Big Residential Services and Table310.15(B)(6)

[dnem]

david

I am suprised to hear you say you NEVER get load calculations on residential, what they do without a permit is there problem, how do you enforce art 220?
load calcs. must be done at plan review to verify the service is adequate for the load, (new or existing). requiring load calcs. covers me as an inspector.
and if they make major changes to a house that increases the load, I require a new load calc, updated plans and permit. I have had architects design additions that are bigger than the house and have no clue what the existing service size or load is, IMO it is a must

The entire time I've worked as an inspector, I have yet to see someone try to put in a service of less than 150amps on even the smallest house. I've seen a 100amp fuse or damaged panel be replaced by a 100amp breaker panel, but that's it. Maybe it's just the common practice in this area of the country, but attempts at small services are just unheard of. Maybe this is the reason that we never look at calcs.

Most houses I inspect have 200amp services and a few have 400amp. Over 400amp is very very rare but I see a few. Under 150s are never seen at all.

Since the services we see are installed according to Table310.15(B)(6) and the actual load when every item in the whole house is on is about one quarter of the main breaker amperage, I can't see what we would accomplish by looking at residential calcs.

David

 

[dnem]

Note 3 is the older version of 310.15(B)(6). In the older editions all of the rules in 310.15(B) were in the form of notes to Table 310.16.
Don

Thanks Don
I was on a whole other wave length and not even thinking about past codes.

 

[dnem]

David if the calculated load is less than 560 amps you are all set as 230.90(A)Exception No. 3 allows the ampacity of the OCPs to be greater than the rating of the conductors as long as the calculated load is not exceeded.

I've been thinking about the 230.90(A)x3 tie-in today and I've concluded that it's not as big of a problem as I originally envisioned.

Since 230.90(A)x3 is an exception to 230.90(A), it's only modifying the rule in 230.90(A) which requires the OC to have a "setting not higher than the allowable ampacity of the conductor". So x3 allows multiple mains to have a total combined ampacity that can be "higher than the allowable ampacity of the conductor". But this doesn't mean that there is no limit to how much higher their combined ampacity can be greater than the conductor ampacity.

If there was no limit, then someone could try to install single [non-paralleled] 1/0 Al service entrance conductors, with a 120amp calculated load, and install three 200amp main breaker panels in the basement supplied by those 1/0 conductors. 230.90(A)x3 would prevent 230.90(A) from stopping that installation.

But 230.90(A)x3 is not an exception to 240.4(B) which when applied along with 240.6(A) only allows you to go as low as 501amp conductors that are supplying 600amps of breakers. The service entrance conductors for the three 200amp panels would have to have an ampacity of 501amps required for the 600amps of breakers.

240.4(B) would require 501amp wire for three 200amp main panels.
230.90(A) would require 600amp wire for one 600amp main panel.

Does anybody disagree with these statements ?
Since I haven't explored the interaction between these specific code references before, I'm not completely confident in my answer. But I think I've got it right.

David

 

[iwire]

If there was no limit, then someone could try to install single [non-paralleled] 1/0 Al service entrance conductors, with a 120amp calculated load, and install three 200amp main breaker panels in the basement supplied by those 1/0 conductors. 230.90(A)x3 would prevent 230.90(A) from stopping that installation.

David IMO that would be a code compliant installation.

There is 'no limit' other than expense of the equipment.

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.

I want to add I will have this running in my mind all day.

I believe the intent was no limit but your 240 references have me thinking.

 

[George Stolz]

240.4(B) would require 501amp wire for three 200amp main panels.

I agree.

230.90(A) would require 600amp wire for one 600amp main panel.

I disagree. Exception #2 makes no bones about a single panel.

 

[Pierre C Belarge]

David

"If there was no limit, then someone could try to install single [non-paralleled] 1/0 Al service entrance conductors, with a 120amp calculated load, and install three 200amp main breaker panels in the basement supplied by those 1/0 conductors. 230.90(A)x3 would prevent 230.90(A) from stopping that installation."


I agree that the installation could not be limited by 230.90(A) as long as the calculations are correct.
Remember code making panels mostly think of electrical work being performed by people who know what they are doing.

I think that Ex #2 & Ex #3 are separate and should be treated separately... which means that 240.4(B) does not apply to exception # 3.

 

[dnem]

230.90(A) would require 600amp wire for one 600amp main panel.

I disagree. Exception #2 makes no bones about a single panel.

Good catch !
I missed it because ..... uum ..... it was right in front of my face

 

[dnem]

David

"If there was no limit, then someone could try to install single [non-paralleled] 1/0 Al service entrance conductors, with a 120amp calculated load, and install three 200amp main breaker panels in the basement supplied by those 1/0 conductors. 230.90(A)x3 would prevent 230.90(A) from stopping that installation."


I agree that the installation could not be limited by 230.90(A) as long as the calculations are correct.
Remember code making panels mostly think of electrical work being performed by people who know what they are doing.

I think that Ex #2 & Ex #3 are separate and should be treated separately... which means that 240.4(B) does not apply to exception # 3.

I think the best way to say it is the other way around.
230.90(A)x3 does not apply to 240.4(B)

 

[Pierre C Belarge]

That works for me.

 

[iwire]

I don't think 240 enters into this discussion.

If it does the exception [230.90(A) EX. 3] is worthless in all cases.

I am busy right now gotta run on a service call but....

I'll be back... in my best Arnold S. voice.

For now check 240.3

 

[dnem]

I don't think 240 enters into this discussion.

If it does the exception [230.90(A) EX. 3] is worthless in all cases.

I am busy right now gotta run on a service call but....

I'll be back... in my best Arnold S. voice.

For now check 240.3

I don't see any of the 240.90(A) exceptions as worthless.

If 230.90(A)x3 is worthless for multiple mains, then 230.90(A)x2 is worthless for single mains. But neither of them are worthless, they allow wire ampacity to be less than main breaker ampacity as long as you stay within the limits of 240.4(B) & 240.6(A).

David