100 amp breaker within 100amp panel


  #1  
Old 02-29-04, 12:40 AM
neff2k
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100 amp breaker within 100amp panel

Getting ready to run 100 amp service to the garage. Currently I only have 100 amp service into the house. Eventually I will be upgrading to 200 amp. So my thought was to just add a 100 amp breaker to the main panel, then once the 200amp service/panel is installed I would be fine. Currently there is no chance of ever pulling 100 amps from the garage, we are getting a fifth wheel with 50 amp service in it, so I figured while I was doing this I might as well plan ahead. Am I wrong thinking this shouldn't be a problem? Or should I really just stick to a 60amp subpanel in the garage and install a 60amp CB at the main for it?

Thanks
 
  #2  
Old 02-29-04, 09:01 AM
W
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In general there is no problem putting any size breaker in a panel, meaning that in theory you could have 100A service with a 100A main breaker and then install a feeder with a 100A breaker to a sub panel. Your main panel might have manufacturers instructions which limit the total installed breaker capacity. It may also be the case that there are not 100A breakers available for the panel in question.

However, even if you install a '100A' subpanel, with wire suitable to carry 100A, there is no requirement that you feed this circuit with a 100A breaker. A 100A panel is simply a panel _capable_ of carrying up to 100A, similarly wire for a 100A panel is simply wire _capable_ of carrying up to 100A. The breaker that you use to protect the subpanel and the wire can be rated less than 100A.

There are requirements that the breaker be sufficient for the load that you are serving. But you can install a smaller breaker now, sized for your current load, and install the higher capacity wire and subpanel, planning for the future. You may even find that a '100A' panel is cheaper than a '60A' panel, simply because of what is most popular. 100A wire will be more expensive than 60A wire, however. One caveat: the 100A wire is _thicker_ than 60A wire, and you will need to make certain that the breaker that you use is rated to take the thicker wire.

In addition, when you upgrade to the 200A service, you might change panel brands, in which case the breaker that you install to feed your subpanel might not be useable in the new panel. I would recommend that you try to get a sub-panel that uses the same breakers as the upgraded service panel that you will be installing in the future.

-Jon
 
  #3  
Old 02-29-04, 04:09 PM
neff2k
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Thanks for the info.. I actually went ahead and picked up a 60amp circuit breaker to put in the main panel today which . I already have the 100amp breaker for the panel in case it doesn't fit. I am running 4 guage copper from the main to the garage. As for when I upgrade to 200amp service, both panels are Square D QO panels as are the breakers (Really like this brand). So I will be able to just use the same breakers in the 200amp panel I will purchase.

I am looking at having the service changed over sometime this summer so there shouldn't be a problem. Again I appreciate your information.

Thanks,
Darrell
 
  #4  
Old 02-29-04, 06:36 PM
W
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Darrell,

I'd suggest that you read the thread http://forum.doityourself.com/showth...hreadid=160442

In short, it is highly unlikely that the 4ga wire that you've run would be suitable for a 100A subpanel. It will be good for at most 70A, possibly 85A.

-Jon
 
  #5  
Old 02-29-04, 06:48 PM
neff2k
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Hmm ok, now I am thrown. So I guess I should be going to #2. Well there is $60 down the drain for the other cable. It is either that, or scrap the whole 100amp to the garage and go with 60amp. So when going to #2 should I be using alum or copper?

Thanks,
Darrell
 
  #6  
Old 02-29-04, 07:04 PM
W
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The recommendations in the other thread were for copper. In general and very approximately, the current capacity of copper and aluminium two sizes thicker are the same.

#2 copper has an ampacity of 95A and can be protected with a 100A breaker.
#1/0 aluminium has an ampacity of 100A.

If you have any special circumstances (eg. a long run, high ambient temperatures, etc.) then you might want to speak up now prior to buying any more wire.

Also, what kind of wire did you get? How many separate conductors?

-Jon
 
  #7  
Old 02-29-04, 07:36 PM
neff2k
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The run is a max of 40-45 ft. I purchased three seperate conductors of 4 guage insulated twisted copper from Menards. As far as ambient temps, we live in northeast Illinois so nothing too dramatic at all, and the cable will be buried 24" within 1 1/2 conduit so plenty cool. But I will head out this week and pick up some 2 guage of the same.

There seems to be a couple different sites with conflicting information about the ground. Should I be running a seperate ground bar at the subpanel? (This is what I have planned for) Or should I be running the ground back to the main panel? Or should I be doing both running a ground back to the main panel and dropping a ground bar at the subpanel as well?

I really do appreciate all the time and help you have offered. Greatly apprecaited.

Darrell
 
  #8  
Old 02-29-04, 07:48 PM
J
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There is one very critical piece of information we need before we can answer any of your questions. Is the garage detached or attached?

If the answer is "detached", then there is one more very criticial piece of information needed. Are there any other metallic paths between the buildings? This includes water pipes, gas pipes, TV cable, phone lines, air ducts, etc.

You may be getting conflicting answers for one of two reasons: (1) Different people are making different assumptions about the answers to the above questions, and/or (2) There is more than one legal option.

There are about 200 things you need to know to do this job correctly. Don't buy anything else or do any more work before you learn the rest of them.
 
  #9  
Old 02-29-04, 08:01 PM
W
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Based upon distance and ambient conditions, I figure that #2 wire is just fine.

Make certain that the wire that you get is rated for wet conditions. It will have type letters of THWN, THWN-2, XHHW, or XHHW-3, among others.

You _must_ install a grounding electrode ("ground rod") at the garage.

As far a running an equipment grounding conductor back to the main panel, you have a choice:
(1) you can run a ground wire back to the main panel. In this case you will need to have _separeate_ ground and neutral bars in the subpanel. For your installation, you would need a #8 CU ground wire at a minimum.
(2) if you have _no_ grounded metallic paths between the garage and the main building (eg. no pipes, no fence, no conduit), and if the garage is _detached_ then you can install a ground rod at the garage, and bond it to the neutral at the subpanel as if this were a service entrance panel.

I believe that the consensus on this board is that (1) is the desired method. My personal opinion is (1) is best unless the garage is much further away (hundreds of feet).

More details on grounding are beyond my personal experience.

-Jon
 
  #10  
Old 02-29-04, 08:04 PM
neff2k
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The wire I have is type RWH-2.

Completely detached garage with only a single 12-3 wire coming in through a small conduit. But this wiring will be completely removed once this panel is brought in.
 

Last edited by neff2k; 02-29-04 at 08:25 PM.
  #11  
Old 02-29-04, 08:16 PM
J
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Winnie gave you the gory details. I'll summarize your best option (not your only option--just what I think is your best option).
  • Run three insulated conductors (#2 copper THWN) plus a grounding wire (#8 copper).
  • Bury them 18" deep in 1.5" PVC electrical conduit.
  • Keep neutrals and grounding wires separate at the subpanel (i.e., throw away the green screw and buy a grounding bar).
  • Install a grounding rod and connect it to the subpanel grounding bar with #6 copper.
 
  #12  
Old 02-29-04, 08:34 PM
neff2k
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Alright gotcha..

1 - Going to get the #2 copper THWN

2 - Have the 1 1/2" electrical conduit and already have the 24" trench dug.

3 - Already have the seperate grounding bars

4 - Already have the grounding rod and #4 copper to run to the rod from the sub panel.

Now if I could just get the weather back that we had this weekend.

Darrell
 
  #13  
Old 02-29-04, 08:34 PM
W
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"The wire I have is type RWH-2"

RWH-2 should also be acceptable for this application. There are a whole slew of different type codes for wires. RWH-2 is a wire that is rated to 90C in wet locations.

-Jon
 
 

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