Installing 125A Service to Outbuilding

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Old 10-02-08, 05:53 AM
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I have a 200 amp underground service to my house. I am constructing a detached workshop that is approximately 250 feet from the house and will have a 125A sub-feed from a 200A meter center on the house. At the workshop I will have a 125A disconnect on the outside and 125A load center inside. I have 300 feet of 1/0 Alcan three conductor cable suitable for direct ground burial I was planning to use to connect the building. I realize I will need a 4th conductor for ground. I have questions regarding that 4th wire.

Is a #6 copper sufficient or would I need a #4?

Does this ground wire have to be insulated?

Can it be in the same trench as the primary feed?

Also, regarding ground. Is a ground rod required at the workshop. If so, how deep should it be?
 

Last edited by ibpooks; 10-02-08 at 08:59 AM.
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Old 10-02-08, 09:02 AM
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I'd like to know this as well. I was under the impression that it had to be in the same cable. If you were to run individual conductors in conduit, then I'm also under the impression that it has to be in the same raceway, not "along side".

BTW, what size breaker to you intend to use for the feeder cable?

250 feet is a LONG way (voltage drop) plus I don't think you can use the max rating for 1/0 AL on sub panels. You typically derate on subs for one reason or another which I still haven't found out why.

A ground rod is required. 8 feet deep, just below grade. Also.... I'm under the impression you need 2 ground rods at a detached structure.

Sorry to hijack with the 3 questions of my own but they are directly related to your setup. Hopefully someone can chime in about all that.
 
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Old 10-02-08, 09:10 AM
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Originally Posted by cdavies View Post
250 feet from the house and will have a 125A sub-feed from a 200A meter center on the house. At the workshop I will have a 125A disconnect on the outside and 125A load center inside.
Given this distance, I would not exceed 100A on the #1/0 aluminum conductor.

I have 300 feet of 1/0 Alcan three conductor cable suitable for direct ground burial I was planning to use to connect the building. I realize I will need a 4th conductor for ground. I have questions regarding that 4th wire.
It would be up to your electrical inspector if you would be allowed to use the three-conductor cable and bury a separate ground wire alongside it. I suspect that most reasonable inspectors would allow this, but it's not certain. The previous post you referred to was in a rural Alaskan area which does not have code enforcement. Therefore, I recommend talking with the inspector before proceeding with this plan.

Is a #6 copper sufficient or would I need a #4?
#6 copper or #4 aluminum ground would match the #1/0 aluminum hots.

Does this ground wire have to be insulated?
I would go with an insulated conductor rated for direct burial. It may be a special order item, but they do make single conductor UF-B cable. There is also single conductor USE-2 available.

Can it be in the same trench as the primary feed?
It should be.

Also, regarding ground. Is a ground rod required at the workshop. If so, how deep should it be?
At least one ground rod is required. It should be a 5/8" x 8' rod driven flush to the earth. Put an acorn clamp on top and connect it to the subpanel ground bus using #6 copper wire.
 
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Old 10-02-08, 09:15 AM
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Originally Posted by madpenguin View Post
then I'm also under the impression that it has to be in the same raceway, not "along side".
True, but in some situations you could consider the trench as a raceway. The other consideration is that hots and neutrals must be kept together to avoid inductive heating caused by the current, but the ground wire does not carry current so this is not a concern.

You typically derate on subs for one reason or another which I still haven't found out why.
The 2008 edition of the code changed the wording of article 310.15 to explicitly say that subpanel and main panel feeders are calculated using different tables. In previous versions, it was assumed that you could use the same table for both.
 
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Old 10-02-08, 09:34 AM
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Thanks for the information.

If I choose to run the 1/0 cable in PVC conduct, as opposed to direct burial, could a convential single conductor #6, not rated for direct burial, be used in the same conduit?

Your point is well taken on the voltage drop. What is an acceptable voltage drop?

As some background on the operating conditions of the building. Concurrent equipment demand would be several 220V units (Air Compressor, Dust Collector, 2.5 Ton A/C, Table Saw) plus 110V lighting.

Would 100A be sufficient to support this demand?

In reponse to:

BTW, what size breaker to you intend to use for the feeder cable?
I was planning to use a 125A breaker in the meter center. The meter center will have a 200A main breaker.
 
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Old 10-02-08, 09:54 AM
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Originally Posted by ibpooks View Post
True, but in some situations you could consider the trench as a raceway.
Well, I suppose... I just got a good laugh from Art 100. Has anyone ever tried to argue with an inspector that a trench could be considered a raceway? I suppose dirt is a "nonmetallic material"... It also says "but are not limited to". Somehow I don't think they had that in mind tho.

If this is being inspected, I'd ask your local inspector first just to be safe.

The 2008 edition of the code changed the wording of article 310.15 to explicitly say that subpanel and main panel feeders are calculated using different tables. In previous versions, it was assumed that you could use the same table for both.
Oi... I'm not seeing it. I'll re-read it again. I don't know whether there is an entirely new table than 310.15(B)(6) or whether your supposed to use 75c on 310.16..... Or something tricky along those lines.

Again, sorry to hijack cdavies... I've been insanely frustrated with my apprenticeship so far. My IEC instructor basically sucks and it's like pulling teeth asking people at work. That's the whole reason I lurk around here... Peeping On U2

It would seem gone are the days of one to one apprenticing under a journeyman or master electrician. I'm quite peeved I'm not under someone wing as it were.

I also seem to remember reading about a second ground rod "6 feet or more" away from the first in the NEC... I'll have to try and hunt that one down as well.

Thanks Ben for the comments. Every bit helps.
 
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Old 10-02-08, 09:58 AM
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Originally Posted by cdavies View Post
I was planning to use a 125A breaker in the meter center. The meter center will have a 200A main breaker.
I wouldn't do that. 1/0 AL for Services is rated at 125 amperes. As ben said, you need to derate somehow for sub panel feeders. Still trying to figure that one out.

Plus, pulling 125 amp over 250' feet will give you crazy voltage drop. I could figure that one out fairly easy. You should stay under 3% at your sub panel, not to exceed 5% on feeder plus branch circuits in the shop.

Load demands aren't my forte'... Someone else can help you out with that and determine what the best breaker is for your main panel feed.
 
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Old 10-02-08, 10:32 AM
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Originally Posted by cdavies View Post
Does this ground wire have to be insulated?
Can it be in the same trench as the primary feed?
The ground wire (equipment grounding conductor) must be in the same cable assembly as the 2 hots and neutral (grounded conductor). Ref NEC 2008 Article 300.3(B) Conductors of the Same Circuit.


Originally Posted by cdavies View Post
Also, regarding ground. Is a ground rod required at the workshop. If so, how deep should it be?
Yes. In fact you have to use 2 ground rods. The ground rod is driven to a depth just below grade. The ground rod clamp must be rated for direct burial.


Originally Posted by cdavies View Post
If I choose to run the 1/0 cable in PVC conduct, as opposed to direct burial, could a convential single conductor #6, not rated for direct burial, be used in the same conduit?
That's a question you should ask your inspector.


Originally Posted by cdavies View Post
Your point is well taken on the voltage drop. What is an acceptable voltage drop?
For 3% voltage drop, I used 50 amps for maximum load @ 250 feet with 1/0 aluminum & came up with a voltage drop of 5 volts, which is acceptable. According to the VD calculator, you could have a maximum load of 70 amps and still be okay.


Originally Posted by cdavies View Post
As some background on the operating conditions of the building. Concurrent equipment demand would be several 220V units (Air Compressor, Dust Collector, 2.5 Ton A/C, Table Saw) plus 110V lighting.
You're okay as long as the combined loads do not exceed 50 - 70 amps. This includes 125% for a continuous duty load, AC for example. Continuous duty is 3 hours or more.

Can you supply us with the equipments nameplate data?
 

Last edited by thinman; 10-02-08 at 10:56 AM.
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Old 10-02-08, 10:55 AM
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Originally Posted by cdavies View Post
If I choose to run the 1/0 cable in PVC conduct, as opposed to direct burial, could a convential single conductor #6, not rated for direct burial, be used in the same conduit?
Yes it could, but the issue of allowing an equipment ground outside the cable assembly still remains up to the inspector.

Your point is well taken on the voltage drop. What is an acceptable voltage drop?
Generally speaking you do not want to exceed 3%. At 125A load, your voltage drop was up near 6%. With a 100A load the voltage drop is 3.5%. A tad out of tolerance, but probably acceptable. Voltage drop is not regulated by the code, it only suggests that you take it into consideration.

As some background on the operating conditions of the building. Concurrent equipment demand would be several 220V units (Air Compressor, Dust Collector, 2.5 Ton A/C, Table Saw) plus 110V lighting.
Probably all just fine on 100A. Do you have estimates on the HP of the tool motors in question?
 
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Old 10-02-08, 11:06 AM
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Originally Posted by madpenguin View Post
Has anyone ever tried to argue with an inspector that a trench could be considered a raceway?
I was thinking more along the lines of using individual conductors of USE-2 or other XLPE condcutors directly buried in the same trench without conduit which is done all the time. This situation is fundamentally the same thing.

I don't know whether there is an entirely new table than 310.15(B)(6) or whether your supposed to use 75c on 310.16..... Or something tricky along those lines.
It does involve 310.15(B)(6) and 310.16. The wording for 310.15(B)(6) changed to specify that it only applies to the first panel which carries the entire load of the dwelling or something like that which therefore excludes subpanels, especially those to outbuildings which are clearly not part of the "dwelling". I don't have a code book right here to give an exact quote.

That means you're left with table 310.16 for sizing the subpanel feeder just like you would with a branch circuit or non-residential panel. The 60 column would be used for wiring methods like UF or NM, and the 75 column would be used for the remaining wiring methods and terminations that support that temp rating.

I also seem to remember reading about a second ground rod "6 feet or more" away from the first in the NEC...
Technically, you must have one electrode with less than 25 ohms resistance to ground or two grounding electrodes. Since actually measuring or proving the 25 ohms is nearly impossible, installing two rods is easier for everyone involved. The exception to this is a UFER (concrete rebar) ground which can be used as the sole method. If the second rod is installed, it must be at least 6' from the first. Many (most?) inspectors seem to not care about the two electrode rule for outbuildings though, so perhaps another thing to ask.

Thanks Ben for the comments. Every bit helps.
No problem.
 
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Old 10-02-08, 11:15 AM
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A _trench_ is a _raceway_ if single conductors are used.
 
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Old 10-02-08, 11:21 AM
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Originally Posted by thinman View Post
The ground wire (equipment grounding conductor) must be in the same cable assembly as the 2 hots and neutral (grounded conductor). Ref NEC 2008 Article 300.3(B) Conductors of the Same Circuit.
Awsome. Bookmarked.

Yes. In fact you have to use 2 ground rods.
At the risk of being annoying, do you have an ART handy? 250.56 is the only thing I'm seeing which leads to 250.53(B)

Unless he is going to do a resistance test on the ground rod to be installed, how do you know if a second one is required?
 
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Old 10-02-08, 11:27 AM
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Originally Posted by thinman View Post
A _trench_ is a _raceway_ if single conductors are used.
Understood. But in context, the OP is using 3 conductor cable which is what I was referring to.

Thanks Ben and Thinman for the informative posts... I'll try to keep my nose out of this thread now....

For the OP if your interested:

http://ecmweb.com/mag/electric_cares_ohms_less/
 

Last edited by madpenguin; 10-02-08 at 11:48 AM.
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Old 10-02-08, 11:52 AM
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Originally Posted by ibpooks View Post
Technically, you must have one electrode with less than 25 ohms resistance to ground or two grounding electrodes. Since actually measuring or proving the 25 ohms is nearly impossible, installing two rods is easier for everyone involved.
Ben summed it up.


Originally Posted by ibpooks View Post
Many (most?) inspectors seem to not care about the two electrode rule for outbuildings though, so perhaps another thing to ask.
In Washington state, we're required to always drive two grounds if we don't do the ground resistance test.
 
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Old 10-02-08, 01:31 PM
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Originally Posted by ibpooks View Post
Yes it could, but the issue of allowing an equipment ground outside the cable assembly still remains up to the inspector.



Generally speaking you do not want to exceed 3%. At 125A load, your voltage drop was up near 6%. With a 100A load the voltage drop is 3.5%. A tad out of tolerance, but probably acceptable. Voltage drop is not regulated by the code, it only suggests that you take it into consideration.



Probably all just fine on 100A. Do you have estimates on the HP of the tool motors in question?

I just met with the inspector and he gave me the approval to use the 1/0 and a #6 both in the same conduit. He said he would approve it up to 125A. It's my call as to whether I want to go that high or be conservative and go with 100A.

On the equipment side, here's what's planned.

Concurrently operating equipment example
Table Saw: 3HP 220V
Dust Collector: 2HP 220V
Air Compressor: 7HP 220V
A/C: 2.5 Ton

I have other woodworking equipment (Planer, Jointer, Sander, Lathe, etc) with power requirements similar to or less than the Table Saw, but they would not run concurrent to the Table Saw. This is not a production shop, but rather a sinle user hobby shop.
 
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Old 10-02-08, 02:16 PM
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Originally Posted by cdavies View Post
I just met with the inspector and he gave me the approval to use the 1/0 and a #6 both in the same conduit. He said he would approve it up to 125A. It's my call as to whether I want to go that high or be conservative and go with 100A.
That's all good news. Personally, I would stick to the 100A on the principle that if you do actually use that extra 25A it will be at the expense of your motors running too hot. I'd rather have the breaker trip and tell me I'm using too much power than run everything at once and overheat the bearings in my motors.

--> Air Compressor: 7HP 220V <--
Usually air compressor motors are vastly overstated, but if it is actually a 7HP motor this will take up a big chunk of your power. The running current on an actual 7HP motor would be about 40A. The truth is that the motor is probably more like 2-3HP.
 
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Old 10-02-08, 02:38 PM
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Originally Posted by ibpooks View Post
That's all good news. Personally, I would stick to the 100A on the principle that if you do actually use that extra 25A it will be at the expense of your motors running too hot. I'd rather have the breaker trip and tell me I'm using too much power than run everything at once and overheat the bearings in my motors.



Usually air compressor motors are vastly overstated, but if it is actually a 7HP motor this will take up a big chunk of your power. The running current on an actual 7HP motor would be about 40A. The truth is that the motor is probably more like 2-3HP.
You're absolutely correct about the compressor. The 7HP rating is a peak rating. The unit is only rated at [email protected] So from your perspective, a 100A service should be more than suficient, correct?
 
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Old 10-02-08, 03:45 PM
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Yes I agree that the 100A panel is sufficient to meet your needs.
 
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