320/400 amp service

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ofourcobra

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Hey guys I combed through the forums before asking this but still didn't understand . I'm having a problem figuring out what conductor size I need for a service . The equipment will consist of a milbank 320/400 amp meter socket with 2-200 amp CBS with 2-200 amp sub panels not directly behind panel but far enough away to justify the disconnects being outside . My load calc comes out to 290 amps . now I know you would size it to the load but this customer will be doing future upgrades so I would like to give him the most out of the service upgrade . I've never really done one of these just 200 amp under ground and over head . Thanks in advance .


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ofourcobra said:
Hey guys I combed through the forums before asking this but still didn't understand . I'm having a problem figuring out what conductor size I need for a service . The equipment will consist of a milbank 320/400 amp meter socket with 2-200 amp CBS with 2-200 amp sub panels not directly behind panel but far enough away to justify the disconnects being outside . My load calc comes out to 290 amps . now I know you would size it to the load but this customer will be doing future upgrades so I would like to give him the most out of the service upgrade . I've never really done one of these just 200 amp under ground and over head . Thanks in advance .


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You may have an issue here with supplying a structure with 2 feeders. Are the service disconnects on a pedestal remote from the house? Can you describe the meter, service disconnects, and sub panel locations in a little more detail?

Getting past that though just to answer the original question: The key thing to note is that each feeder doesnt not supply the entire dwelling unit so you cannot use the reduced "residential wire sizes". This would mean 250 Al or 3/0 CU, for a full 200 amps. Note however that one can use the next size up rule with smaller wire if the calculated load is less than the ampacity of the wire. Thus, unless this a a very large home, one could still use 4/0 Al for each.
 
ofourcobra said:
Hey guys I combed through the forums before asking this but still didn't understand . I'm having a problem figuring out what conductor size I need for a service . The equipment will consist of a milbank 320/400 amp meter socket with 2-200 amp CBS with 2-200 amp sub panels not directly behind panel but far enough away to justify the disconnects being outside . My load calc comes out to 290 amps . now I know you would size it to the load but this customer will be doing future upgrades so I would like to give him the most out of the service upgrade . I've never really done one of these just 200 amp under ground and over head . Thanks in advance .


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As of right now you don't need more then 290 amps of conductor on the supply side of the meter. Load side you do need 200 amp conductor (or can round up 176A + up to 200 as long as load calc isn't over the conductor ampacity) to/from each individual 200 amp breaker.

1/0 copper or 3/0 aluminum for the common parallel supply would be suitable with 290 amp load calculation.

I believe you could apply 310.15(B)(7) to this common parallel supply because it does feed the entire dwelling load - and it would still have to be 1/0 copper (because you can't parallel anything smaller then 1/0) or 1/0 aluminum (borderline to having to round up to 2/0 aluminum though).

That said, nearly everyone just runs parallel 4/0 aluminum in most cases though. As long as your load calculation isn't over 400, it works with 310.15(B)(7), and 3/0 aluminum isn't always easy to find like 4/0 is anyway.
 
The meter socket it mounted to the house and has 2 200 breakers and the panels are directly behind the meter and next to each other but about 8 feet away , when I spoke with the inspector he said he would prefer I put the disconnects outside as he felt the panels were not close enough to the point of entry to run 4/0 SEU , so I will be running 4/0 SER from the 2-200 amp breakers to the sub panels .


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ofourcobra said:
The meter socket it mounted to the house and has 2 200 breakers and the panels are directly behind the meter and next to each other but about 8 feet away , when I spoke with the inspector he said he would prefer I put the disconnects outside as he felt the panels were not close enough to the point of entry to run 4/0 SEU , so I will be running 4/0 SER from the 2-200 amp breakers to the sub panels .


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As long as the calculated load on each panel is 180A or less, this would work and the inspector is correct about the discos needed.
 
I understand the rationale of the discos but the parallel we are talking about is the wires from the line side of the meter to the utility hand hole correct ?


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I understand the 4/0 SER from the load side of the breakers to the panel my question is for cables from line side meter to utility hand hole , sorry for the confusion


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ofourcobra said:
I understand the rationale of the discos but the parallel we are talking about is the wires from the line side of the meter to the utility hand hole correct ?


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Yes, I messed up.

ofourcobra said:
I understand the 4/0 SER from the load side of the breakers to the panel my question is for cables from line side meter to utility hand hole , sorry for the confusion


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Yeah the service, I was noy paying attention. Two 4/0 SE, no derating. Works.

Handhole? Is this a lateral/underground service?
 
320/400 amp service

Yes underground , it is a newer development with pad mounts , the feed from pad mount transformer goes to a hand hole which is essentially the line of demarcation , so I will be running my conduit to the hand hole and poco is making connections .


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ofourcobra said:
Yes underground , it is a newer development with pad mounts , the feed from pad mount transformer goes to a hand hole which is essentially the line of demarcation , so I will be running my conduit to the hand hole and poco is making connections .


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In that case, assuming 1 conduit, 4 CCC, neutral would not be a CCC, 80% derating needed.

4/0 CU = 230. 230 x 2= 460. 460 x .8 = 368.
 
Load calculation was 290. Because there is multiple service disconnecting means the minimum conductor ampacity on the common conductors ahead of the service disconnecting means only needs to be at least equal to the load calculation, after applying any ampacity adjustments if necessary. The individual conductors to each 200 amp overcurrent device must be sized based on that 200 amp rating, rounding up to next standard size is allowed, as long as load doesn't exceed the conductor ampacity.

Most people will still supply such a situation with parallel 4/0 aluminum, if anything it does allow for minor additions without having to re-do the service conductors.
 
This all depends on what code cycle you are under. The 2017 allows ser to be used at 75C but other cycles have restrictions. If you can use 4/0 ser at 75 C then you are good to go as long as the calculated load is not more than 180 amps in either panel. If possible I would run pvc to the interior panels and then the 75C issue is guaranteed.
 
Dennis Alwon said:
This all depends on what code cycle you are under. The 2017 allows ser to be used at 75C but other cycles have restrictions. If you can use 4/0 ser at 75 C then you are good to go as long as the calculated load is not more than 180 amps in either panel. If possible I would run pvc to the interior panels and then the 75C issue is guaranteed.
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SE cable can't be used for his underground service conductors, which is what he is inquiring about.
 
Dennis Alwon said:
No it cannot, he would need USE but I thought he knew that but used the wrong terms.
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he is running SE cable for his feeders to the panel's, but his inquiry is about sizing the parallel underground service conductors. It did take a few posts to clarify that though.
 
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