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

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dnem

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Table310.15(B)(6) lists residential service sizes up to 400amps. If you're installing a residential service that's larger than 400amps, can you use Table310.15(B)(6) or are you forced over to the lower amperage allowances of Table310.16 ?

I was inspecting a service trench that had parallel 350 Al conductors in para 3" PVCs. Services over 400amps require CTs, so the electricians plan was to bring the para 350s into the CT cabinet and then use the multiple load lugs to come out of the cabinet 3 times to 3 different 200amp main panels in the basement.

If you take the ampacity for the Al 350s from Table310.15(B)(6), you get 300x2, which gives you the needed 600amps for the three 200amp mains. But if you're forced to use Table310.16, Al 350s can be used for 250amps on 75?C CT lugs [250x2=500amps] or 280amps on 90?C CT lugs [280x2=560amps], which falls short of the needed 600amps for the three 200amp mains.

So what do you think ? I know what my conclusion was but I'd like to find out how others see it.

David
 
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David IMO Table 310.15(B)(6) can not be used for parallel conductors.

You must use the table as shown or move to 310.16.

Above 400 amps we have no choice but to use 310.16.

Admittedly not all agree with my view on this.
 
dnem said:
But if you're forced to use Table310.16, Al 350s can be used for 250amps on 75?C CT lugs [250x2=500amps] or 280amps on 90?C CT lugs [280x2=560amps], which falls short of the needed 600amps for the three 200amp mains.
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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.
 
iwire said:
David IMO Table 310.15(B)(6) can not be used for parallel conductors.

You must use the table as shown or move to 310.16.

Above 400 amps we have no choice but to use 310.16.

Admittedly not all agree with my view on this.
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But some do. :)

Above 400A, it's not a house any more, it's a (expletive) hotel, I don't care who lives there. They can use 310.16.

IMO. :)
 
Bob,
David IMO Table 310.15(B)(6) can not be used for parallel conductors.
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CMP 6 does not agree with your opinion. Proposal 6-74 in the 95ROP was to prohibit the use of the reduced wire sizes in parallel. Panel 6 rejected the Proposal with this statement: "Conductors 1/0 and larger are permitted to be paralleled by section 310-4. This would apply to Note 3."
Don
 
I agree with Bob. You'll need to use 310.16. But the service lateral is only required to be sized according to the connected load not the sum of the 3-200 panels.
 
iwire said:
David IMO Table 310.15(B)(6) can not be used for parallel conductors.

You must use the table as shown or move to 310.16.

Above 400 amps we have no choice but to use 310.16.

Admittedly not all agree with my view on this.
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I can understand where you're coming up with that conclusion.

The last sentence of 310.15(B)(2)(a) says, "Each current-carrying conductor of a paralleled set of conductors shall be counted as a current-carrying conductor.", which would result in a count of 6 for the para 350s. The heading for Table310.15(B)(6) uses the words "3-Wire".

I just don't see that conclusion as an obvious call. It's possible that the term "3-Wire" in the Table310.15(B)(6) heading is there only to specify a single split phase system and rule out using the Table for a 3 phase install.

When there's doubt about the meaning of a code phrase, I try to give the benefit to the contractor unless I see an actual hazard resulting from using the more liberal interpretation.

David
 
iwire said:
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.
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I certainly see 230.90(A)x3 applying to commercial permits and projects but not residential.

We NEVER get calculated loads submitted for plan review for residential. There's never any electrical of any type at all on residential plans. Homeowners are constantly changing things "on the fly" and sometimes we get called back for a rerough just to check changes that come after the rough has been approved.

Commercial jobs get inspected by us according to the approved plans but residential is strictly by NEC.

This house with the para 350s, has plans for one outbuilding, so far, being feed from a house main and various undetermined extensive patio and outdoor pool equipment. They couldn't give me a load calc that would stand longer than a couple of days.

Homeowners are notorious for doing things like, finishing their basement without a permit. And how many times do they do a new calc, and then replace service conductors if necessary, when the finishing the basement "under the radar" ? Even if they tried to submit a calc on a residential project, I don't think the residential plan reviewer would accept it. The only electrical item that he asks for and considers is voltage drop calcs for long service entrance laterals.

David
 
don_resqcapt19 said:
Bob,

CMP 6 does not agree with your opinion. Proposal 6-74 in the 95ROP was to prohibit the use of the reduced wire sizes in parallel. Panel 6 rejected the Proposal with this statement: "Conductors 1/0 and larger are permitted to be paralleled by section 310-4. This would apply to Note 3."
Don
Click to expand...

What's note 3 ?
 
dnem said:
I certainly see 230.90(A)x3 applying to commercial permits and projects but not residential.

We NEVER get calculated loads submitted for plan review for residential.
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David regardless of how you feel about it that exception applies equally to all occupancies. That is not IMO disputable.

If the contractor shows you NEC complaint load calculations that are under 560 amps you will have to accept the parallel 350s with three - 600s unless you have an amendment otherwise.

For what it is worth I am not a fan of that exception either.

It relies on the next electricians to make sure they do not add load beyond the service conductor ratings.

I have seen commercial service with 1200 amp conductors 'protected' by 3000 amps of OCP.
 
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Bob brought up the issue of parallel conductors and Table310.15(B)(6).
I anticipated that issue one also one other issue.

Do you use Table310.15(B)(6) based on the service rating or the conductor size ? Is there a difference of opinion about this ?

If you use it based on service [or feeder] size, than it can't be used for services over 400amps.

If you use it based on conductor size, then you can pick up an ampacity of 300 for each 350 Al.

What do you think ?

David
 
don_resqcapt19 said:
Bob,

CMP 6 does not agree with your opinion. Proposal 6-74 in the 95ROP was to prohibit the use of the reduced wire sizes in parallel. Panel 6 rejected the Proposal with this statement: "Conductors 1/0 and larger are permitted to be paralleled by section 310-4. This would apply to Note 3."
Don
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I know, I have seen you post that before. :) I actually tried to find our last discussions on this but had no luck.

Their statement does not match the wording of the NEC and is not as you know an official interpretation.

part of 310.15(B)(6)
6) 120/240-Volt, 3-Wire, Single-Phase Dwelling Services and Feeders. For dwelling units, conductors, as listed in Table 310.15(B)(6), shall be permitted as 120/240-volt, 3-wire, single-phase service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. ...
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As listed IMO is clear even if that was not the intent.

I do not seen anything in the table listing other combinations.

I also was forthcoming with David that not everyone shares my view.:)
 
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iwire said:
It relies on the next electricians to make sure they do not add load beyond the service conductor ratings.
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Residential has traditionally been the place where new electricians "cut their teeth". How many of them will do a service calc before doing a basement refinish ? And us inspectors never look for a load calc on a basement refinish. We never look at a load calc for any residential job [4 family and up is issued as a commercial permit even if it's strictly dwellings].

If the service conductors are sized according to the main OC, then the main will trip if they overload the house. The lack of a load calc isn't critical.

If someone insisted on submiting a load calc, then we would require extensive electrical plans just like a commercial job and require them to follow the plans just as written and not make any changes. Then we'd probably make a special note of that property that would flag any future work at that house on into the future and subject it to tighter scrutiny.

So I guess it could be done if someone pushed the issue.

David
 
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
 
Something to keep in mind about table 310.15(B)(6).

This is not a table listing the ampacities of the various conductors shown in the table. For that you need to see 310.16 (-.20 as appropriate).

Instead 310.15(B)(6) permits the conductors shown in the list to be used for services of a particular rating.

310.15(B)(6) permits 2/0 Cu to be used as the feeder for a 200A residential service. But the ampacity of 2/0 Cu is still 175A.

I believe that 310.15(B)(6) would be far less confusing if, rather than listing specific conductor sizes, simply stated that a residential service of 200A _rating_ may be served by conductors of 175A _ampacity_. 310.15(B)(6) recognizes that for residential services, the _peak_ power requirements may be far in excess of the median or RMS power requirements. A 200A service will almost never be loaded to the full 200A, and my guess is that it will commonly be loaded at less than 20A. 310.15(B)(6) should be seen as permission to use protection rated at more than the ampacity of the conductors because the characteristics of the _load_ ensure that the conductors won't be overloaded, in a fashion analogous to protection for motors.

Power companies commonly size feeders to residences which are far smaller than the NEC required conductors.

(As an aside, the _average_ household power consumption in the US averages about 1.2kW. (Calculated from data at http://www.eia.doe.gov/emeu/reps/enduse/er01_us.html ) That's right. 5A. )

(This leads to a further aside: I what sort of safety, reliability, and cost benefits/issues would be found if we used 'overload heaters' as found in motor starters for service protection. I'd bet that normal '200A' services would do just fine with 125A service conductors and heaters set for 100A full load current. Not NEC compliant at all, but something to explore as a way to save copper a couple of code revisions hence.)

-Jon
 
Winnie I am sure you are correct that OLs would work for overloads but what about short circuit and ground fault protection?

FWIW In the past some of the utility guys have mentioned that they assume about 50% of the NECs service calc.

Take that along with free air / higher temp conductors and they quickly end up with much smaller conductors than the NEC requires.
 
iwire said:
Winnie I am sure you are correct that OLs would work for overloads but what about short circuit and ground fault protection?

FWIW In the past some of the utility guys have mentioned that they assume about 50% of the NECs service calc.
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Like I said, just musing on the point. But I'd bet that a 'service entrance protective device' consisting of a 200A circuit breaker and overloads set for 100A, fed with 125A conductors, would be safe and work just fine for a residential service which under present code would require a 200A breaker and conductors sized per 310.15(B)(6)

I agree that a magnetic breaker would still be required for short circuit protection.

-Jon
 
Technically, parallel conductors (all sets of the same phase) are considered 1 conductor 310.4 the last line in the paragraph.

Is a 3 wire 240v - 200 amp service to a dwelling a 3 wire system?
Is a 3 wire 240v - 400 amp parallel service to a dwelling a 3 wire system?

David
You are correct that the table is only good to 400 amps. So this is almost a moot conversation. Once they go larger, Table 310.16 will be the reference.

Remember the "heat of the conductor is relative to the square of the current"
 
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