With the change to section 338.10 (B) (4) in the 2008 NEC,
1) Is a 4/0 Aluminum SER sill allowed to serve an interior MLO panel installed in a one or two family dwelling that is protected by an exterior 200 amp main breaker.
2) Are two 4/0 Aluminum SER’s allowed to server two MOL interior panels that are protected by two 200 amp main breakers. (400 amp service)
3) Are two 2/0 Aluminum SER’s allowed to server two MOL interior panels that are protected by two 150 amp main breakers. (300 amp service)

I feel #1 would still be permitted due to the following,
1)From NEC 2008 ROP 7-88 Log #2639 NEC-P07 (338.10(B)(4)(a)Substantiation: When Type SE conductors are used for interior wiring, as a replacement for Type NM cable, the ampacity of the conductors should be the same as permitted for NM cable since the insulations used are the same both NM and SE conductors.
This SER cable is not being installed as a replacement for type NM cable in this application.

2) 310.15 (B) (6) would still apply being the more specific requirement for this installation. “For individual dwelling units of one-family, two-family, and multifamily dwellings, 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 each dwelling unit and are installed in raceway or cable with or without an equipment grounding conductor. For application of this section, the main power feeder shall be the feeder between the main disconnect and the panelboard that supplies, either by branch circuits or by feeders, or both, all loads that are part or associated with the dwelling unit.” SER cable installed under this provision would be the “main power feeder”, not an interior feeder or branch circuit that 338.10(B)(4) is referring to.

I am not sure about #2 and #3 but I feel this installation is not allowed due to the following,
1) Each SER cable is not carrying the entire load associated with the dwelling unit. From the 2008 NEC Handbook “Section 310.15(B) (6) permits the main feeder to a dwelling unit to be sized according to the conductor sizes in Table 310.15(B) (6). For the 2008 Code, the panel clarified that this permission to use this table applies only to conductors carrying 100 percent of the dwelling unit's diversified load.” So since each SER is only carrying around 50% of the dwellings load, 310.15 (B) (6) and the Table 310.15 (B) (6) would not be allowed to be used to size each SER cable.

I’m curious how other inspectors are looking at these changes.

Neal Burdick
City of Tampa
Construction Inspector

Hi Neal,

#1 most jurisdictions permit that. I would allow that, but if long run through an attic in Florida I may question the installation, possibly tag it.

#2,3 most jurisdictions do not permit that, for the reasons you point out and it is not a single set of conductors.

Somebody please jump in if I am wrong.

I vote with Nick on 1, 2 & 3 and also agree derating may be necessary in the attic.
For the 310.15(B)(6) issue, the problem is, you may not have diversified loads on each side of the split service.

See the thing with 310.15(b)(6) is that this table is not a new ampacity for those conductors, and does not apply to parallel applications. The conductor ampacity is still the same as 310.16. That is part of the reason why we don't need to do a temperature correction that was talked about in a different post. 310.15(b)(6) is a comparison between a conductor and it's use with an overcurrent protection size, service, or feeder/panelboard ampacity size.

As far as your question with 2 & 3 I also agree with Nick.

Good point Mike. There is no ampacity adjustment cited for 310.15(B)(6). I stand corrected.

I too agree with both Nick and Mike's comments.

I reall don't see too many designers / contractors trying to "take advantaqe" of ampacity rules, rather they simply don't apply them correctly.

The real problem is the lack of instant impact of not complying with the ampacity rules. Unlike most the rules found in the NEC, improper application of ampacities is a long term event. That is, the overload of an insulated conductor may not result in a failure for a really, really, long time, if ever.

That is a good point Bryan. Another issue is growth. An installation that seemed perfectly safe when the computed load was safely under 100a (150 or whatever) can easily get to the point that the original computed load is approached or exceeded on a regular basis as the homeowner adds new equipment. If that feeder was sized to 310.15(B)(6) it will be running hot. I find it interesting that this is a fairly new idea. 310.15(B)(6) popped up in the 93 code. I am looking to see if I still have that ROP somewhere spinning on a disk drive.

From what I understand, the Edison Electrical Institute submitted the proposal which created Table 310.15(B)(6). It appears they studied real load demands on single family dwellings and determined the values in Table 310.16 were too stringent.

Here's some history on the "main power feeder" aspect of the section and table:

1987 – Note 3 to 310-16 stated, “…feeder that carries the total current supplied by the service.

1990 – 89’ ROP proposal to drop above sentence. Submitter indicated that even if some loads are supplied by other than the feeder, the diversity of loads will still be present for the applied demand. CMP approved and dropped the above sentence.

1993 – CMP reversed itself and restored the 87’ wording.

1996 – CMP panel submitted their own proposal and added terms “main power feeder”. They again commented that some loads could be supplied by other than the “main power feeder” as long as diversity is still present in the feeder for the applied demand.

1999 – CMP moves the Note 3 to full text, and added a definition of “main power feeder”. The intent of the 1996 was still intact.

2002 – A proposal was submitted to quantify how much diversity would be needed to apply the demand to the “main power feeder.” The submitter used a value of 40% of total line-neutral load to define the “main power feeder”. The submitter used an example of a 400A service with 2-200A mains. One could use the demand the other could not based on the proposed 40% rule. The CMP commented that neither could use the demand based one the original intent that the “main power feeder” carries all of the diversified loads.

2005 – No significant change to section

Again the key is the diversity of loads. How many times an electrician when asked to provide a load calculation for service size states: "I put my amprobe on and I only have 82 amps on a 200 amp service".

In addition we can also take in to consideration that most conductor ampacity is actually based on the 90 deg. column because of its insulation. Although we can't use that to size conductor ampacity because of equipment ratings, and have to use the 60 or 75 deg. column, the conductor ampacity is actually that.

That is really no solace if the conductor does handle the full 90c ampacity but the lug on the main breaker burns up.

I am only bringing up the "increasing load" issue because people do continue to add loads until the main breaker trips.