Connecting Subpanel to Main Service Panel

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  #1  
Old 07-17-04, 11:56 AM
darrink
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Connecting Subpanel to Main Service Panel

Hello,
This is my first post. I have read a lot on this forum, and in turn have learned a lot. I feel confident that I can wire a subpanel to my shop, 90' away, from my main service panel feeding the house.

I have a 200A panel to install in the shop. This is probably over kill and will most likely never be used to it's full potential. I have 200A service feeding my house, and plan on running some basic hand a tools in the shop. The biggest items being a table saw and a 6 HP air compressor.

When this 2400 sq ft house was originally built in 1998 (not by me), the main service panel did not have enough capacity as far as spaces is concerned. Therefore, a 4 space subpanel is already installed next to the main service panel. This subpanel is supplied by a 60 amp breaker, and feeds both outside AC units. One 3 ton and one 4 ton, and the water well.

My intentions are to remove the original subpanel and replace it with a 200A sub panel, with feed thru lugs and a 200A main disconnect in it, and use the feed thru lugs to feed the subpanel in the shop. I hope all of this is ok so far. My question is, can I tie into the main panel before the main disconnect. This would be done by removing the meter, or should I just replace the 60A breaker feeding the subpanel with a larger one like a 100A or 150A?

Thanks,
Darrin
 
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  #2  
Old 07-17-04, 01:20 PM
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Calculate then plan.

If your estimation of your present and future use of electricity in your shop is accurate then you don't need a two hundred ampere feeder. You need to do a service calculation for the house and for the shop. The results of that calculation will tell you the size of service you need to supply the demand. Here is my suggestion as to how you might proceed.

If the 200 ampere service is adequate to supply both the house and the shop then install a two hundred ampere feed through panel either inside or outside of the house near the meter enclosure. The new two hundred ampere panel becomes the service equipment. Its feed through lugs will supply the feeder to the existing panel in the house. The new panel will then become the home for most of the 240 volt loads. The well, the shop feeder, the two air conditioning units, and any other larger 240 volt load will go in the new panel. The service entry conductors will be terminated on the new panel's main lugs. A new feeder will be run from the feed through lugs to the existing panel.

The decision as to whether to use an indoor or outdoor location for your new service equipment will be governed by whether the meter location makes a good origin point for the shop feeder.

The feeder for the new shop would be sized in accordance with the shops load calculation. I can't imagine that, given the loads you described, you would need more than a forty ampere feeder. If you want to hedge your bets against future expansion of shop use, then put in a sixty ampere feeder. Installing only the feeder that you actually need will save you a lot of money. One good way to hedge your bets would be to install the feeder in conduit that is sized for the largest load that you can imagine needing in the shop building. You then pull in to the conduit only the size of feeder that you need right now. With conduit in place it is very easy to install a larger feeder later if the larger load actually materializes.
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Tom H
 
  #3  
Old 07-18-04, 09:11 AM
darrink
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Hornetd,
Thanks for the reply. I had thought of that idea, but was skeptical, because the wires coming out of the wall into the existing main service panel wouldn't be long enough to reach into the new main service with the feed thru lugs. My main service is on the outside of my house, and it is the most logical place for the feeder for my shop.

I have already have enough 2 1/2" conduit, 200' of 3/0 and 100' of 1/0 copper to supply power to the shop. If I did my research correctly, this should be adequate to supply at least 150A service to the shop if I ever see the need. This would be done by installing another meter for the shop alone.

After reading all of the post on here about a grounding conductor, I am probably going to go ahead and purchase another hundred feet of wire so I can have 2 hots, 1 neutral and a ground. The ground and the neutral will not be bonded at the shop. Right now, the shop is not tied to the house with any metallic non-current carrying material. I do plan on having a phone line and a network cable running to the shop in the near future, so to be safe, I feel it would be best to go ahead and run a grounding conductor now.

I do plan on running a 240V welder in the future, but once again, I will be the only person in the shop at any given time. The only thing that could possibly be running besides the welder would be the air compressor and lights. The welder at full capacity is capable of 50 amps, and the 6 HP air compressor I am not sure of the amp draw.

Now I have several question:

1. If I used this idea, could I splice the wires to make them reach the new main service, or into the new subpanel, depending on where I would install the new panel? If so, how would I splice them? I don't really like to splice wire of any kind.

2. Is the size of the wire I have adequate to supply 150A service to the shop in the future?

3. What size wire would I need for a grounding conductor to match the capacity of the 3/0 and 1/0?

4. This question I really hate to ask, because it seems to be a big controversy in the interpretation of NEC. Should I install a secound ground rod at the shop?

Thanks,
Darrin
 
  #4  
Old 07-18-04, 12:35 PM
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1. If I used this idea, could I splice the wires to make them reach the new main service, or into the new subpanel, depending on where I would install the new panel? If so, how would I splice them? I don't really like to splice wire of any kind.
One good way to do this is to locate your lighting and appliance distribution panel inside the house were the branch circuit wires would already be long enough to terminate in that panel.

If you do have to splice them that would occur in a junction box. Something I did on one panel replacement was to mount a 24" X24" junction box in the adjacent area and mount barrier strips inside of that. Since I used marathon bowe terminal strips which have pressure washers under each screw they were very easy to use. That made extending the circuits go a lot faster than using wire nuts.

2. Is the size of the wire I have adequate to supply 150A service to the shop in the future?
Yes it is. It is large enough to supply a two hundred ampere service if needed.

3. What size wire would I need for a grounding conductor to match the capacity of the 3/0 and 1/0?
The size of the Equipment Grounding Conductor for the largest Over Current Protective Device (OCPD) that the code would permit for that size of conductor is #6. But the code requires that when the conductors are increased in size to compensate for voltage drop that the EGC must be increased in size to the same proportion. Since you will be using a smaller breaker than 200 to protect the feeder the inspector may want you to increase the size of the EGC by the ratio of difference between the size that would be required for the breaker you actually use and the 3/0 wire. Your position should be that the wire was increased in size to allow for future expansion of demand and not to compensate for voltage drop.
4. This question I really hate to ask, because it seems to be a big controversy in the interpretation of NEC. Should I install a second ground rod at the shop?
Yes, you should have a minimum of two ground rods at the shop.

If you dig your trench down to a depth of at least thirty inches you could run a bare number two copper conductor directly buried in the trench to serve as a supplemental grounding electrode for both buildings. You would still need to install two ground rods because the code does not recognize the bare number two as a grounding electrode unless it is installed so as to circle the building. If you strike rock bottom when driving the ground rods you are permitted to drive them at a forty five degree angle or bury them in a trench 30 or more inches deep.
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Tom H
 
  #5  
Old 07-18-04, 03:36 PM
darrink
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Horntd,
It would not really be feasible to mount the panel in the house, this is a two story house and the wall just opposite the outside panel is in a bedroom.

Is Marathon Bowe a brand of terminal strip? I did a search on Google and came up with nothing. I would really hate to have to hang another panel on the side of the house. Is there any other way to splice or lengthen the wires that you know of?

Thanks,
Darrin
 

Last edited by darrink; 07-18-04 at 06:53 PM.
  #6  
Old 07-19-04, 08:40 PM
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I take it that there is no basement or crawl space. You could install a large J-box in the bedroom wall and then cover it with anything that can be removed without taking apart the wall.

Yes marathon is a brand of terminal strip. There is no safe way to splice the wires without mounting a junction box. Paratrooper splices are not a desirable thing there is no way to make a code compliant splice that is supported by nothing but the air.
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Tom
 
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