Sizing of SEC's

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e57

e57

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San Francisco, CA
Got this one today...

"Service needs to be sized for the rating of the equipment - an Inspector told me that..."

My responce. From 02 NEC but 05 is rougly the same…
“230.42 Minimum Size and Rating.
(A) General. The ampacity of the service-entrance conductors before the application of any adjustment or correction factors shall not be less than either (1) or (2). Loads shall be determined in accordance with Article 220. Ampacity shall be determined from 310.15. The maximum allowable current of busways shall be that value for which the busway has been listed or labeled.
(1) The sum of the noncontinuous loads plus 125 percent of continuous loads
(2) The sum of the noncontinuous load plus the continuous load if the service-entrance conductors terminate in an overcurrent device where both the overcurrent device and its assembly are listed for operation at 100 percent of their rating
(B) Specific Installations. In addition to the requirements of 230.42(A), the minimum ampacity for ungrounded conductors for specific installations shall not be less than the rating of the service disconnecting means specified in 230.79(A) through (D).
NEC Commentary on this code
The title and text of 230.42(B) were changed in the 2002 Code for clarity. The basic rule in 230.42(B) requires ungrounded service conductors to be sized large enough to carry the load and to be not smaller than the minimum size of the disconnect as specifically required in 230.79(A) through (D).”

Underlining above is by me, Service Conductor sizes from say a riser need only be sized for the sum of continuous and non-continuous loads alone – i.e. a load calc on the whole building. The underlined part about busways is only for the busway – not the conductors feeding them, and meant only as a note about ampacity – since bus ways are not in 310.15. It is not the intent for the service conductors to be sized for the busway, or buss-bars in the equipment.

SF has no code amendments to this section….
http://www.amlegal.com/nxt/gateway.dll/California/sfbuilding/cityandcountyofsanfranciscobuildingelect?f=templates$fn=default.htm$3.0$vid=amlegal:sf_building

Even then – Dwellings have a number of exceptions to this as well for Service and Feeder size…

So once you have done a load calc, and say you got – 165A the conductors in the riser need only be sized for 165, or higher, whatever the max size for those conductors is – is the size of the service. The next size would be 175A for a residential service that would be 1/0 CU – from Table 310.15(B)(6).
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Anyone see any thing wrong in the above statement?
 
Yeah, kind of confusing......service conductors are sized by the loads, I think we all know that. Are you asking about busway, maybe the whole story would help clarify.
 
This guy thought that if a panel is say rated for say 400a - the riser feeding it also would need to be rated for 400a - reguardless of load.

e.g. 3 meter/main with load at 250A - says because the meter and its busses are rated for 400a the rise needs to be as well.
 
Service only needs to be sized to the load, not the equipment rating. What about allowance for future loads? Sometimes people look at the equip rating and assume they have that much capacity.
 
jbt260 said:
Service only needs to be sized to the load, not the equipment rating. .
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If there is a single service disconnect the conductors serving it must be rated at least as high as the OCPD.

If there are multiple service disconnects the conductors serving them only have to be rated for the calculated load.
 
e57 said:
But is that the right code? I can't think off the top of my head any others....
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What code section did the inspector cite?

Looks to me like you have cited the correct section ('Minimum Size and Rating').

If the inspector is requiring you to size the conductors according to the maximum rating of the equipment then that's a new one to me. For example, if I purchase your basic 125 amp loadcenter and install a 100 amp main; does this mean I must automatically size the conductors @ 125 amps (the rating of the loadcenter) instead of 100 amps (the rating of the OCPD)?

Nope.

*edit*

But you said this is a duplex, so you probably have two mains. In that case you need to size by the calculated load as already mentioned in post 7.
 
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This is what a PM said after I pulled him aside after he described this to a customer who was questioning why he needed a 400a service....

"Service needs to be sized for the rating of the equipment - an Inspector told me that..."
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So I told him that I was 100% positive that was not the case - that it only need be sized for the load alone - not for the equipment....

But now not sure if that is the right code that I sited...

Get it????

(Think some of you know that I have a tendancy to put my foot in my mouth)
 
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I already assumed from previous posts you knew that but went on a rant anyway. I think you cited the the right section.
 
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