basement subpanel

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  #1  
Old 05-23-01, 11:27 PM
MrFrog
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I get the feeling that I am going to re-open a can of worms here(looking at a previous post), but my situation is slightly different.

My plan is to install a 100 amp subpanel in the other half of my basement, about 30 feet from the main panel. This subpanel would supply a 220V/20amp circuit for an air compressor, and 3 or 4 110V outlets, but no light fixtures. I realize that 100 amps is probably overkill, but it leaves me room for expansion (welder, etc.). I figured to use a 2 pole, 100 amp breaker at the main panel as a disconnect, and separate breakers on the subpanel for the outlet circuits. My questions are as folows;

Am I on the right track?

What qauge wire would I need between the main and subpanels? (gulp!)

I have a 200 amp service, but lots of current-hungry stuff (A/C, electric range, dryer). How can I tell if I have enough capacity for a 100 amp subpanel?

The only open spots on the main panel are on the bottom (opposite the main lugs at the top). Is this OK, or do I need to locate the breaker closer to the main lugs?

This forum is great. It lets code-challenged people like me get accurate information ( and probably start a few arguments, too!
 
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  #2  
Old 05-24-01, 08:20 PM
raca
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Your on the right track but i dont think you have the capacity for 100 amp seeing your house has all electric appliances.I would go with a 60 amp sub panel thats fine for what your doing.Youll need #4SER cable aluminun which has 2hot,1neutral,and 1bare ground,if you use copper you can use #6 insted of #4 but good luck trying to find it, its not to common.Mounting the breaker in the bottom of the panel is fine just watch out for murray panels bottom spots might be different breaker type.Also make sure you seperate your grounds and neutrals in the sub panel put them on different buses theres usually a removable jumper to seperate the two in GE panels anyway dont forget the green bonding screw in the ground bus.good luck buddy
 
  #3  
Old 05-24-01, 11:21 PM
MrFrog
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Thanks for the info!

Looking at the main panel, I'm not even sure that there is enough capacity for even a 60 amp panel. How does one determine how much is available? If memory serves (which it usually doesn't!), the breakers on each leg cannot exceed a certain percentage of the rated capacity of the panel(?)
 
  #4  
Old 05-25-01, 03:57 AM
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Do a search for "demand load" in this forum. There are some excellent guidelines to use to determine the capacity of your panel.
 
  #5  
Old 05-25-01, 05:24 PM
Wgoodrich
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If you had a 100 amp main panel you could legally install a 100 amp subpanel off that main panel even if both panels are full of breakers.

The key to you situation is that you are not now adding that much load to that main panel. However if you load that sub panel in the future you may then have to upgrade your main panel to carry the extra load.

If you want to know approximately what you total demand load of your main panel is considering minimum service size required by the NEC answer the following; I will be glad to calculate your demand load for you setting your minimum main service size as you have it loaded be sure to include your current added loads if asked for below.

What is your total square feet of your dwelling by outside demensions excluding unfinished basements or attached garages.

What type heat do you have and what is the amp or KW rating found on the name plate of your heating unit.

What is you amp load rating of your central a/c unit?

Do you have an electric range or electric dryer?

Make yes or no and how many of the following fastened in place appliances.

WTR. HEATER
KIT. DISPSL
DISH WASHER
HOOD FAN
HOT TUB
WHIRLPOL TUB
ATTIC FAN
WTR. PUMP
LIFT PUMP
SUMP PUMP
PADDLE FANS
WINDOW A/C


Anything else fastened in place that I did not ask for like welders, air compressors etc.?

Wg
 
  #6  
Old 05-26-01, 04:43 PM
MrFrog
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basement subpanel

wg,

The answers to your questions are as follows;

What is your total square feet of your dwelling by outside demensions excluding unfinished basements or attached garages.

2259 square feet.

What type heat do you have and what is the amp or KW rating found on the name plate of your heating unit.

oil fuel/hot water baseboard, burner is 5.8 amps, circulation pump is not marked.

What is you amp load rating of your central a/c unit?

25 amps.

Do you have an electric range or electric dryer?

Both

Make yes or no and how many of the following fastened in place appliances.

WTR. HEATER No
KIT. DISPSL No
DISH WASHER Yes
HOOD FAN Yes
HOT TUB No (I wish!)
WHIRLPOL TUB No (Ditto!)
ATTIC FAN small ventilation fan
WTR. PUMP Yes
LIFT PUMP No
SUMP PUMP No
PADDLE FANS 1
WINDOW A/C No


Anything else fastened in place that I did not ask for like welders, air compressors etc.?

No, but the new subpanel will eventually feed a 5-6 HP coppmressor.
 
  #7  
Old 05-26-01, 05:01 PM
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MrFrog;

as to your comment here;

the breakers on each leg cannot exceed a certain percentage of the rated capacity of the panel(?)

the panel should have a sticker, the rating of each buss position should be there, usually it's 100A, 120A. My point is, in introducing your 100 or 60 A breaker, do not let this and the opposing breaker add up to more than said rating.

just my 2 cnts passing thru....
 
  #8  
Old 05-26-01, 05:28 PM
Wgoodrich
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As you answered the questions your demand load as per Article 220 of the NEC is apporximately 160 amp. This is including your new air compressor. Thus requiring your minimum service size of 200 amps. The NEC says you can load that panel to its minimum service size considering the demand load calculation we just estimated for you, then you only have approximately 40 amps left for a safety margin without worrying about kicking your main breaker due to too much load. I would not add any more new loads after your air compressor is installed in your home without upgrading your service size.

Hope this helps

Wg
 
  #9  
Old 05-26-01, 05:31 PM
Wgoodrich
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wirenuts is your comments our thoughts being advisory for his best interest or do you know of a Code article forbidding a sub panel being as large or larger than the main overcurrent device carrying that sub panel in the main service rated panel?

Curious

Wg
 
  #10  
Old 05-27-01, 10:24 AM
MrFrog
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wg & wirenuts,

Thanks for the help. I quess my next question is should I forget the subpaneel idea and just run a branch circuit to the compressor, or go ahead with a subpanel in case I do need to upgrade at a later date. Right now I don't see that need arising, but who knows.

If I run a branch circuit, it would be a run of about 50 ft from the main panel. What size wire would be required?

If I run a subpanel, it would be about 30 ft from the main panel. What size wire would I need?
 
  #11  
Old 05-28-01, 08:14 AM
Wgoodrich
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I suspect it would be your choice while considering anymore future loads. Sub panel versus branch circuit would really depend on what future increased loads in that area is in your mind.

Branch circuit size for a 120 volt / 5 hp motor would be #4 ga copper if a 120 volt / 6 hp motor is used would be a #2 ga copper if a 240 volt / 5 hp motor is used would be a #8 ga. copper if a 240 volt / 6 hp motor is used would be a #6 ga. copper.

Feeder actually would depend on breaker installed in main panel serving that sub panel. If you install a 60 amp breaker in main panel no matter how much bigger rated the sub panel would be, your minimum feeder size required would be #4 ga. copper sized by table 310-16 in 60 degree column. If you install a 100 amp breaker in that main panel no matter how much bigger rated that sub panel owuld be, your minimum feeder size would still be a #4 ga copper as per table 310-15-B-6.

A couple of notes as follows. The sub panel must be rated at least equal to or greater than the ampacity rating of the breaker in the main panel serving that subpanel.

There seems to be a great disparity concerning sizing this feeder to that subpanel by 310-15-B-6 instead of table 310-16. If you size by the other side of this opinion disparity using table 310-16 a #2 copper would be required to serve that sub panel if a 100 amp breaker is installed in the main panel.

Really your choice.

Wg
 
  #12  
Old 05-28-01, 10:49 AM
MrFrog
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Smile

wg,

Again, thanks for all of the help. I suppose the size of the feeder to use is really dependant on what side of the "opinion disparity" the local inspector is on.

I will probably go ahead with the subpanel, figuring that if I am going to pull wire through the length of the basement, I may as well pull wire big enough for any future needs, rather than having to upgrade the feeder later.

With all the helpful info that you have given me, at least now I can put something on paper to take to the inspector and get a permit.

Thanks again!
 
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