Sub-Panel in Detached Garage ..... A Few ???'s

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  #1  
Old 03-15-11, 12:50 PM
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Sub-Panel in Detached Garage ..... A Few ???'s

Hey guys, in the not too distant future I'll be installing a sub panel in my detached garage. I'd like to run my plans by you too see if I'm on the right track. I've also got a few questions.

The subpanel in the detached garage will be 70 feet from the main 200Amp panel. The electical usage will likely never exceed 40 amps at any given time, its going to be a woodworking shop.

I plan to install a 100A panel (overkill) in the garage and plan to have the following circuits. Also listed is thier actual draw according to the manufacturers. All 220V and 110V circuits will be 20A.
220V
Dust Collector - 10A
Table Saw - 8A
Belt Sander - 10A
Shaping Table - 8A
Air Compressor - 18A

110V
General Shop - 20A MAX
Lighting - 20A MAX

The most I see myself drawing is 35-40A, and thats a high estimate. I would only use one woodworking machine at a time with the dust collector and the air compressor would always be used by itself. Even if I have a dehumidifier, a fan, small fridge, and all the lights on I don't see myself exceeding 40A while operating a huge belt sander and dust collector.

That said, I was planning to use 6/3 wire to run the 70'. When it exits the house it will buried at least 18" in PVC conduit before going into the 100A panel.

My understanding is that 6/3 is rated for 60A. Using the CM = (25*I*L)/V equation I am going to have a voltage drop of [email protected]

Should I install a 50A or 60A breaker at the main panel?

Thanks and I look forward to your suggestions!
 
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Old 03-15-11, 01:03 PM
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100 amp panel....main breaker or main lug??? The 6/3, please advise what this is. Are you referring to 6-3wg romex or 4 runs of single conductor?? The romex is rated 55 amps, and in conduit perhaps less.

Moderator Addendum: On a detached garage if cable is used it would have to be UF not Romex for the portion outside.
 

Last edited by ray2047; 03-15-11 at 02:32 PM. Reason: Add information.
  #3  
Old 03-15-11, 01:13 PM
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Originally Posted by BigOldXJ View Post
I plan to install a 100A panel (overkill) in the garage
It's no problem to use a large panel for the additional space. This will need to be a main breaker panel to supply the mandatory building disconnect. You will also need to isolate the neutral bar by removing the bonder; install a ground bar kit; and drive two ground rods with acorn clamps connected to the ground bar with bare #6 copper.

and plan to have the following circuits.
Each of those machines sounds okay. You may need to bump up to 30A circuit with #10 wire for the compressor for startup.

That said, I was planning to use 6/3 wire to run the 70'. When it exits the house it will buried at least 18" in PVC conduit before going into the 100A panel.
If you're going with conduit, go ahead assemble the full conduit system then pull individual THHN conductors in black, red, white and #10 green. If you want to use cable, get a direct burial cable like UF-B or USE-2. Romex NM-B cannot be used outdoors or underground.

My understanding is that 6/3 is rated for 60A...Should I install a 50A or 60A breaker at the main panel?
The 6-3g UF-B is rated at 55A which you can round up or down to 50A or 60A, your choice. I see no reason not to do 60A in this case.

If you decide to pull THHN through the conduit go with the 60A breaker.
 
  #4  
Old 03-15-11, 01:34 PM
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Originally Posted by JimElectric View Post
100 amp panel....main breaker or main lug??? The 6/3, please advise what this is. Are you referring to 6-3wg romex or 4 runs of single conductor?? The romex is rated 55 amps, and in conduit perhaps less.
The 100A panel is a main lug, I didn't know there was a choice. I didn't think your ordinary subpanel could be a main breaker.

6/3 is your typical electrical wire found at your local big box store, looks like this:


I'm realizing Romex is not prefered (but apparently safe) when pulled through conduit. I'm noticing that the NEC2008 code does say that 55A is the rating for 6 gauge copper per Southwire's specs. How do you determine the derating when in conduit?

I'm not opposed at all to using 4 individual wires in conduit. I suppose I could install a 50A breaker at the main panel and run 6/3 Romex to a junction box at the conduit and then run the individual wires from there. I was just hoping to buy a single product.

Keep the suggestions coming! Thanks again!
 
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Old 03-15-11, 01:46 PM
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Originally Posted by ibpooks View Post
It's no problem to use a large panel for the additional space. This will need to be a main breaker panel to supply the mandatory building disconnect. You will also need to isolate the neutral bar by removing the bonder; install a ground bar kit; and drive two ground rods with acorn clamps connected to the ground bar with bare #6 copper.
Didn't know this..thank you. I'm just begining to research installing a sub panel and most of my reading has contradicted what you just said. You're not wrong, I am...most of my reading was regarding subpanels within a few feet of the main panel, not in a detached building.

If you're going with conduit, go ahead assemble the full conduit system then pull individual THHN conductors in black, red, white and #10 green. If you want to use cable, get a direct burial cable like UF-B or USE-2. Romex NM-B cannot be used outdoors or underground.
I guess I shot my mouth off a bit too early with the Romex. I read that Romex through conduit was okay, but as you mentioned...not outdoors or underground. Is it okay to run 6-3 Romex to the conduit, and then THHN through the conduit and still install a 60A breaker?

Thanks for the reply! Excellent response!
 
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Old 03-15-11, 01:51 PM
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Originally Posted by BigOldXJ View Post
I'm realizing Romex is not prefered (but apparently safe) when pulled through conduit.
The problem is that it is very stiff and can easily be stripped pulling through conduit fittings. Also NM-B romex (pictured) is not approved for outdoor or wet locations as it is not waterproof or rated for sunlight exposure.

How do you determine the derating when in conduit?
Derating only applies when more than one circuit is in the conduit. In this case the only factor that matters is the temperature rating of the wire which is the 60C column if you are looking at table 310.16 in the NEC.

I'm not opposed at all to using 4 individual wires in conduit. I suppose I could install a 50A breaker at the main panel and run 6/3 Romex to a junction box at the conduit and then run the individual wires from there.
That is a good option. Another option would be to use UF-B for the whole run, which can be routed like romex through the house, but also direct buried. It only needs a short conduit sleeve to protect entrance/exit from the ground. This cable is very stiff though so really tough to pull through a full conduit. You could use 50A or 60A breaker at your discretion.

add: the major difference between the UF-B method and the THHN+conduit method is burial depth. 24" for UF-B or 18" for THHN in PVC conduit.
 

Last edited by ibpooks; 03-15-11 at 01:54 PM. Reason: addition
  #7  
Old 03-15-11, 03:42 PM
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Thanks guys...you've ironed out all the details for me! I've got two final questions.

1. I'm reading the NEC chart and it looks like the Ampacity for #6 THHN is 65A and #8 THHN is 55A. Does that sound correct?

2. Hopefully this doesn't sound too dumb....but does a detached garage count as outdoors? Or (as I suspect) outdoors means outside of a building structure?

Thanks again!
 
  #8  
Old 03-15-11, 04:15 PM
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Originally Posted by BigOldXJ View Post
1. I'm reading the NEC chart and it looks like the Ampacity for #6 THHN is 65A and #8 THHN is 55A. Does that sound correct?
If you use all conduit and THHN panel-to-panel you can use the 75C column which gives 50A for #8 copper and 65A for #6 copper. If you use a cable like UF-B, you are limited to the 60C column which gives 40A for #8 copper and 55A for #6 copper. The 90C column cannot be used in this situation.

Breaker sizes can be rounded up to the next standard size (e.g. 55A -> 60A).

does a detached garage count as outdoors?
Inside the garage is an indoor ("normally dry") location. The conduit and trench is considered an outdoor ("wet") location due to soil moisture and condensation.
 
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