3000A bus too small

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hhsting

Senior Member
Please see sketch below. I have main service switchboard incoming 8 sets of 750 kcmil Al. There are six main service breakers all 800A.

All horizontal and vertical buses are 3000A. The total calculated demand load comes out to 2950A at the moment but can increase in future.

However, the total six 800A comes out to 800x6= 4800A. How does this protect the 3000A bus? Is their any code regarding this in NEC 2017?

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Hv&Lv

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How does your 200 amp main breaker in your house protect the bus or the wire between the utility transformer and the breaker?

I think every post that starts out with “Please see sketch below” should be treated like a homework question from any other student…
 

roger

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How does your 200 amp main breaker in your house protect the bus or the wire between the utility transformer and the breaker?

I think every post that starts out with “Please see sketch below” should be treated like a homework question from any other student…
I think that's an excellent idea. The OP needs to tell us what he thinks and why.

Roger
 

hhsting

Senior Member
How does your 200 amp main breaker in your house protect the bus or the wire between the utility transformer and the breaker?

I think every post that starts out with “Please see sketch below” should be treated like a homework question from any other student…

. If i knew that why post this question. How?
 

roger

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Its not protected in my opinion.
Based on what? See HV&LV's example. If you have a 200 amp 40 circuit panel loaded with 20 amp breakers what's the difference in your example besides size.

Roger
 

infinity

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Look at the calculated load first. Is the bus and wire size large enough? If so then that's it. Future loads are not your concern.
 

hhsting

Senior Member
How does your 200 amp main breaker in your house protect the bus or the wire between the utility transformer and the breaker?

I think every post that starts out with “Please see sketch below” should be treated like a homework question from any other student…

Hold on 200A breaker would protect the 3/0awg copper cable line side between utility and main service Panelboard

In my case you have 3080A 8 sets 750kcmil AL service conductors into main service switchboard with total breaker size of 4800A trip. How does that protect the bus or the cable?
 

roger

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Hold on 200A breaker would protect the 3/0awg copper cable line side between utility and main service Panelboard
The POCO could feed it with a #4 if they chose to. The designer knows what he's doing.

Roger
 

hhsting

Senior Member
The POCO could feed it with a #4 if they chose to. The designer knows what he's doing.

Roger

That is not in my case. In my case 3000A bus is internal to switchboard its not poco provided??

How exactly is all this protected? 4800A trip main service breakers total with 3000A switchboard
 

infinity

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Hold on 200A breaker would protect the 3/0awg copper cable line side between utility and main service Panelboard

In my case you have 3080A 8 sets 750kcmil AL service conductors into main service switchboard with total breaker size of 4800A trip. How does that protect the bus or the cable?
Did you see post #6? Adding the OCPD sizes is not going to tell you anything other than that you know how to add.
 

hhsting

Senior Member
Did you see post #6? Adding the OCPD sizes is not going to tell you anything other than that you know how to add.

Yes switchboard total demand load is less than 3000A.

So your saying I can even have 1000A six breakers in that switchboard with 3000A bus as long as the total load on that switchboard is less than 3000A?
 

roger

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Yes switchboard total demand load is less than 3000A.

So your saying I can even have 1000A six breakers in that switchboard with 3000A bus as long as the total load on that switchboard is less than 3000A?
Yes, glad this got figured out.

Roger
 

infinity

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Location
New Jersey
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Journeyman Electrician
Yes switchboard total demand load is less than 3000A.

So your saying I can even have 1000A six breakers in that switchboard with 3000A bus as long as the total load on that switchboard is less than 3000A?
Yes that's correct when you have more than one service disconnect. So as I said before look at the calculated load and then ensure the wire and bus is large enough for that number.

6-800 amps switches does not mean that you have 4800 amps of load, you've already stated that the load is actually 2950 amps.
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
Yes switchboard total demand load is less than 3000A.

So your saying I can even have 1000A six breakers in that switchboard with 3000A bus as long as the total load on that switchboard is less than 3000A?
Exactly. Just as Post #6 states.
See 230.71 and 408.36 Exception
 
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