Sub Panels and Current Load

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Old 02-07-13, 05:38 AM
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Sub Panels and Current Load

Hi guys, first post here been reading threads for awhile! I just bought a house, a slight fixer-upper and I'm really doing my best to learn the ins and outs of all things repair. I've got a few books on different topics and I've been learning electrical. I have an electrician doing this current project but I want to atleast understand the how and why.

First and foremost I'm renovating a room into a laundry room. I also have a 80 gallon compressor that I'm putting into the garage. My service is 100amp. The dryer has a current draw of 28amp and the compressor 22amp. I'd like to run a sub-panel that is closer to the garage/laundry room. When I looked at them my options were a 60amp/70amp/100amp sub-panel/main breaker. So my questions are...
  • If my service is 100 amp would it even make sense to do a 100amp sub-panel? (The 70amp only has room for 2 breakers and I'd like the compressor on a breaker, the dryer on one and the outlets/lights on another)
  • If I do a sub-panel, will I need to run 4gauge wire for the panel? My reading tells me that 70amp load requires 4 but my whole house is powered by 4
  • I may be thinking to literal but being as the dryer and compressor take up almost 60amp combined does that mean I should look to upgrade my service? This same logic says that there's 250amps of breakers in my main breaker, this doesn't make sense to me. Basically what I'm saying is, is it even possible to run both the dryer and compressor along with the rest of the house on 100amp service?
  • Lastly, can I use a 100amp main lug panel as a sub panel? and if so, does it just wire into a breaker on the main panel?

Sorry for all the questions and being so wordy! I appreciate the help.
 
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  #2  
Old 02-07-13, 05:54 AM
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Your subpanel in the same building as the service panel does not require a main breaker. You can install a 100 amp 20 space panel if you like. The size of the feeder will depend on the loads expected on the subpanel.

Adding up the breaker handle ratings is not the way to figure out the demand on the service. It totally disregards the diversity factors of things like heat and A/C that are not used at the same time. It also does not mean that the 20 amp kitchen appliacne circuits are using only a portion of the circuit to power the clock on the coffee maker until you brew a pot. The correct procedure is called a demand load calculation.
 
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Old 02-07-13, 06:02 AM
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Thank you pcboss. I'll read up on this. Do I wire the sub-panel directly from a circuit breaker on the main breaker?
 
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Old 02-07-13, 07:57 AM
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Yes.....you will need to connect to a two pole breaker in the main panel to power the sub panel.
 
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Old 02-07-13, 10:11 AM
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Awesome, thanks PJmax. Without knowing too much do you see running these two items and a room of lights and outlets being an issue with a 50amp breaker and 6 wire? Also.. say I have a dryer running at 28amp draw on a 50amp sub panel breaker and other peripherals around the house all drawing a combined 80amp for a total 108amp draw, 8 over rating. I'm assuming it's possible to trip the main breaker this way without ever tripping an individual breaker?
 
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Old 02-07-13, 10:17 AM
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You will be better off leaving the large loads in the main panel and moving smaller ones to the sub.
 
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Old 02-07-13, 10:28 AM
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Thanks, my only issue with this is that the two large loads are literally as far away as possible from the main panel making for 2 times the wire. Also, it would be nice to have the breaker for the compressor in the garage. I appreciate all the help and researching is really helping me. I think the most confusing part to me is know what size breakers to use and when and how to determine demand load. I think to litteral and say 30 amp breaker for compressor, 30 amp for dryer 20 amp for outlets 15 amp for lights = 95 amps in a sub panel.
 
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Old 02-07-13, 10:42 AM
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Also, I'm seeing varying pages saying what count wire to use, 3 or 4. I know I need two hots to run a 240/220 but do I need 2 hots a Neutral and a Ground or 2 Hots and a Neutral? How do I determine what I need?
 
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Old 02-07-13, 10:47 AM
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To a 240 volt only load you need to hots and a ground. Most stoves and dryers have 120 volt components so therefore need two hots and a neutral plus ground. They are properly called 120/240 appliances. Almost all residential subpanels supply both 240v and 120v and therefore need four wires.
 
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Old 02-07-13, 12:06 PM
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Thanks Ray, So my sub panel will run 220 and 120 volt components. I'd like to run 2 220's from the sub panel and also 2 120's from the panel. So I'm assuming I need 4 wire from main to sub.
 
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Old 02-07-13, 12:29 PM
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Correct on the wires but technically this is 120/240 not 120/220.
 
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Old 02-07-13, 01:37 PM
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Yes, a 4 wire feeder to the subpanel.
 
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Old 02-07-13, 02:36 PM
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Check the inside of the subpanel before you hit the checkout. If it only has 1 bussbar with the small setscrews that is mounted on plastic standoffs at each end, you will have to purchase an additional ground bussbar (around $7) and screw it fast to the inside of the box so it is electrically in contact with the box.

All your bare copper ground wires connect to that bussbar and the white neutrals go to the one isolated from the metal box.

Even though they both are likely on the same bussbar in your main panel, they must be kept separate in the sub panel.
 
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Old 02-07-13, 06:32 PM
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I think to litteral and say 30 amp breaker for compressor, 30 amp for dryer 20 amp for outlets 15 amp for lights = 95 amps in a sub panel.
The 95 amps you have come up with means nothing, those are just the breaker ratings and not load. You need to do a load calculation to size the subpanel and feeder to it.
 
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