100 amp panel ready for wiring


Old 02-04-05, 06:30 PM
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Question 100 amp panel ready for wiring

I have recently had a house built with a three car garage. I expanded the third bay (single bay) in width and length to accomodate my wood working tools. I have studded-up the walls seperating the two car area from the single bay. I had an 100amp panel put in the third bay and am ready to begin the wiring. I have wired several times in the past but I have never started out with a "fresh" panel meaning that before I would use 12 gauge romex throw in a twenty amp breaker into an existing box and have a nice day. I understand the amp draw on all of tools but I guess my question is the 100 amp panel will deliver 100amp load? Meaning that if I have five machines running drawing 20 amps per machine it equals 100 amp and if I exceed this it will trip the main breaker of the 100amp panel? correct?

The main panel in the house is 200amp and this one is 100amp. Does this mean that the 100 amps are feeding off of the 200amp and maintaining it's 200amp along with feeding the 100amps to the sub panel?

I know this is a fundemental question, but I would appreciate some help.

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Old 02-04-05, 07:29 PM
Join Date: Sep 2000
Location: United States
Posts: 18,497
The 100-amp panel can provide anything up to 100 amps. How much it actually provides depends on the size of the feeder you supply it with. That depends on the size of the wires you run between the panels, and that in turn determines the size of the breaker you put in the main panel to protect the feeder. You don't need a main breaker in your subpanel unless you want one to serve as a manual disconnect.

The total power available to your house and shop combined remains at 200 amps.

The thing you have to determine is how many of those 100 amps you are going to need. The more you need, the more the feeder wire will cost, and the greater difficulty it will be to run it. But if the distance between the panels is not great, that may not be a big consideration. Of course, if the distance is great, you will want to increase the feeder size to limit voltage drop.

So the two big questions are: (1) how much power do you need from the subpanel (in KW)? and (2) how far apart are the panels?
Old 02-05-05, 03:25 AM
Join Date: Oct 2004
Posts: 84
John N

There again I assume John means 200 "running amp", Right John!!

Old 02-05-05, 07:25 AM
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100 amp panel ready for wiring

Thanks for the repsonse. The 100 amp panel is installed without a disconnect at the panel. I have an empty 100 amp panel (I believe it is square D). My plan is to outlets to all of my machines (lets say six) with 20 amp breakers. Also install recepticle's every 32" just to have them. I plan on putting those recepicles (20amp) on one breaker itself. All my lighting on a 20 amp breaker as well. So when I add up the six 20's for the machines, one 20 for the recepticles and one for the lighting that equals 160 amp's at full load. Correct? This is a 100amp panel with 160 amp's worth of wiring to it. This is o.k. for NEMA code? I never invision (or could) put the system under full load conditions.

I don't want to burn the shack down yet and I don't want to pay. But I also want to be within the code to assure all that I haven't discussed (insurance, re-sell etc.)

Thanks again
Old 02-05-05, 11:25 AM
Join Date: Sep 2000
Location: Fayetteville, NY, USA
Posts: 1,052
It is common, almost universal I should say, for the amperage numbers on your individual breakers to exceed the amperage rating of the main breaker controlling the whole panel. I think you meant NEC (National Electrical Code) instead of NEMA (National Electrical Manufacturer's Association). Yes, this is acceptable to the NEC. They figure the same as you - you will probably never even be able to run every single light, machine and have every single receptacle filled with running equipment at one time.

The size of your wiring between the main and sub-panel, which John Nelson mentioned, will need to be either #4 copper with #8 copper ground or #2 aluminum with a #6 aluminum ground if the panel is fed by a 100 amp breaker. You can use "SER" cable, which is a single cable with three insulated conductors and a bare ground in an overall sheath, or you can certainly pull individual conductors in conduit. (1-1/2" conduit will handle 4 conductors of either copper or aluminum.)

Being a sub-panel, you must not use the bonding screw that comes with the panel. It is used to bond your neutral bus to the steel panel enclosure, which will bond neutrals and grounds together. Without the screw the neutral bus in your panel is insulated from the enclosure. That's what you want in a sub panel. Only neutrals can go on that bus. You must have a separate ground bus bar, screwed directly to the steel enclosure. If your panel did not come with a separate ground bus then you can buy one for $5 to $8 at Home Depot and install it. You must sand the paint off the area where the ground bus will be installed so it makes good electrical contact with the steel itself. You must not use sheet metal screws, you must drill pilot holes and use self-tapping screws. The NEC says that in your main panel all your grounds and neutrals MUST be on the same bus, and in a sub panel they are NOT permitted to be on the same bus.

Hope that helps.


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