230.90(A), Exception 3

[charlie b]

Can I have a service load calculated value of 2,500 amps, a set of service conductors rated for 3000 amps, a service switchboard rated for 3000 amps, and a service disconnecting means that comprises a set of 6 circuit breakers that add up to 4,225 amps?

I am reviewing a coworker's design that has this situation. I was about to throw a flag on the play, but I wanted to check the actual code language first. I saw nothing in 408 that explicitly deals with this question. There is Table 450.3(A) and its Note 2. But the utility owns the transformer, and we do not yet know what KVA rating it will have. Then I read 230.90(A), and thought I had the basis for criticizing the design. However, Exception 3 has me confused. It looks a little bit like the rule about using the next higher standard breaker rating, so long as you are under 800 amps. But this Exception does not put an 800 amp limit into play.

Can anyone clear this up?

 

[electrofelon]

Can I have a service load calculated value of 2,500 amps, a set of service conductors rated for 3000 amps, a service switchboard rated for 3000 amps, and a service disconnecting means that comprises a set of 6 circuit breakers that add up to 4,225 amps?

I am reviewing a coworker's design that has this situation. I was about to throw a flag on the play, but I wanted to check the actual code language first. I saw nothing in 408 that explicitly deals with this question. There is Table 450.3(A) and its Note 2. But the utility owns the transformer, and we do not yet know what KVA rating it will have. Then I read 230.90(A), and thought I had the basis for criticizing the design. However, Exception 3 has me confused. It looks a little bit like the rule about using the next higher standard breaker rating, so long as you are under 800 amps. But this Exception does not put an 800 amp limit into play.

Can anyone clear this up?

I believe this is fine. I have done this frequently. Note there is an exception for panelboard protection for this situation (408.36 E#1). Also note switchboards have no protection requirement like panelboards do.

So to summarize, when using the 2-6 rule, the service conductor AND panelboard protection requirements are amended so the sum of the OCPDs can exceed these ratings. Note in the case of a SDS using the 2-6 rule, transformer protection must be EQUAL TO OR LESS than the sum of the OCPDs. It is interesting that it is more restrictive for SDS's.

 

[Dennis Alwon]

I don't see an issue with the install as long as the load is under 3000 amps. This is always an issue because most people add things and never do a service calculation. Also there is no way to shut down the panel so the buss bar is not energized.

Exception No. 3: Two to six circuit breakers or sets of fuses shall be
permitted as the overcurrent device to provide the overload protection.
The sum of the ratings of the circuit breakers or fuses shall be permitted
to exceed the ampacity of the service conductors, provided the calculated
load does not exceed the ampacity of the service conductors.

 

[charlie b]

My concern is that the service calculation process allows us to size a service based on the expectation that not everything connected will be operated at the same time. That is all well and good most of the time. But there are days (the fourth Thursday in November comes to mind) when all residents of an apartment building will be cooking and cleaning most of the day. So there can be periods in which the actual load exceeds the calculated load. In the example I describe above, there is nothing that would protect the 3000 amp bus within the switchboard against a total load of 4000 amps. So perhaps instead of being able to throw a "code violation flag," I will have to settle for a "poor design choice flag."

 

[electrofelon]

Well we all know how conservative the nec load calcs are. I think you are overthinking this a bit. This isnt some loophole, it is used in practically every situation where 2-6 switches are used (in my experience). What kind of occupancy is this?

 

[charlie b]

Mixed use (mostly apartments and some office space), a renovation of a very old building.

 

[George Stolz]

So perhaps instead of being able to throw a "code violation flag," I will have to settle for a "poor design choice flag."

Agreed. I wouldn't worry about Thanksgiving, even in the unlikely event that everyone snubs Grandma's and cooks in, the elements are all cycling anyway...

 

[electrofelon]

Mixed use (mostly apartments and some office space), a renovation of a very old building.

I have found the rule of thumb that NEC load calcs are 2 to 2.5 times over actual use to be spot on every time I have looked at utility demand data. This always seems to have been for commercial/light commercial though. Has anyone compared apartment buildings actual vs NEC? I have always wondered if they come out to be less conservative. I do know that I am usually surprised how small the services are in such buildings.

 

[don_resqcapt19]

Well we all know how conservative the nec load calcs are. ...

I was just looking at the power service to a 5 story 50' x 150', office building with high leg delta service. The utility transformers are a 25kVA and a 100kVA. I bet the load calcs show a lot more than that, especially since the high leg is only used for the air conditioning equipment. The lighting load alone would be 131 kVA based on Article 220.