Still trying to make sense of SER wire size.

[racerdave3]

We have tossed this question around before, and as I read the code book I still have some questions about this. It has previously been discussed that a 2-2-2-4 SER cable must be protected by a 90 amp breaker (the SER cable is aluminum of course), and that table 310.15 (B)(6) only applies to the main feeder into the building (it states that a #2 wire can be protected by a 100 amp breaker). However, when you look at 310.16, both the 75 C column and the 90 C column list aluminum XHHW insulated wire. In the 75 C column it is listed as 90 amps and in the 90 C column it is listed at 100 amps. Now my only thought on this is that we are looking at the 75 C column because that is the rating of the equipment we are using?

 

[stew]

the table says sevices and feeders. they are 2 different animals. one is your service entrance conductor and a feeder is just that, a cable which feeds another panel. thus # 2 can be used at 100 amps and breakered for same when used as a feeder, IMHO

 

[stew]

the table refers to service conductors and feeders. this amperage rating can be used for both. not sure when the forum consensus was that the table is for service entrance only because it clearly says services AND feeders. 100 amps is correct.

 

[suemarkp]

I think its more the other way around. The equipment has terminals for 75C, so you can't normally go over 90 amps. Section 310.15(B)(6) allows you to exceed that for services and main power feeders, and the implicit assumption here is that there is a load diversity present in a service or feeder that supplies an entire dwelling. So if the service calc says 100 amps, you in all likelyhood won't ever reach that, so 90A wire is permitted to be used as if it were 100A.

 

[dnem]

the table refers to service conductors and feeders. this amperage rating can be used for both. not sure when the forum consensus was that the table is for service entrance only because it clearly says services AND feeders. 100 amps is correct.

"not sure when the forum consensus was that the table is for service entrance only"
Is that the forum consensus ?
I guess I've never been on one of those threads. . I would have to dissent if that was the conclusion. . It says feeder and you can use it for a feeder.

I've been in discussions about the supply to a 240v AC unit. . If there's OCPD in the AC disconnect, then it's a feeder that can use 310.15(B)(6). . If it doesn't contain OCPD, it's a branch circuit and has to be wire sized off 310.16 and lug termination temp rating enters the decision. . I understand the load diversity concept but the code doesn't say you can't use 310.15(B)(6) for a feeder to a panel that has only one load.

David

 

[iwire]

but the code doesn't say you can't use 310.15(B)(6) for a feeder to a panel that has only one load.

In a way it does say that.

The feeders referenced in 310.15(B)(6) are "feeder conductors that serve as the main power feeder to a dwelling unit"

IMO a feeder supplying a panel that supplies only one load can not be the 'main power feeder' it must be something less than the main power feeder.

 

[dnem]

You're right, Bob !
Altho the beginning of 310.15(B)(6) seems straight forward enough, as the paragraph goes on it starts to add doubt. . The argument in favor of using 310.15(B)(6) for the AC disconnect was based on the sentence in 310.15(B)(6) that says, "For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboards."

But now I've just read thru all of 408.34 and I don't see that these AC disconnect / service plug combo units with a breaker for the AC can qualify as a lighting and appliance branch-circuit panelboard. . The OCPD for the service plug is back in another panel.

I guess I allowed myself to be conned. . I should have looked it up.

David

 

[iwire]

David you found your own way to that conclusion.

I agree an HVAC unit is not a lighting and appliance branch circuit panel.

However before we get to that issue we see that an HVAC unit is not a dwelling unit

"feeder conductors that serve as the main power feeder to a dwelling unit"

These 310.15(B)(6) feeders must supply a dwelling unit.....not just an HVAC unit located at a dwelling unit.

 

[suemarkp]

You must be seeing some pretty large houses if you can apply 310.15(B)(6) to an airconditioning feeder. That table starts at 100 amps.

I think you can make a good argument that any L&A panel past the main disconnect in a dwelling could have this table applied. But others have tried to dig up the old ROP/ROC's for this section and imply the intent is that it be only the feeder that carries the entire dwelling load. Supposedly the reasons for the plural panelboards is for multi-dwelling buildings.

I wish the CMP would write more clearly.

 

[George Stolz]

I wish the CMP would write more clearly.

Well, I tried and failed to help. If I had gotten a comment off on my proposal we might have seen a change next cycle.

See what I get for sleeping in?

 

[racerdave3]

Well, the tone of this thread certainly has been different from some others in the past, or I should say the concensus of it has been. I have used a 2-2-2-4 SER cable in the past for a sub-panel and fed from a 100 amp 2 pole breaker, and the last time this was discussed, there seems to have been disagreement on whether this was correct or not. I have always felt the same way that it is in fact a feeder to the sub-panel, so it would fall under the table in 310-15 (B)(6).

 

[dnem]

David you found your own way to that conclusion.

I agree an HVAC unit is not a lighting and appliance branch circuit panel.

However before we get to that issue we see that an HVAC unit is not a dwelling unit

"feeder conductors that serve as the main power feeder to a dwelling unit"

These 310.15(B)(6) feeders must supply a dwelling unit.....not just an HVAC unit located at a dwelling unit.

You’re right about that first section sounding straightforward and easy to apply. . The problem is the panel didn’t stop there but kept going until they confused the situation. . They didn’t help things by adding, "For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboards."

I think you can make a good argument that any L&A panel past the main disconnect in a dwelling could have this table applied. But others have tried to dig up the old ROP/ROC's for this section and imply the intent is that it be only the feeder that carries the entire dwelling load. Supposedly the reasons for the plural panelboards is for multi-dwelling buildings.

If they wanted to say multi-dwelling buildings then I can’t understand why they didn’t just say it.

You must be seeing some pretty large houses if you can apply 310.15(B)(6) to an airconditioning feeder. That table starts at 100 amps.

I’ve only seen a 100amp AC unit on a house once. . The standard for the monster houses is multiple smaller ACs. . That’s probably why this hasn’t come up before and why I’m just now working thru this concept on this thread.

David

 

[dnem]

Well, the tone of this thread certainly has been different from some others in the past, or I should say the concensus of it has been. I have used a 2-2-2-4 SER cable in the past for a sub-panel and fed from a 100 amp 2 pole breaker, and the last time this was discussed, there seems to have been disagreement on whether this was correct or not. I have always felt the same way that it is in fact a feeder to the sub-panel, so it would fall under the table in 310-15 (B)(6).

The sentence that would allow you to use T310.15(B)(6) for something other than the building supply feeder/service would be, "For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboards."

That doesn't apply to an AC disconnect w/OCPD but it might apply to the installation that you're talking about. . It all depends on the specifics.

David

 

[dnem]

Well, I tried and failed to help. If I had gotten a comment off on my proposal we might have seen a change next cycle.

See what I get for sleeping in?

I was mostly surprised that they didn't just 'accept in principle' and add the omitted text on their own. I thought this proposal had a high probability of success for it's clarity.

Hopefully, they accept it with the changes. I don't mind adding some (of what I consider to be) nonsense if the overall section is improved.

Why don't you add a sentence in your proposal next time that says
"Please accept in principle and change whatever you need to change in order to get this upgrade done !"

 

[George Stolz]

The sentence that would allow you to use T310.15(B)(6) for something other than the building supply feeder/service would be, "For application of this section, the main power feeder shall be the feeder(s) between the main disconnect and the lighting and appliance branch-circuit panelboards."

IMO, the "main power feeder" needs to start at the main disconnect, and terminate in a L&ABCPB. That interpretation would preclude using table 310.15(B)(6) for a feeder between two L&ABCPB's, IMO.

 

[George Stolz]

Why don't you add a sentence in your proposal next time that says
"Please accept in principle and change whatever you need to change in order to get this upgrade done !"

Good idea - I just wish I had made a comment, I feel like an idiot for missing the boat.

 

[dnem]

It appears I need to edit the first sentence to read "all the following conditions". I also need to add: "(f) The conductors are installed in a raceway or cable, with or without an equipment grounding conductor."

I don't understand what they are trying to prohibit by these statements, however. All installations under the sun are either with or without an equipment grounding conductor, so it seems like a waste of ink to me. :?:

Services and feeders are required to comply with Chapter 3, so what does the "in a raceway or cable" statement add? :?:

6-65 Log #1410 NEC-P06 Action: Reject
(310.15(B)(6))
Panel Meeting Action: Reject

Panel Statement: The proposed text does not require compliance with all the list items and omits some existing requirements (?and are installed in raceway or cable with or without an equipment grounding conductor?). The panel appreciates the submitter?s efforts to improve the readability and clarity of the Section but any revised text must require that the installation comply all of the existing requirements.

Add to your substantiation: . ?This proposal purposefully omits some existing language that is of no value and adds no useful direction of any kind such as: installed in raceway or cable with or without an equipment grounding conductor.

Please accept in principle and change whatever you need to change in order to get this upgrade done !"


If they can?t use their own brains to figure that out then there?s no help for them.

I?m still pissed that their response to my proposal was that it was a ?design? issue. . Some of these guys need to grow a brain.

David

 

[suemarkp]

IMO, the "main power feeder" needs to start at the main disconnect, and terminate in a L&ABCPB. That interpretation would preclude using table 310.15(B)(6) for a feeder between two L&ABCPB's, IMO.

But most houses have a combination main disconnect and circuit panelboard. Runing feeders off of that are still between the main disconnect and an L&A panelboard. What I think you mean is the main disconnect needs to be just a disconnect and not a panelboard.

Dnem: If you put a 15A breaker in that air conditioning panel (to supply the required maintenance receptacle), then you'd have a lighting an appliance panelboard and open up this can of worms again.

 

[dnem]

Dnem: If you put a 15A breaker in that air conditioning panel (to supply the required maintenance receptacle), then you'd have a lighting an appliance panelboard and open up this can of worms again.

I was thinking the same thing myself. . But I don't think such a thing is made. . An AC disconnect with overcurrent for the 240v 2pole AC is common but I haven't seen one that also has a 120v single pole OCPD for the plug.

But if it did have that it.
408.34(A) more than 10% overcurrent on lighting & appliance branch circuits.
408.34 lighting & appliance branch circuit has a neutral & 30a or less.
The 15/20a plug is under 30a, has a neutral, and would be 50% of the OCPD.

Now if it's also supplied from the main and not another subpanel and the AC is on a 100 or higher OCPD, then I'm thinking you could use T310.15(B)(6).

For a 100a AC, in the end we're only talking about the difference between using #2 AL according to T310.15(B)(6) and #1 AL according to T310.16. . But it's the mental gymnastics of thinking thru the whole scenerio that keeps the brain sharp and provides practice at dealing with "codespeak".

David

 

[suemarkp]

Just install a 4-slot outside panel instead of an AC disconnect. These are common for spas (I think that is how I got the one I have). An AC disconnect doesn't have to say AC Disconnect on the box you buy it in.

I think this example is what gives people the willies about 310.15(B)(6). I wouldn't be comfortable with using that table in this situation because there is no calculated load required for the Service receptacle. So the feeder to that panel needs to be rated for the MCA of the AC unit. Complicating this example is whether a feeder will work because of the huge breaker required for an airconditioner this size (I don't think the upsize of an HVAC breaker can be applied to a feeder, only a branch circuit). I could see some homeowner buying a storable pool and plugging the pump into the Service receptacle. That is probably going to exceed the load on the wire if you used 310.15(B)(6).

This isn't a common occurrence, but it could happen. I'd like to see some minimum number of circuits in the panel that have a calculated load associated with them in order to apply that table. Otherwise, it is difficult to assure diversity.