Bus Bar Rating

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TheElectrician

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Hey guys,

I have a load center to accommodate 8 x 150A breakers which provides continuous load to the bus and 2 x 15 A breakers which will draw current from the bus. In this case can I just use a 1200A Bus (8 x 150A) or the bus rating calculation should also include the two 15A breakers although they draw current and are not a load on the bus. Does NEC call out for this?

Thanks
 
Tony S said:
Are these busbars 100% loaded? 8x150A=1200A with no diversity. Think about it, is an extra 30A or 2.5% overload going make any difference?
Click to expand...

The 8 breakers are from 8 inverters with 120A max output, and the breakers for each are 150A (120A x 1.25). The other two 15A are drawn by the monitoring system. I thought this 30A is not going to be a load on the bus additional to the 1200A as the 30A is drawn from the bus unlike the other 8 breakers which is the load on the bus. correct me if i'm wrong.
 
If I understand your setup correctly:
The total current is no more than the sum of the inverter outputs connected to the back-fed breakers. A maximum of 30A flows through the two 15A breakers, reducing the current through the main lug outputs by the same amount. If these 15A breakers are just for monitoring devices then I suspect the actual load could be significantly less than 30A.
The only thing is the additional heating on the bus from the contact resistance of the connections of these 15A breakers to the bus. But I don't think this is normally taken into account. For example when a breaker is back-fed to act as the main breaker and feeds all of the current to a MLO panel this current isn't added with the load from the other breakers.
I'm thinking there's really no problem here, and even if there was as Tony said it's a 2.5% one under the worst case scenario.
 
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synchro said:
If I understand your setup correctly:
The total current is no more than the sum of the inverter outputs connected to the back-fed breakers. A maximum of 30A flows through the two 15A breakers, reducing the current through the main lug outputs by the same amount. If these 15A breakers are just for monitoring devices then I suspect the actual load could be significantly less than 30A.
The only thing is the additional heating on the bus from the contact resistance of the connections of these 15A breakers to the bus. But I don't think this is normally taken into account. For example when a breaker is back-fed to act as the main breaker and feeds all of the current to a MLO panel this current isn't added with the load from the other breakers.
I'm thinking there's really no problem here, and even if there was as Tony said it's a 2.5% one under the worst case scenario.
Click to expand...

I understand, Thanks for the explanation. Does it make any difference if it is a MBO Panel with a 1200A Main Breaker?
 
I do not really understand what you mean when you say they the 15A breakers just draw current but do not load the bus. They would actually be a load on the bus, though small and the others would be supplying the bus.
If there is an upstream breaker in the distribution path that protects the bus, there is no limit on the total of breaker ratings, just on the actual connected or calculated load. If this is a service panel, you have too many handle throws to disconnect.
If you are doing a 120% rule calculation and there is any "real" load on the bus you must add the size of the upstream breaker to the sum of the inverter outputs, and that will not comply. And the 150A breakers would have to below the 15A breakers (opposite end of bus from ML terminals)
Your only hope is that your AHJ reasonably agrees not to consider the 15A breakers as loads.

Sent from my XT1585 using Tapatalk
 
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It's unfortunate that your 15A violates 705.12(B)(2)(3)(c). But here's an idea that might save you most of the cost of upgrading to 1600A: use of of those 150A breakers to feed a 200A subpanel, and put the monitoring loads in there.

Feel free to PM me regarding my consulting fee. :lol:
 
GoldDigger said:
I do not really understand what you mean when you say they the 15A breakers just draw current but do not load the bus. They would actually be a load on the bus, though small and the others would be supplying the bus.
If there is an upstream breaker in the distribution path that protects the bus, there is no limit on the total of breaker ratings, just on the actual connected or calculated load. If this is a service panel, you have too many handle throws to disconnect.
If you are doing a 120% rule calculation and there is any "real" load on the bus you must add the size of the upstream breaker to the sum of the inverter outputs, and that will not comply. And the 150A breakers would have to below the 15A breakers (opposite end of bus from ML terminals)
Your only hope is that your AHJ reasonably agrees not to consider the 15A breakers as loads.

Sent from my XT1585 using Tapatalk
Click to expand...

I meant that it will draw from the bus and not supply the bus. But what you say does makes sense, and the initial idea was to connect the 150A breakers the way you recommended at the opposite end of the bus, But was not sure if I have to upgrade to the next size bus.
 
jaggedben said:
It's unfortunate that your 15A violates 705.12(B)(2)(3)(c). But here's an idea that might save you most of the cost of upgrading to 1600A: use of of those 150A breakers to feed a 200A subpanel, and put the monitoring loads in there.

Feel free to PM me regarding my consulting fee. :lol:
Click to expand...

Your idea seems to be smart! :lol:

Consulting fee? LOL (Hope you are not serious :p)
 
jaggedben said:
It's unfortunate that your 15A violates 705.12(B)(2)(3)(c). But here's an idea that might save you most of the cost of upgrading to 1600A: use of of those 150A breakers to feed a 200A subpanel, and put the monitoring loads in there.

Feel free to PM me regarding my consulting fee. :lol:
Click to expand...

Even if I do this, I have to combine them in order for the monitoring system to read all the 8 inverters. Even if I put the current reference on the main bus bar and voltage reference on the 200A sub panel, the 200A subpanel will have 150A from 8th inverter)+ 15A + 15A = 180A, so I cannot use a 150A breaker to feed the 200A subpanel. and ultimately still violate 705.12(B)(2)(3)(c). Correct me if i'm wrong.
 
TheElectrician said:
Even if I do this, I have to combine them in order for the monitoring system to read all the 8 inverters. Even if I put the current reference on the main bus bar and voltage reference on the 200A sub panel, the 200A subpanel will have 150A from 8th inverter)+ 15A + 15A = 180A, so I cannot use a 150A breaker to feed the 200A subpanel. and ultimately still violate 705.12(B)(2)(3)(c). Correct me if i'm wrong.
Click to expand...

I don't know what the constraints are on installing your monitoring system, but the subpanel breaker will not draw 150+15+15. It will either draw max 30 when then inverter is not producing, or output max 150 if the inverter is fully producing and the monitoring gets turned off. The source and load cannot add to each other on the feeder to the sub. That's part of why this works from a code point of view. 705.12(B)(2)(3)(c) looks only at the breaker connecting the 200A sub and it ignores what's in the sub.
 
Jaggedben, I'm thinking there shouldn't be a 200A main breaker in a subpanel that feeds one of the 150A breakers in the 1200A panel, because then the inverter could experience a max load 150+30=180A before it's protected.
But if the subpanel had a 150A main breaker and feed-thru lugs to drive the 150A breaker in the 1200A panel, I think it should work out. A MLO subpanel with a back-fed 150A breaker should also work.
 
But then again, if the actual load from the monitoring circuits is small enough then it could be a negligible additional load beyond the 150A level for the inverter. It all depends on what the inverter's requirements are for overcurrent protection.
 
Yes what I was proposing was one of the 150A breakers in the main panel feeding a 200A MLO sub. (Code doesn't care if the sub has a superfluous main breaker, but that'd be wasting money.)

The monitoring draw is no doubt negligible and ought to be able to be ignored in this case, but unfortunately the code goes by overcurrent device ratings in this case. After all, the rule also covers cases where loads might be much higher.
 
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