230.80 combined rating of disconnects

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anbm

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Does anyone know the reason behind this 230.80 rule? (i.e. the combined rating of breakers in main switcboard SHALL NOT BE LESS THAN a single main service disconnect to protect this switchboard). Thanks!
 
230.80 is not talking about protecting a switchboard, a panel, or anything else. It is simply adding a constraint to the rule given in the paragraph above. 230.79 tells us that various types of facilities must be served by a minimum amount of power. For example, a single family dwelling unit must have at least 100 amps provided to it. If you do a load calculation and conclude that the total load is only 55 amps, you still must provide a 100 amp service, at the minimum.

Continuing this example, what 230.80 is telling us is that if we were to chose to use a set of up to six breakers as the disconnecting means for that single family dwelling unit, then the ratings of those six breakers must add up to at least 100 amps. In other words, you must give the building at least 100 amps, whether your service disconnect is a single breaker or a set of six breakers.

Does that make sense?
 
Let take an example, I have a service entrance main panel with 1,000A main breaker, downtream of this main breaker we have following breakers(A,B,C,D,E):

A. 100A
B. 200A
C. 300A
D. 400A
E. 500A

According to the rule, total downstream breakers size is (100+200+300+400+500) = 1,500A which is greater than 1,000A (main board breaker size) and this is allowed by code.

However, the real load draws by each above breaker as following:

A. 60A
B. 155A
C. 210A
D. 290A
E. 380A

The total Amp we will see at main panel bus is:
(60+155+210+290+380) = 1,095A

So, the question is the downstream breakers are still fine with above load, but will the main breaker (1000A) in the board be tripped cause the load see at its main bus is larger than breaker size.
 
anbm said:
Let take an example, I have a service entrance main panel with 1,000A main breaker. . . .
Click to expand...
At this point, I would say that you are not discussing article 230.80. So may I ask what article you are really talking about?

anbm said:
According to the rule, total downstream breakers size is (100 + 200 + 300 + 400 + 500) = 1,500A which is greater than 1,000A (main board breaker size) and this is allowed by code.
Click to expand...
Once again, what rule are you talking about (it?s not 230.80)?
anbm said:
So, the question is the downstream breakers are still fine with above load, but will the main breaker (1000A) in the board be tripped cause the load see at its main bus is larger than breaker size.
Click to expand...
I suspect that a load of 1095 amps would never trip a 1000 amp breaker. It would be wrong of us to design or install a configuration in which that circumstance would arise. But that was not your question.
 
anbm said:
Let take an example, I have a service entrance main panel with 1,000A main breaker, downtream of this main breaker we have following breakers(A,B,C,D,E):
Click to expand...


The rule does not apply to your situation, you have a single main breaker that exceeds the rating required by 230.79

230.80 applies to service equipped with 2 to 6 service disconnecting means. There is nothing in article 230 that applies to breakers supplying feeders.
 
charlie b said:
For example, a single family dwelling unit must have at least 100 amps provided to it.
Click to expand...

I have to disagree here, 230.79 requires nothing more then a 100 amp disconnecting means, that 100 amp disconnecting means may have a 20 amp over current device associated with it if the load calculations support that.
 
You are right, I think I refer to a wrong article. Anyway, what your opinion in my example above? I saw a lot of these similar designs, and in one of actual projects the main breaker was tripped but non branch breakers are tripped.
 
iwire said:
I have to disagree here, 230.79 requires nothing more then a 100 amp disconnecting means, that 100 amp disconnecting means may have a 20 amp over current device associated with it if the load calculations support that.
Click to expand...
I have to agree with your disagreeing with me. ;)

Mea culpa.
 
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