Table 310.15(B)(6)?

[Smart $]

When you use just two hots and one neutral from a wye system the neutral carries more than the imbalance due to the phases being 120 degrees apart.

Actually it is still the imbalance, but because of the 120 degree voltage shift and only two of three "hots", the line currents never cancel to zero current.

 

[kwired]

That is the reason (per 2014 ROPs); there are three CCCs, not two. If this makes sense to you would you mind explaining this to the rest of us for which this makes no sense?

No, what I mean is that three CCCs require 0% consideration for 99.9% of the code, but somehow 310.15(B)(7) will catch a building on fire with more than two.

Actually it is still the imbalance, but because of the 120 degree voltage shift and only two of three "hots", the line currents never cancel to zero current.

I don't know how the table ever came up with the values that are in it. My understanding is it is likely been determined that a single dwelling has enough load diversity that someone decided the supply conductors can be lowered in ampacity. You can ask why the difference between 2 vs 3 current carrying conductors. Well what about the difference between 4 and 5? I see this as just a common application that some study must have went into, maybe with whatever changes are coming up it has been studied again and they come up with different diversity on average today than they did however long ago this was first put in there. Just some speculation, I have no idea for certain.

 

[George Stolz]

When you use just two hots and one neutral from a wye system the neutral carries more than the imbalance due to the phases being 120 degrees apart.

I didn't notice the "more than" in your post. Pains me to say it but Smart's right, provided the loads are linear. Harmonics are about all that can drive the neutral load beyond the phase conductors supplying it.

 

[mark32]

But this rule only applies to 120/240 volt three wire feeds. OP has 120/208 volts. This means 310.16 applies. 2005 did not have any rules stating to use type SE cable at 60 deg ratings. 2008, required to treat it like NM cable for interior installations effectively making it a 60 degree cable, 2011, now only says you need to treat it as 60 degree cable where installed in thermal insulation.

Oh wow, it's so astute of the inspector to catch that, now I'm impressed. The only hope the OP has now is if the cable is not embedded in thermal insulation, thus allowing him to use the 75c deg ampacity rating of the cable. He's under the 2011 nec right?

 

[Smart $]

Oh wow, it's so astute of the inspector to catch that, now I'm impressed. The only hope the OP has now is if the cable is not embedded in thermal insulation, thus allowing him to use the 75c deg ampacity rating of the cable. He's under the 2011 nec right?

No, he stated earlier the installation is under 2008 NEC... that's why I suggested he request a deviation for compliance with 2011 NEC.

 

[jflynn]

Hold on, I don't see what the issue is. Smarter people than I have already made their comments which makes me think I'm wrong but hear me out. According to 310.15(b)(6) one can use aluminum #2 for a 100a install in this multi-family dwelling. The article even states type SE so what's this guy's problem with it being fused at 100amps? (I'm taking this from the '05 code)

We have a very similar project-incoming service is 120/208 3ph 4w.Panel schedule for the dwelling unit(s) says 120/240v 1ph 4w,it also calls out for the panel feeder to #2 ser al,breaker @ the meterbank is called out to be a 2p 100amp.This is an engineered project, plans and specs-but there is also the "classic note" on the plans that says all work done per NEC.The engineer has done his homework and list(s) all the load calc;s lowest being 78amps -highest being 87.These are 1,2,and 3 bedroom apartment in a multi family building.

 

[kwired]

We have a very similar project-incoming service is 120/208 3ph 4w.Panel schedule for the dwelling unit(s) says 120/240v 1ph 4w,it also calls out for the panel feeder to #2 ser al,breaker @ the meterbank is called out to be a 2p 100amp.This is an engineered project, plans and specs-but there is also the "classic note" on the plans that says all work done per NEC.The engineer has done his homework and list(s) all the load calc;s lowest being 78amps -highest being 87.These are 1,2,and 3 bedroom apartment in a multi family building.

To be code compliant you either will need 90 amp feeder breakers or 1 AWG conductors.

If 120/208 is incoming then his panel schedules are wrong if they say 120/240.

 

[augie47]

To be code compliant you either will need 90 amp feeder breakers or 1 AWG conductors.

If 120/208 is incoming then his panel schedules are wrong if they say 120/240.

Or, depending on the install and the local AHJ's call on 338.10, a 75 amp breaker (as discussed ad-infinitum)

 

[kwired]

Or, depending on the install and the local AHJ's call on 338.10, a 75 amp breaker (as discussed ad-infinitum)

His highest calculated load was 87 amps.

 

[augie47]

His highest calculated load was 87 amps.

I understand, but going back to the 1st post of the thread, the OP had encountered an inspector who enforces the wording of 338.10 over 310.15(B)(6).
That interpretation has been discussed at length here and there remain two camps of thinking. I was simply trying to caution jflynn (post 26) that he could possibly encounter an AHJ who enforced the 60 deg "rule" and limits his OCP to 75 amps.

 

[dana1028]

It appears you have it right to me.

I don't know why the first inspector is quoting you Article 334 (NM) if you are use SE Cable (Article 338.)

His supervisor who said that Table 310.15(B)(6) only applies to single phase feeders on a 208/120V, 3ph service has it backwards. T310.15(B)(6) applies ONLY to 120/240V services, NOT to single phase feeders from a 208/120V, 3ph service.

In addition, SE is one of the conductor types specifically listed in Table 310.15(B)(16).

The inspector should have been more descriptive in his 'Article 334' requirement.........this has nothing to do with ampacity, thermal insulation, etc.

338.10(B)(4) - says SE installed indoor must comply with Art. 334 [excluding 334.80 - ampacity].

Now look at the code section the inspector mentioned: 334.12 - wrong code section.....prob. meant 334.10 - SE [like NM] is limited to Types III, IV, & V construction...the OP mentioned 5 story construction....Types III, IV & V are only allowed to go 3 stories....4 stories if fully sprinklers....with the 5th story it means the first floor [and underground parking] are Type I or II construction........NM [and thus SE] is not permitted in Type I construction.

The OP does not indicate where he is....but with this structure already to the 5th floor, chances are he is on the 2008 NEC [the code applied for inspection is based on when the plans were submitted].

Even IF he were on the 2011 - SE must be installed in raceways approved for Type I construction.....very unlikely this was done...i.e. they probably pulled the SE cable out of a Type I construction vault.

This is all supposition....but this is a very common screw up in 5-story condo's [vault in Type I construction].

 

[dana1028]

Two additional comments from my post.

Typically - the plans will show MC cable as the wiring method and the electrical contractor installs SE to save money.

- I have to wander - who was the inspector that missed/ignored the SE cable being installed through 4 stories of construction before finally making such a call?

This becomes a coin toss - should you let it go because the inspector missed it? or make the call - every jurisdiction that I have worked in would let it go because they [the jurisdiction] had erred in not catching the wrong wiring method till so much of the project was already completed.

 

[vanvan]

Hello all- so our final resolution is as smart$ suggested. Req a deviation to '11. This def makes all parties at ease (especially us!). It def raised a lot of ?'s for us for future installs. As dana1028 mentioned - we were actually designed for MC and switched to SE once we left the type I construction and entered type V. (Our building is 5 wood framed stories over 3 poured- 1 level parking below grade, retail at grade, mixed use / apts L2, then the wood framed apts on up). Once structure topped out and sealed, we worked our way down from L7. It was at L3, last of wood framed that inspector memtioned problem with feeders.
Thanks again to all for taking time to look at our issue and offer suggestions and insight. By far my favorite electrical resource.