Subpanel question

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Old 11-17-13, 08:36 PM
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Subpanel question

I have 150A service in my house, and a 100A breaker in the house's panel that feeds the garage. The garage has a 100A panel (one of the cheap $20 6 position boxes with no main disconnect). This panel currently has a breaker for lights, 2 breakers for outlets on a tandem breaker, and a 30A 240 breaker for a compressor. I have 2 spaces left. I'm wanting to put another 100A breaker in the remaining space and place another one of the same panels by the furnace to power it.
The question is can I put a 100A breaker in a 100A panel to feed a subpanel? The lights and outlets breakers are barely pulling a load as it is. My other option is to replace the panel in the garage with a full sized 100A panel, but I'm trying to save about $80.
Thanks
Greg
 
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Old 11-17-13, 08:47 PM
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The question is can I put a 100A breaker in a 100A panel to feed a subpanel?
Yes ...........................................
 
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Old 11-18-13, 08:23 AM
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Read the label on the 100A garage panel. Look for stab rating and feed through lug kit. These determine your answer.

Stab - the piece of metal formed from the power bus that the breaker slides onto. Abutting left and right breakers slide onto the same stab. The sum of those two breakers cannot exceed the stab rating.

Feed through lugs are cheaper and more durable than 100A breakers. They attach at the ends of the power buses.

If this is an attached garage then a junction in the wires just ahead of the breaker is possible.
 
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Old 11-18-13, 09:22 AM
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I'd prefer to feed through this panel onto the next panel. The description on Square D's website says "Feed Through Lugs: Yes", although there is only one lug for each hot bar and one lug for the neutral. Not sure how I'd attach a second set of wires to the bus bars. The panel is a Square D HOM612L100SCP.

[ATTACH=CONFIG]21485[/ATTACH]
 
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Old 11-18-13, 09:39 AM
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You do not need a 100 amp sub panel to feed a furnace. One circuit should be enough. Is the furnace gas fired or is this actually part of a heat pump?
 
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Old 11-18-13, 09:57 AM
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If saving money is the goal you would install one 20A breaker & a few feet of 12ga cable to the furnace rather than a 100A breaker, + 6ga. wire + the lug panel + a 20A breaker + a short length of 12ga cable to the furnace.

There must be more to the story?
 
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Old 11-18-13, 10:01 AM
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I haven't got the furnace yet. I'm looking for a residential electric furnace, hoping to get one off a jobsite within the month. Probably somewhere between a 15kw and 20kw unit. I will need a 15A circuit for the blower and control panel. Then depending on the unit, I could need several double pole breakers, depending on how many elements are in the furnace.

I live in the middle of nowhere. No natural gas, heatpump wont work because I need my garage heated in very cold weather, and electricity is cheaper than propane or oil (at least in my area). So resistance electric heat is my best, cheapest option.

BTW the main house is heated with a woodburner only (so no worry about overloading the main with two electric furnaces running simultaneously)

Thanks again.
 
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Old 11-18-13, 10:41 AM
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With thoughts of adding that kind of load you might be looking at a service upgrade.

I would keep large loads like that in your service panel, not a sub panel, especially with other loads.
 
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Old 11-18-13, 11:07 AM
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You still haven't told me if this is an attached garage. I need to know.

The description on Square D's website says "Feed Through Lugs: Yes"
Really!? Far out. Get the feed thru lugs kit and it will be obvious how they attach.
Not sure how I'd attach a second set of wires to the bus bars.
My guess is you take the wire capture screw out of the lug, which exposes the screw that holds the lug on. It's usually a torx or allen head. Then you replace the lug with a double lug.

x2 with pcboss/service upgrade. At the very least you need a residential load calculation and a sub panel load calculation.
 
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Old 11-18-13, 12:23 PM
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It's a detached garage, appx 100 ft from the house.
I know I'm going to be very close with only 150A service total, and only 100A feeding the garage. I'm trying to make due with what I've got though, and depending on the furnace I get, I might be able to rewire to eliminate one of the stages, thus saving power. If everything was on at once, I can definitely see needing more power. However, I understand I would have to switch the heat off to fill the compressor tanks or use a welder. I'm not worried about that, really.

So i can't put the furnace in the service panel because I'm too far away from the house. My options at this time seem to be to feed through the current subpanel in the garage and place another subpanel next to the furnace. Or, replace the current subpanel with a 100A panel with plenty of spaces. The first option is MUCH cheaper ($20 box vs $80 box, short runs from breakers to furnace, I already have extra 100A wire to install new box)

Thanks
Greg
 
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Old 11-18-13, 12:46 PM
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The 20kw of heat is over 80 amps alone.
 
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Old 11-18-13, 12:53 PM
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Yeah I know. I'm actually shooting for around 15kw.
I know having the heat on will almost max out the 100A panel. As I said I'll shut the heat off to run the compressor or welder. Other than those two things, the only other thing on the 100A panel is a few lights and a radio.
 
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Old 11-18-13, 02:55 PM
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Probably somewhere between a 15kw and 20kw unit.
Most 15 KW furnaces today are two circuit furnaces; 1 - 60 amp and 1 - 30A. A single circuit 15 KW usually requires either a 90 or 100 amp breaker.

Most 20 KY furnaces are two circuit furnaces too; 2 - 60A circuits. A single circuit 20 KW furnace usually requires a 125A breaker.
 
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Old 11-18-13, 09:03 PM
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Sounds like ill be looking for a 15kw. The panel I have does not support a feed through lug so I plan on putting a 100a breaker in the existing panel, and put another 6 space panel next to the furnace.
Thanks.
 
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Old 11-19-13, 07:56 AM
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Sounds like ill be looking for a 15kw. The panel I have does not support a feed through lug so I plan on putting a 100a breaker in the existing panel, and put another 6 space panel next to the furnace.
I think you'll be sorry within a few years to have used the cheapy aluminum bus panels with such a heavy load. I think I'd bite the bullet now and get a decent copper bus loadcenter to replace the existing subpanel and forget adding another subpanel.

I'm trying to save about $80.
$80 is nothing really when you compare your future utility bills using a 15 KW electric furnace in a garage.
 
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Old 11-19-13, 08:29 AM
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Another question is what size/type of cable will you be using for the 100' run? 100 amps on 2 gauge aluminum works out to 600 watts of loss.
 
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Old 11-19-13, 12:27 PM
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The 100A feed from the main panel to the garage is 2-2-2 aluminum. It's actually the 100a service wire that used to feed our single wide trailer. It's already installed and in the ground with a concrete pad and a deck on top of it... not too easy to fix that now.

I'm strongly considering a propane furnace instead.
 
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