assuming you are referring to 310.15(B)(6) in the '08 Code....
(if not, my mistake)
310.15(B)(6) can only be used for service conductors and feeders that supply ALL loads that are associated with the dwelling unit.
If you have a single set of service conductors as a riser to the meter they carry ALL loads so (B)(6) can be used.
If the loads are split between two feeders, neither carry ALL the load associated with the dwelling and (B)(6) can't be used.
 
So it looks like from the table if I'm feeding a 400 amp meter all in one I go by the OCPD. So if I wanted this to be able to have 2 -200 amp breakers I size it at 400 amps. So I can use 400mcm. Is this correct and using that table all 3 conductors must be same size it looks like?
 
augie47 said:
assuming you are referring to 310.15(B)(6) in the '08 Code....
(if not, my mistake)
310.15(B)(6) can only be used for service conductors and feeders that supply ALL loads that are associated with the dwelling unit.
If you have a single set of service conductors as a riser to the meter they carry ALL loads so (B)(6) can be used.
If the loads are split between two feeders, neither carry ALL the load associated with the dwelling and (B)(6) can't be used.
Click to expand...

so is the drawing correct? Sorry, how do I get the larger image attached?
 

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Edward if you have a multi family apt and the feeder to each apt serves the entire load of that unit that 310.15(B)(7) can be used. The riser may be sized based on the calculated load.

Gus-- I didn't think you could use the calculated load for the riser if there is one unit
 
Dennis Alwon said:
Edward if you have a multi family apt and the feeder to each apt serves the entire load of that unit that 310.15(B)(7) can be used. The riser may be sized based on the calculated load.

Gus-- I didn't think you could use the calculated load for the riser if there is one unit
Click to expand...

OK.

So on the bottom of the drawing 310.15(b)(7) does not apply?
 
Dennis Alwon said:
.......................................................

Gus-- I didn't think you could use the calculated load for the riser if there is one unit
Click to expand...
I agree. I didn't mean to infer otherwise...
sometimes my mind does not know what my fingers type.
 
I am afraid to post on this topic as everyone will realize just how dumb I am but here goes....

First a common denominator here is for a 200 amp panel use 3/0 and for 400 use 400 mcm. Since the OP says it is a residential single service why not 2/0 and for the 200 amp according to 310.15(b)(6) 2008 edition.

Second for 400 residential services I use the 320m meter fittings. I would think it would be much less expensive.

If I am all wet on this I would like to know it. Everyone can go ahead and laugh now.

Bob
 
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