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normbac

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I am installing per plan a metered 400 amp residential panel on a home and then feeding a ( metered for homeowner to read only) outdoor semi flush 200 amp res. panel on a guest home does table 310.15 B6 apply for sizing the feeders Should this be treated as a sub meaning an isolated ground at the 200 amp am confused how to treat this 200 do I need to pull a egc from the 400 and what size. A seperate ufer will be applied on the guest home.
Thanks for any clarity on this.
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Yes 310-15(B)(6) can be used (225-5); this is a feeder also 215, 220 & 225 apply at a minimum. You will have to install ground electrodes at each building in keeping with 250-24 & 250-32(A) and as per 250-52 for types. Your feeder circuit needs to be 4W even with electrodes at each end unless it meets the criteria of 250-32(B)(2) because ?a grounding connection shall not be made to any grounded circuit conductor [neutral] on the load side of the service disconnecting means except as otherwise permitted in this article.? (250-24(A)(5)) & ?an equipment grounding conductor shall be run with the supply conductors?ANY installed grounded conductor shall NOT be connected to the equipment grounding conductor [neutral] or to the grounding electrodes.? (250-32(B)(1), your equipment-grounding conductor must be sized from table 250-122 and in your case it is #6 cu.
To be a 3W feeder you must meet the criteria of 250-32(B)(2) rather than (B)(1) in other words: no equipment grounding conductor with feeder, no continuous metallic paths, and no ground fault protection at the service. If you use this method you will be using the grounded conductor (neutral) for both the grounding system and the unbalanced load which is safe and common practice but the 4W circuit may be cleaner for today?s electronics in that your unbalanced load does not mingle at all with the equipment grounding possibly causing objectionable current.
In keeping with 225-33 you will most likely need a main breaker sub and it must be located at the outbuilding as per 225-32, usually outside.
 
normbac said:
does table 310.15 B6 apply for sizing the feeders Should this be treated as a sub meaning an isolated ground at the 200 amp am confused how to treat this 200 do I need to pull a egc from the 400 and what size. A seperate ufer will be applied on the guest home.
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Norm, If the power supply to guest home is fed from the main panel and not the meter, it will be considered a sub panel and you'll need to seperate the grounds and neutrals in the guest home panel.The EGC will be based on the 200 amp breaker from table 250.122.There are some exceptions in 250.32 (B)2 as to whether you need to run the EGC, but if unsure, then run it.
Rick

You'll also need to keep the grounds and neutrals seperated through the meter base on the guest home.
 
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RUWired said:
Norm, If the power supply to guest home is fed from the main panel and not the meter, it will be considered a sub panel and you'll need to seperate the grounds and neutrals in the guest home panel.The EGC will be based on the 200 amp breaker from table 250.122.There are some exceptions in 250.32 (B)2 as to whether you need to run the EGC, but if unsure, then run it.
Rick
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That is not the case if you are running a 3 wire feeder to the guest house. It may be a moot point since there is probable a metallic pathway between the buildings. If there is then you would need a 4 wire.

In the 2008 code you will need a 4 wire to the detached building so you probably ought to do that-- I bet you need to anyway. Then you need to dive your ground rod(s) and tie the gec to the equipment ground which would be separate from the grounded conductor.
 
Dennis , i did mention that in my reply.I am not so sure if table 310.15(B)6 can be used whether or not its a 3 or 4-wire feeder, because this feeder is not the "main power feeder".This is being fed from the main panel.Just like any other sub it needs to follow table 310.16.
Rick
 
RUWired said:
I am not so sure if table 310.15(B)6 can be used whether or not its a 3 or 4-wire feeder, because this feeder is not the "main power feeder".
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IMO this feeder may use T310.15(B)(6) as it is the 'main power feeder' to a dwelling unit.
 
iwire said:
IMO this feeder may use T310.15(B)(6) as it is the 'main power feeder' to a dwelling unit.
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Bob , you and Dennis (and tryinghard) are saying that the difference between a sub in the house and a sub to a seperate garage is the word dwelling.
Rick
 
Art. 310.15(B)(6) states that the feeder conductors that serve as the main power feeder to each dwelling unit and are insatlled in raceway or cable with or without an egc. The main power feeder shall be the feeder(S) between the main disco and the Lightng and appliance branch panelboards(S).

This tells me that it can be more than one feeder as in the case of a 6 disco panel that feeds more than one L&A branch panelborad.
 
Dennis Alwon said:
This tells me that it can be more than one feeder as in the case of a 6 disco panel that feeds more than one L&A branch panelborad.
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IMO that is true sometimes, not all times.

If you have 6 feeders to six separate dwelling units (as you might in an apartment building) you could use T310.15(B)(6) as each of these feeders is the 'main power feeder' to a dwelling unit.

However if for some reason each of these feeders supplied the same dwelling unit you have to use T310.16 as none of these feeders would be 'the main power feeder' to a dwelling unit.
 
I always thought that if the feeders came from the meter to a disconnect(s) and then to the main panel(s),they are the main feeders.But if they originate from the main panel, they are not the main feeders anymore.But you are all saying they are.
 
RUWired said:
Bob , you and Dennis (and tryinghard) are saying that the difference between a sub in the house and a sub to a seperate garage is the word dwelling.
Rick
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In a nut shell.....yes.

The reason Table 310.15(B)(6) exists in the first place is the NFPA knows just as the power companies know that the NEC service calculations result in a much larger service then is really needed for a dwelling unit.

So they give us a break and let us use Table 310.15(B)(6) and it reduced conductor sizes, the reason they can do this is they can count on the load diversity of an entire dwelling unit.

Once you start subdividing the loads, as happens with a feeder to a detached garage they can no longer count on the load diversity to prevent the cables from being overloaded.
 
RUWired said:
But you are all saying they are.
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What I am saying in regards to using Table 310.15(B)(6) is that it only applies to feeders that provide all the power to dwelling units. :smile:

It does not apply to feeders that supply only part of the power to one dwelling unit.
 
RUWired said:
So if i had to run a 4-wire to this same dwelling, then T310.15(B)6 would not apply?
Rick
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I think it would apply-- if you mean the guest house. I may have not been clear-- the 3 wire or 4 wire has no affect on the use of that table.

Here is my understanding of art. 310.15(B)(6) which controls the use of Table 310.15(B)(6)---

The feeder is allowed to use the Table as long as the feeder is run on the line side of the L&ABP. You can have 2, 3 or up more L&ABP feed from the same service and their feeders may use the Table as long as the panels are feed from a point before any L&ABP. In other words I can feed a sub panel and use the Table if it is fed from the service disconnect that is not an L&APB. If I fed a panel from the sub panel then I cannot use Table 310.15(B)(6).

Since art 310.15(B)(6) allows more than one feeder ( it uses the plural) then the feeder going to a guest house from the main panelboard (In this case it is not a L&ABP) can use Table 310.15(B)(6).

Now someone can argue if it were a detached garage then the feeder is not for a dwelling unit therefore you must use Table 310.16. I don't see it that way but I guess someone can.

I am hoping the 2008 NEC will spell this out better.
 
iwire said:
It does not apply to feeders that supply only part of the power to one dwelling unit.
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I guess we are not in agreement here from my previous post.


Edit--- I have always been unclear about this art.--- you may be correct but it is up for interpretation.
 
Dennis Alwon said:
Now someone can argue if it were a detached garage then the feeder is not for a dwelling unit therefore you must use Table 310.16.
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A garage is not a dwelling unit, it does not apply....little load diversity.

Please take a look at my post #14


I am hoping the 2008 NEC will spell this out better.
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My understanding is it does clear this up and supports my position. I will try to find the info.
 
iwire said:
A garage is not a dwelling unit, it does not apply....little load diversity.
Please take a look at my post #14
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I did but it was after I posted since we were posting about the same time.
You are probably correct .

iwire said:
My understanding is it does clear this up and supports my position. I will try to find the info.
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I would appreciate that info.
 
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