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#87706 04/11/04 12:31 PM
Joined: Apr 2002
Posts: 197
T
Member
Roger, how many times do I have to acknowledge my understanding that writings outside the actual NEC are not code? Between this thread and previous ones I must have said it 10 times! Therefore, when you write, “do you really believe everything you read or hear” you already know my standing. Obviously you are making that statement because its audacity makes me sound ignorant.

Tell me Roger, did you learn everything you needed to know about your electrical career by simply reading the codebook? In other words, did you one day just pick up a copy of the NEC, read it cover to cover, and then call yourself an electrician? Of course you didn’t. Like most (all?) electricians you probably began your career by working under those already experienced in the trade. You learned not only the procedures involved but also the code from (gasp) the electrician you worked under. Shame on you for not ignoring those outside sources!

This is the way I learned. I have since found that some of what I was taught was incorrect but most was not. I know that if all my electrical related info came straight from the NEC, I would have misinterpreted and misapplied about 50% of what I read! I salute you if you’ve been able to independently implement the NEC from day one on the job.

For argument sake, let’s say the 100 top NEC savvy individuals in the US (subjective, I know) were asked to weigh in on this issue. If all 100 agreed that subpanels did not fall under the residential table then would you be willing to accept this ruling from outside the NEC? Or would you feel more comfortable with your own interpretations? I’m not making a comment on your standing on the table issue (especially since I believe you stated you agreed with me) but rather on your apparent disinterest in outside sources.

Don’t get me wrong. I’m not saying that the residential wiring book I received my information from has to be correct. I don’t know the author from Adam. For all I know, he could be a kook with an agenda involving the spreading of misinformation for his own amusement. For every expert that leans this way, there could be 10 others that lean the other. What I was hoping to accomplish here was to get others involved in doing a tiny amount of research. Apparently I am asking too much.

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#87707 04/11/04 12:40 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Dude, Chill out a little bit.

Forget about the outside sources and read the article out of the NEC itself and tell me (or us) what part supports this view you are getting so upset about.

No one here thinks your ignorant, you obviously care a great deal about your trade.

But please forget about my opinion, Rogers opinion and your books opinion and read carefully word for word what the NEC article says.

I know a lot of people share the view you do, I just do not see anything in the code book to support it.

The more I read it it looks like both your 320 example and mine would be a violation because neither of those applications supplies all the main power.

But... we have those darn (s) to deal with. [Linked Image]

Bob, [Linked Image]


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#87708 04/11/04 12:54 PM
Joined: Apr 2002
Posts: 197
T
Member
BTW Roger, are you comparing your decision-making methods to those of Tesla's? You also "need a published document to substantiate your direction". That document is the NEC! You are not dreaming up your standing all on your own.

On another point, I didn't go in search of a document that would "substantiate" my "direction". Actually, I had always used the residential table to wire subpanels until I read on a forum that this was not code compliant. Only then did I search out an outside source (after repeated attempts to decipher the NEC myself). I had to change my "direction" after doing this research. Like I said before, I very much want to use table 310.15(B)(6) for subpanels so it's not like I set out to debunk my previous view.

#87709 04/11/04 12:56 PM
Joined: Feb 2002
Posts: 840
C
Member
Simple question, hopefully a simple answer. (I doubt it though.) What is the danger of using this table for "subpanels" in a residence? As I stated in my intro, this is commonly done around here. I know that doesn't make it right, but common sense should prevail.

Peter


Peter
#87710 04/11/04 05:47 PM
Joined: Mar 2001
Posts: 345
T
Member
The danger that I see in using the table in question for anything less than "serve as the main power feeder to a dwelling unit" would be the possibility that the lack of diversity in the load thus served could cause chronic overloading of the conductors. The example I would give is the service call I did in a dwelling with a feeder supplied panel that supplied the air conditioning loads for a home having three separate AC systems. All the loads on that panel were 240 volt. During a particularly brutal hot spell the feeder faulted out. Since the feeder had been run in flexible metallic conduit the failure was confined to the feeder itself. Rather than just repair the feeder by replacing the conductors I checked the size against the ampacity tables and found they were sized in accordance with Table 310.15(B)(6) using number four copper RHW conductors for the feeder that was protected by a 100 ampere breaker. When first called out on the sunday afternoon I had used a 2/0 Al service entry cable as the temporary feeder to that panel. It was the only wire that I had on hand that seemed safe enough for a one day fix. Once I was able to research the problem and found that the original feeder was overloaded I purchased number three THHN - THWN as the replacement conductors. Since my suspicion had been aroused I megged the feeders for the kitchen panel and the panel to the detached garage. The feeder to the all electric kitchen failed at the 500 volt test level. It was also sized at number four copper for a 100 ampere breaker. This was also replaced with number three THHN. The feeder to the garage workshop was number 2 Al UF. Since it had megged clear at 500 volts I simply replaced the breaker with a 70 ampere breaker which was within the ampacity of that wire type under table 310.16. I think that diversity is the critical factor here. If the circuit in question supplies "the main power" to a dwelling unit then it is unlikely that so many of the loads will be used at once that the dwelling feeder sized circuit will be overloaded. On the other hand if you feed only part of the load with the feeder diversity is not assured or even perhaps likely depending on the type of load that the feeder supplies.
--
Tom Horne


Tom Horne

"This alternating current stuff is just a fad. It is much too dangerous for general use" Thomas Alva Edison
#87711 04/11/04 06:04 PM
Joined: Sep 2003
Posts: 650
W
Member
The danger applying 310.15(B)(6) to subpanels:

IMHO this all comes down to load diversity and the current versus time statistics of a given load.

A residence has a rather different load characteristic than an industrial plant, and the whole residence has a rather different load characteristic than the electric heaters inside that residence.

Depending upon the load characteristics, the OCPD protecting a set of conductors may or may not be matched to the continuous ampacity of those conductors. An example would be the use of oversized OCPD to protect an electric motor, where the thermal overload protection of the motor itself is permitted to provide the overload protection for the conductors, and the OCPD is used more for short circuit protection.

IMHO 310.15(B)(6) assumes the time characteristics of residence loads to mean that the conductors will be properly protected where the OCPD rating is greater than the continuous current rating of the conductors as determined using 310.16.

Thus IMHO if the load has a time characteristic that is anything like that of a normal residence, then there will be essentially no danger from using 310.15(B)(6) for a subpanel...it may not be 'code' but it will be okay.

It is only if the load is very different from an average residence, very high (in proportion to the feeder rating) and continuous that I believe that you will see problems.

In the worst case, the conductors will end up at 90C or above, and the insulation will degrade rapidly.

-Jon

#87712 04/11/04 06:51 PM
Joined: Jan 2003
Posts: 4,391
I
Moderator
Jon & Tom understand I have no inside knowledge on this subject and do not have the option of using this table at my job. [Linked Image]

You both bring up some interesting points to think about, the problem I have is there is nothing in that article that talks about diversity.

Another problem I have is if you calculate the loads for both panels in the same way it will not make a difference.

As an example a subpanel sized to feed only two outside air conditioner compressor condenser units.

They both have a minimum circuit ampacity of 50 amps, we install a 100 amp subpanel using Table 310.15(B)(6) which requires 4 AWG CU with 75 C or 90 C conductors.

This panel at most see a load of 80 amps, as the tag on the HVAC units already includes the 125% increase required by 440.

310.16 lists 4 AWG CU @ 75 C as 85 amps, so how would this be a problem?


Bob Badger
Construction & Maintenance Electrician
Massachusetts
#87713 04/11/04 11:33 PM
Joined: Jan 2004
Posts: 615
J
Member
triple,

I understand what you are saying about outside sources. I think they are important too. Afterall every code ever written was once the opinion of an outside source.

With all do respect, guys, your point is pretty clear. But triple's point is a separate, but valid point.

Sorry to stick my nose in if it doesn't belong, but I understand triple's frustration even if it is seen as him needing to chill out.

#87714 04/12/04 11:44 AM
Joined: Sep 2003
Posts: 650
W
Member
Now Bob,

I don't think you could call a panel with 2 50A air conditioner circuits and nothing else on it a 'lighting and appliance panelboard' [Linked Image] (just yanking the chain a bit)

I agree that the code in 310.15(B)(6) doesn't mention load diversity or any sort of load time characteristics; I guess that I am reading this into the code based upon the similarity to things like the explicit reduced ampacity of conductors for welders and the like, related to the duty cycle of the welder...remember though that I am a theory guy rather than someone with experience (again *grin*, Tom was writing his post while I was writing mine, and my jaw dropped when he described as a service call that he did, something that I'd theorized was possible...)

It is not as though the electrons going into a residence are somehow different from those going into an office building, such that the actual continuous amapcity of the conductors changes (again, tongue firmly in cheek). The difference in the situation has to be that conductors into residences are not expected to be used with the same time distribution as conductors in other occupancies. Again, this is just me reading into code.

In the example that you gave, I think that _in general_ there would be a problem putting a _continuous_ 80A load on 85A conductors with 100A OCPD. The general requirements are that both the OCPD and the conductors be increased in size to meet the 125% of continuous load requirements. For motors the requirements are even stricter; you don't have the '100% rated OCPD' exception. The only time that it is okay to have conductor ampacity > load amps but < OCPD rating 'in general' is when you are using the round-up rule for the OCPD.

Where to go with this: I still believe that applying 310.15(B)(6) to anything that doesn't look like a residence _as a whole_ is at the very least a 'bad idea', and depending upon interpretation violates code. As Bob noted, however, the '(s)' in the actual code text leave a big hole open for interpretation, as does the concept of 'main power feeder'. Clearly the air conditioning panelboard that Tom described is fed by a feeder in a residence, and if there were a couple of general purpose lighting circuits on it then it would be a lighting and appliance panelboard.

Here is perhaps a different interpretation of 'main power feeder' to multiple panels...not saying that it's right, just presenting another possibility. If 310.15(B)(6) is used to size multiple feeders to multiple panels in a residence, then _all_ of the feeders must be sized using 310.15(B)(6) for the ampacity of the _entire_ service.

This clearly does not match 'common sense', unless the use of the '(s)' was intended to address multiple panels and feeders in series rather than in parallel.

-Jon

#87715 04/12/04 12:47 PM
Joined: May 2002
Posts: 1,716
R
Member
My opinion of the (s's)is as follows.

This article specifically allows reduction to the first lighting appliance
branch-circuit panel or panels immediatly down line of the service equipment
.

Using a lateral in this example

400A service
2-200 amp lighting appliance branch-circuit panels.

Service lateral --400KCM copper per article 310.15(b)(6) to a 400A OCPD.

We have not made it to our lighting appliance branch-circuit panels (note plural) yet.

For sake of coversation we install 2-200 amp enclosed breakers adjacent to
this 400 amp OCPD and feed these from parallel lugs.

We use 2/0 copper per 310.15(b)(6).

We still have not made it to our lighting appliance branch-circuit panels
yet.


We feed 2-200 amp lighting appliance branch-circuit panels with 2/0 copper
per 310.15(b)(6)and make all terminations.

We have finally made it to our lighting appliance branch-circuit panels.

Now 310.15(b)(6) has seen it's end and if we feed any sub panel, say a 100 amp, from one of these aforementioned lighting appliance branch-circuit
panels, this 100 amp sub panel can not use 310.15(b)(6)

Just my unplublished opinion. [Linked Image]

Roger



[This message has been edited by Roger (edited 04-12-2004).]

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