Which Table?

[pierre]

Sizing conductors situation:

Installing a 100 amp 'subpanel' in the detached garage of a single family dwelling. The service is a single phase 120/240 volt system.
Which table do you size the conductors from, Table 310.16 or Table 310.15(B)(6)?

 

[jwelectric]

Re: Which Table?

Table 310.16

 

[stud696981]

Re: Which Table?

310.16 for me as well. I have heard of some inspectors considering this a feeder as well, but I prefer to error on the side of caution and pass inspection.

 

[websparky]

Re: Which Table?

Hi Pierre,

"service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit"

Garages aren't included!

 

[luke warmwater]

Re: Which Table?

Pierre,

to be safe, I would use 310.16.

That said, 100 out of 100 that I've ever seen were wired using 310.15(B)(6).

Now if you want some fun reading from some older threads, check these out:

.15(B)(6) or .16 #1

.15(B)(6) or .16 #2

.15(B)(6) or .16 #3

.15(B)(6) or .16 #4

.15(B)(6) or .16 #5

How many remember this from the last link:

The following is from an old thread, substitute your numbers as applicable.


quote:

This article specifically allows reduction to the first lighting appliance
branch-circuit panel(s) down line of the service equipment.

We will use 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 400A OCPD.

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

For sake of conversation 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)

 

[iwire]

Re: Which Table?

Originally posted by websparky:
Hi Pierre,

"service-entrance conductors, service lateral conductors, and feeder conductors that serve as the main power feeder to a dwelling unit"

Garages aren't included!

Pierre I agree with Dave here, a garage is not a dwelling unit.

Part of 310.15(B)(6)

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

 

[pierre]

Re: Which Table?

Hello Dave, thanks guys for the answers

But... I am still gathering info from you guys before I make a decision on a job. I find this an interesting situation.


Installing a 100 amp subpanel in a second structure (used as a dwelling unit)on the same propery. The service in the first dwelling is 120/240 volt system.
Which table do I use, 310.16 or 310.15(B)(6)?


The reason I am asking, is one of my students called me and told me one of the inspectors told him that he could use table 310.15(B)(6) and run smaller conductors...hence saving some money. If he is correct, then I say great, but if not, I do not want to see the practice continue. When I have read this section in the past, I always thought it meant the "first" panel after lets say a service disconnect mounted outside and then the supply to the panel. Not to any other panels after that.

And as has been mentioned here, does a garage really have the distinction from lets say another dwelling being supplied on the same property from the first dwelling?

Thanks!

 

[jwelectric]

Re: Which Table?

Originally posted by pierre:

When I have read this section in the past, I always thought it meant the "first" panel after lets say a service disconnect mounted outside and then the supply to the panel. Not to any other panels after that.

Thanks!

I agree with what you have said in the above statement.

 

[stud696981]

Re: Which Table?

quote:
--------------------------------------------------------------------------------
Originally posted by pierre:

When I have read this section in the past, I always thought it meant the "first" panel after lets say a service disconnect mounted outside and then the supply to the panel. Not to any other panels after that.

Thanks!
--------------------------------------------------------------------------------

That is the same thing I was taught as well.

 

[charlie b]

Re: Which Table?

I don't see any choices here. 310.15(B)(6) says more than just that the table applies to a feeder that is the main power source to the dwelling unit. Indeed, it goes on to say the feeder must be between the main disconnect and the panel. Here, we are talking about a feeder between one panel and another. So 310.15(B)(6) clearly does not apply.

 

[George Stolz]

Re: Which Table?

What is a main disconnect? I don't see that defined anywhere. I think people read that and assume "Service Disconnect", but that's not the term used.

If the conductors feed a dwelling unit, and are the main power feeder for that dwelling unit, then what is the problem?

 

[jwelectric]

Re: Which Table?

George
That is a good call. There is a big difference between the service disconnect and the main disconnect.

310.15 (B)(6) does state;

--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--

If we read on a little future down we find out what these feeders are;

--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(s).--

It clearly states that these feeders are the conductors between the ?main disconnect and the ?branch circuit panel.?
Now we need to understand just what a ?main disconnect? is.
I think that the best example is found in 250.142;

250.142 (A)
(1) On the supply side or within the enclosure of the ac service-disconnecting means
(2) On the supply side or within the enclosure of the main disconnecting means for separate buildings as provided in 250.32(B)
(3) On the supply side or within the enclosure of the main disconnecting means or overcurrent devices of a separately derived system where permitted by 250.30(A)(1)

In #2 we see the main disconnecting means at a second building meaning that the derating of the feeders would have to take place between this main and the branch circuit panel. A good example would be the disconnect on the outside and the main lug panel on the inside.
The other main disconnect we see is at a separately derived system. So we could use this rule for the main on a generator if we switch the grounded (neutral)

Based on the findings of fact outlined above I must say that 310,15 (B)(6) could not be used as outlined in the original post.

 

[kottd]

Re: Which Table?

Hi Pierre,
Think about the main service disconnect being at the outside of your house because it's on an addition, and you run power to the panel that's back into the basement, that's a subpanel in the basement. Are you going to size the service conductors according to 310.15(B)(6) and the conductors to the panel in the basement (feeders)according to Table 310.16?
Dennis

 

[websparky]

Re: Which Table?

George,

How many "main" disconnects are there in any given dwelling unit?

 

[websparky]

Re: Which Table?

jwelectric,

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

not to a sub panel or anything else that you can think up.

 

[benaround]

Re: Which Table?

Possibly there could be 6!! Not probally.

frank

 

[pierre]

Re: Which Table?

After reading these posts, and talking to some inspectors (Brian McPartland is one), I have come to the conclusion that the conductors supplying a panel in a separate building that is being supplied from the first building has to be sized according to Table 310.16.

 

[don_resqcapt19]

Re: Which Table?

All feeders are between the service disconnect and the lighting and applicance branch circuit panel. They may not be directly between, but they are between.
Don

 

[iwire]

Re: Which Table?

Originally posted by don_resqcapt19:
All feeders are between the service disconnect and the lighting and appliance branch circuit panel. They may not be directly between, but they are between.
Don

But are they all 'main' feeders?

However this feeder feeds a detached garage which is not a dwelling unit.

 

[electricmanscott]

Re: Which Table?

On the original question the answer I'd go with is 310.16. What panels you can or can not feed with 310.15 B 6 seems to be debatable to some. However since we are talking about a garage B6 should not even be in the conversation.