Table 310.15(B)(6)

[iwire]

I would like to talk about Table 310.15(B)(6).

My question is this.

Does Table 310.15(B)(6) change the ampacity rating of the conductors or does it simply change the required OCP?

Table 310.15(B)(6) shows 400 amp OCP on 400 Kcmil copper.

Table 310.16 rates 400 kcmil 335 amps @ 75 C.

When we apply 240.4(B) to a circuit we can move up to the next standard size breaker but we can not load the cable past the 310.16 rating.

If I use 400 kcmil copper per Table 310.15(B)(6) can I load it to 400 amps?

If yes, why is that?

If yes IMO that undermines the importance of Table 310.16 and the all the derating and adjustment factors the go with it.

In my mind it can lead to this sort of statement out in the field.

"Heck the NEC overloads the conductors why shouldn't I?"

Any opinions are welcome, I can not use that table on the jobs I do, maybe I am just jealous. :roll:

Thanks in advance, Bob

[ February 08, 2005, 08:23 AM: Message edited by: iwire ]

 

[wirenut1980]

Re: Table 310.15(B)(6)

I have wondered the same thing, and I guess I don't like using table 310.15(B)(6) either. IMO, according to that table, you can use 400 KCMIL copper with a 400 Amp OCPD as long as the Article 220 calculation is below the ampacity of the conductors, so less than 335 Amps at 75 deg C. But what happens if loads are added later that pushes the load above 335 Amps, but below 400 Amps? Overlaod?

 

[iwire]

Re: Table 310.15(B)(6)

Thanks for the reply wirenut.

Originally posted by wirenut1980:
. IMO, according to that table, you can use 400 KCMIL copper with a 400 Amp OCPD as long as the Article 220 calculation is below the ampacity of the conductors,

That is what I am unsure of, does the calculated load have to be less than the Table 310.15(B)(6) info or less than that of 310.16.

But what happens if loads are added later that pushes the load above 335 Amps, but below 400 Amps? Overlaod?

This is less of an issue for me as 230.90(A) Exception 3 allows the same possibility.

230.90(A) Ungrounded Conductor. Such protection shall be provided by an overcurrent device in series with each ungrounded service conductor that has a rating or setting not higher than the allowable ampacity of the conductor. A set of fuses shall be considered all the fuses required to protect all the ungrounded conductors of a circuit. Single-pole circuit breakers, grouped in accordance with 230.71(B), shall be considered as one protective device.

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.

When exception 3 is used any future additions could be a problem.

It seems that the NEC is counting on qualified people working on the system.

 

[bphgravity]

Re: Table 310.15(B)(6)

I have long been questioning this section as well.

I think it indicates a few things. One, dwelling calculations must be too conservative. This is also proved by permitting optional calculations that reduce the required service to dwellings even greater. Two, studies must have shown that loading of dwellings is different than other types of occupancies. There are probably only a handfull of days in a year that the loading of dwelling will be near capacity. Christmas, Thanksgiving, and maybe even Super Bowl day.

I think your interpretation is correct. You can arbitrarily decide to install a 400A service to a dwelling and you are only required to use 400 kcmil conductors. I'm not exactly sure how long this section has been in the NEC, but I am certain there has not been any case studies that have determined a problem with it.

Does it undermind 310.16 and the derating rules? Well, maybe. I think in most cases, derating is ignored for typical service installations and more frenquently used on brnach circuits and some feeders.

 

[cpal]

Re: Table 310.15(B)(6)

310.15 (B) (6) is a limited permissive rule (dwellings only).
It does not apply to network derived dwelling services (it's limited to single phase 240 Volt )

Generally the lion share of service entrance conductors are exterior to a building with limited lengths interior. There have been many discussions regarding the location of the service equipment.

Conductors have a very high withstand rating (5 seconds for every 42.25 Cir Mil at 75 deg C)when compared to the ampacities recognized in Table 310.16.A 14 AWG Copper is rated for about 105A for 5 Seconds.

The Code traditionally is conservative in it's application of conductor sizing and overcurrent protetction.The NEC is a minimum standard and not a design manual. 310.15 Allows ampacities other than those listed in 310.16 UNDER ENGINEERING SUPERVISION.The Neher Mc Grath Controversy in 84 to some degree bares this out.


Last the Feeder and Service Demands applied by Article 220 are also extremely conservative and such installations that comply with the sizing requirements of Art. 220 and Conductor capacity of 310.15 (B) (6) do not indicate a history of failure.

Charlie

 

[wirenut1980]

Re: Table 310.15(B)(6)

That is what I am unsure of, does the calculated load have to be less than the Table 310.15(B)(6) info or less than that of 310.16.

IMO, table 310.15(B)(6).

240.4 refers me to 310.15, which says use 310.16 through 310.19 and tables 310.20 through 310.23 (for 0-2000 V) except as modified by 310.15(B) (1) through (6). So to me, it seems that table 310.15(B)(6) is an exception to table 310.16 for residential only. It makes sense to me to apply it like I was derating for number of conductors or ambient temperature (except it increases the allowable ampacity of course). just my interpretation

[ February 08, 2005, 10:36 AM: Message edited by: wirenut1980 ]

 

[BAHTAH]

Re: Table 310.15(B)(6)

310-16(b)(6) as mentioned is a permissive rule and only applies to the main power feeder to a dwelling. For a single dwelling this would apply to only that portion of the feeder that carries the total load for the dwelling such as between the main disconnect and a sub panel. For a multi-family dwelling where the service voltage is 120/240 1ph3w (table cannot be used on 208/120 1ph3w)the table can be used for each feeder going to a individual dwelling unit. I see no deration required for a single feeder since the neutral on a 120/240 1ph3w system is not counted as current carrying unlike the neutral on a 208/120 1ph3w system and you never have four current carrying conductors in either case. As far as the load I would say you do your load calculation and are then allowed to select the conductor from the table because of the load diversity that has been established in dwellings. I am seeing more and more homes with services over the 400amp rating at which point the table would not apply. I think the reason allowing the use of the table for the main feeders only assures that the total load diversity of the dwelling will be relected in the feeder. Just my opinion.

 

[charlie]

Re: Table 310.15(B)(6)

Table 310.15(B)(6) is indeed an ampacity table under the parameters given. You may load the conductors up to their maximum ampacity listed with impunity. It doesn't matter whether you use the "standard" or "optional" calculations and size the service a tight as you wish. Even if the service entrance conductors are overloaded for a period of time, the residential load profile is such that a high amount of overload will not be sustained that could damage the conductors.

This table is the result of a study done by a group with the cooperation of EEI, which was a part of that study. It was derived from actual demand figures from electric utilities across the country and the reason 120/208 V, 1? was not included is the dearth of services of that voltage configuration at the time.

For what it is worth, most residential services are oversized to impress the customer and/or to have increased circuit capacity. It is seldom that a home with heat other than electric needs more than a 100 ampere service or a total electric home needs more than a 200 ampere service. It is typical for most new homes around here to have a 200 ampere service because the additional cost is marginal. Also, most larger homes will have 2 - 200 ampere services (400 total) so they can have 60 to 80 circuits.