Did I size this right?

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billdozier

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Hey guys boss asked me to size wire and pipe for a 225a mb singlephase subpanel. Just want to make sure I did this correctly as I havent done much of this. Started at 310.15 ampacity table. Found xhhw wire which would be 200mcm alum I then went to index where you have conduit fill Found the schd 80pvc calc called for 4 200s in a 2 and 1/2 pipe. Then went to 250 equipment grounding table and ordered a #4 al ground if I remember correctly. We chose not to downsize the neutral. Could we have downsized the neutral? what code reference for the correct size neutral if downsized? Thanks for your help
 
I just want to clarify my terminoligy real quick A panel any panel that is fed after the service is a subpanel is this correct. The reason why I ask is the panel in question is fed from the service and then feeds two seperate additions that are divided by a swimming pool. One side of the additions is freestanding while the other side connects to the origional house. So there are no branch circuits in these panels only feeds out of it to the indoor panels in the additions
 
Sorry, but no, you didn't size it right.

Exactly which table are you using? Table 310.15(B)(6) is not an option, as 480 Sparky just said. Table 310.16 does not contain any 200 MCM conductors, so you can't be talking about that one either. Also, in case you didn't already take this into account, you can't use the 90C ampacity column.

I think you need a 300 MCM aluminum conductor, to get an ampacity of at least 225 amps. You can get four of those in a 2.5 inch schedule 80 PVC, but getting an EGC in as well might be a problem.

Speaking of the EGC, you need a #2 AL, not just a #4 AL.

As to the question of reducing the size of the neutral, that would require you to calculate the neutral load.
 
225a resi service would likely invoke the need for 400a frame size service equipment. Mainly the service disconnect and meter socket.
 
billdozier said:
A panel . . . that is fed after the service is a subpanel. Is this correct?
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The term "subpanel" is not an official NEC term. But your understanding is correct.

It does not matter whether the panel you are talking about has branch circuits, or just feeders to other panels. If you plan to use all of its 225 amp rating, then you have to size its feeder (from the service) to handle all of the 225 amps. If you wish to use a smaller feeder, and if you will protect the smaller feeder with a breaker (in the service panel) that is smaller than 225 amps, then you need to prove (by calculation) that the load on your new panel is lower than the value you wish to use.
 
billdozier said:
Hey guys boss asked me to size wire and pipe for a 225a mb singlephase subpanel.
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There's two ways to approach the situation. The first is to do a load calculation for the subpanel and size the feeder conductors to handle the load. In this case, the feeder may have an ampacity not more than the panel rating (which isn't necessarily the size of the main breaker). However, the OCPD at the supply end of the feeder will have to be sized to protect the feeder. If the feeder ampacity is less than the main breaker in the subpanel, the breaker serves more so as a disconnect than an OCPD.

The second way is to assume the subpanel is desired to have the full 225A available at the subpanel.

In either case, you can choose to use 90?C-rated conductors, but their "applied" ampacity cannot be greater than the 75?C-rated counterpart, because of termination temperature requirements.
 
unfortunately I left my code book on the work truck Duoh! So I cannot verify exactly what size al wire I suggested. I know the chart I used is right around 310-15/16 area may have been 16. It discusses all wires has 3 charts and reverts you back to the a smaller chart for downsizing wires. This is why I need two code books
 
Ok just googled nec 310-16 this is the formula we used. Now following what youre saying smart I would have to use the 70deg column according to this column I would be required to run 300mcm Is this correct? Thanks for your help guys
 
billdozier said:
Ok just googled nec 310-16 this is the formula we used. Now following what youre saying smart I would have to use the 70deg column according to this column I would be required to run 300mcm Is this correct? Thanks for your help guys
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If you want the panel to have 225A available, yes... basically.

However, it's a little more complicated than that, if you are going to use the word "required". The NEC only "requires" the feeder ampacity to equal or exceed the the load(s) it serves. For example, your panel has a 225A main breaker but may only have loads which need 120A of "continuous" line current. You would only be required to have a feeder ampacity of 120A ? 125% or 150A.
 
Smart and 480 as well as charlie thanks for all of your help I think I see the error of my thinking now. Am I safe to assume I should almost always use the 70deg table when sizing with with a mindset of reverting back to the ambient temp rules?
 
Hey smart im with you as far as sizing these loads I need to find my old apprentiship books and revisit sizing loads on a feeder. Because we probaly just fought 3in conduit and ordered big wire when it wasnt needed However I was giving oh 3 min to get the boss an answer So guess what ill be doing this weekend.
 
billdozier said:
Am I safe to assume I should almost always use the 70deg table when sizing with with a mindset of reverting back to the ambient temp rules?
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I cannot in good conscience answer your question with a simple yes. There are several requirements in this regard, and none leave room for assumptions.
 
Smart $ said:
I cannot in good conscience answer your question with a simple yes. There are several requirements in this regard, and none leave room for assumptions.
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Yep. That's a question with a gray area answer. Notice all the gray areas in the '08?
giggle.gif
 
billdozier said:
I just want to clarify my terminoligy real quick A panel any panel that is fed after the service is a subpanel is this correct. The reason why I ask is the panel in question is fed from the service and then feeds two seperate additions that are divided by a swimming pool. One side of the additions is freestanding while the other side connects to the origional house. So there are no branch circuits in these panels only feeds out of it to the indoor panels in the additions[/quote
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Bill,
What are you calling the service? If you are calling the meterbase(with no oc protection) a service your first panel is not a subpanel.

Then went to 250 equipment grounding table and ordered a #4 al ground if I remember correctly.
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If this is the first panel after the meter you woul not pull a egc.
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