sub-sub panel questions

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  #1  
Old 03-10-07, 10:51 PM
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sub-sub panel questions

It's a long story, but I need to find a place to connect 8 additional circuits of NM wiring already run to a subpanel that is already completey full and will not accept tandem (dual) breakers.

MY plan is to install a sub panel to the sub panel to accept these circuits. SEveral questions:

1. Can I have a sub to the sub?
2. While I'm at it, it makes sense to put the new subpanel about 20' away from the existing subpanel. I imagine I need a box of some sort next to the existing subpanel inside of which I can terminate and extend the wirinmg to the new panel. What do you call this box?
3. I'd like a reccomdation for the ampacity of the breakers in the existing panel that will be protecting the new sub panel. These 8 circuits in the new subpanel are for my 1-man, hobby woodworking shop and include: 220v 20 amp circuits for 3 hp saw, 110v 15 amp dedicated for 1.5hp dust collector; 110v 15 amp dedicated for air purifier; 5 110v 15 amp ciruits for all of the many wall, bench and ceiling outlets . Plus one or two spaces for future circuit expansion? All lighting circuits will stay in the existing subpanel.
4. Based on breaker reccomendation--what wire size and type would be more than adaquete to feed the new subpanel?
5. I know it will be redundant, but I'd like to have a disconnect / main switch in the new sub-panel. Any problem with having it breakered twice? Is the best way to accomplish this a 100 amp, 8 space/16 circuit panel with a main lug?

Thanks much
 
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Old 03-10-07, 11:02 PM
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First off, What is your main service? If it's not 200, I would upgrade to a 200 amp with a 40 space panel. It sounds like you have things very ... crowded. After it's all said and done putting in a 40 slot main panel and hopefully elimanting the subs may be easier and cheaper.
 
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Old 03-11-07, 07:25 AM
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To make a long story longer: The main panel is 200 amp / 40 space located in the original basement of the home. It feeds the 100 amp / 30 space subpanel in the new basement 60' away. These panels were both recently installed when we added 1000 square feet to our home.

Original quote from contractor called for 100 amp / 20 space subpanel to service the new addition (large kitchen, 2 baths, bedroom, family room, dining room, study, basement storage and shop).

During the walk-thru with the electrician (where I sited outlets, switches, AC pad, etc) I informed him that I would be building-out most of the basement for a wood / hobby shop and needed at least 10 spaces for stationay tools, lot's of outlets, lots of lights, a small welder and some future expansion. He said 100 amp panel was fine but he suggested a 30 space for a small up-charge.

Because of delays with other trades, it was 6 weeks from the point he installed the subpanel and when he completed the job. In the mean time I pulled my own permit for the shop build-out and I installed all my receptacles and ran all my wire back to the box. I trimmed everything to length in the panel and tested the circuits with a breaker.

Imagine my surprise when they finished the electrical and there were only 4 spaces available. I reminded the electrician about the 30 space panel upgrade and 10 spaces for the shop and he said it took more spaces that he expected for the addition. He suggested I get tandem breakers which this panel will not support.

Anyways--I'd appreciate someone looking at my original 5 questions.

Thanks
 
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Old 03-11-07, 07:58 AM
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I will take a stab at your questions with a caviet.

1. Can I have a sub to the sub?

Yes

2. While I'm at it, it makes sense to put the new subpanel about 20' away from the existing subpanel. I imagine I need a box of some sort next to the existing subpanel inside of which I can terminate and extend the wirinmg to the new panel. What do you call this box?

A junction box. It will need to be sized in accordance with article 314. You may be better off to use several smaller boxes located in the ceiling in various locations.

3. I'd like a reccomdation for the ampacity of the breakers in the existing panel that will be protecting the new sub panel. These 8 circuits in the new subpanel are for my 1-man, hobby woodworking shop and include: 220v 20 amp circuits for 3 hp saw, 110v 15 amp dedicated for 1.5hp dust collector; 110v 15 amp dedicated for air purifier; 5 110v 15 amp ciruits for all of the many wall, bench and ceiling outlets . Plus one or two spaces for future circuit expansion? All lighting circuits will stay in the existing subpanel.

Lets look at this in list form:
Saw...17amps at 240v = 4080
dust...20 amps at 120v = 2400
air... we do not know
general purpose recs. we do not know how many. I would use 180va each, the code does not require any load for this in a residential building.

4. Based on breaker reccomendation--what wire size and type would be more than adaquete to feed the new subpanel?

Lets get the answers to the questions above then take another look at this.

5. I know it will be redundant, but I'd like to have a disconnect / main switch in the new sub-panel. Any problem with having it breakered twice? Is the best way to accomplish this a 100 amp, 8 space/16 circuit panel with a main lug?

If you want the extra disconnect means at the new sub panel, I would use a main breaker panel, not a main lug panel.

The caviet is that you have already added alot to your 200 amp service and you are now adding even more. I reccomend that you do a new demand load calculation on your entire house, with a breakdown demand load calc for the new panel in the addition. I have concern that the entire system, or the new sub panel, may already be near capacity, and it may not be safe to add these loads.
 
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Old 03-13-07, 10:23 AM
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In that particular situation, I'd put a sub-panel in the shop proper, and run it is a sub off the main.
 
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