# Looking for some help on garage sub panel

Discussion in 'Electrical Forum discussion & Blog' started by knied1, Aug 31, 2012.

1. ### jwelectricElectrical Contractor/Instructor

Joined:
Jun 14, 2007
Occupation:
Instructor
Location:
North Carolina
Ask for a code reference as this is not true. You could install a 1000 amp feeder and overcurrent device in that 100 amp panel if you so choose

2. ### HomeownerinburbMember

Joined:
Jan 14, 2012
Location:
Los Angeles, CA USA
Finally! Gawd! Yes? Why did I not just speak in watts? Possibly because the OP sounded a little less up to speed on the whole concept?

My point: almost every time I have put a sub in a garage for a home handyman or hero gear head, and I tell him a 30 amp (or 50) is going to serve his demands, he wants to challenge me until I explain that he actually has two legs of power at 30 or 50 amps in a three wire circuit, not one, as might at first blush appear to be the point.

3. ### HomeownerinburbMember

Joined:
Jan 14, 2012
Location:
Los Angeles, CA USA

Could be a local rule? Many of my local cities would not permit NM in single family homes up till about 10 years ago.

4. ### jwelectricElectrical Contractor/Instructor

Joined:
Jun 14, 2007
Occupation:
Instructor
Location:
North Carolina
This is very true. I remember a few years ago working in Fl. and the local would not permit ground rods due to the water table. They required a concrete encased electrode.

5. ### jwelectricElectrical Contractor/Instructor

Joined:
Jun 14, 2007
Occupation:
Instructor
Location:
North Carolina
I find myself complicating issues sometimes simply due to the fact that I stand in the classroom.

When I teach a class on transformers, which is where we are at now, and how they relate to a dwelling unit service we discuss 120 verses 240 volts that is supplied by the transformer.

I present the question to the class which is correct for a 200 amp service; the service will be 200 amps or 400 amps if the two legs are added together. Most answer that it would be 400 amps if added together which is incorrect.

The most 120 volts that can be achieved would be 200 amps. When the second leg is added to it the addition would be +200 added to -200 or if both legs were loaded to the max there would be no 120 being used at all. When we load one side to 10 amps and then the other side to 5 amps even if what are used are 120 volt appliances then the service will only see 5 amps at 120 and 5 amps at 240.

After going back and reading your post I see that what you were saying is that if each 120 volt appliance being used at one time that the total draw on the feeders would be just that and you did not say that each leg would be carrying twice the rating of the conductors or breaker. As a matter of fact you were only addressing each branch circuit instead of the total amperage being on the feeder.

My mind went to the load on the neutral which I guess was easy to see and how that any balanced portion of the 240 volt feeder would be a series 240 volt circuit instead of the parallel 120 volt circuit as you were addressing.

I am slow but I usually get there.

6. ### BrianKNew Member

Joined:
Jun 4, 2012
Location:
Calgary, Alberta, Canada
It was interesting watching/reading the 'discussion' between the two posters. While it 'seems' obvious that a 1 amp/120 VAC load on each side of the panel would add up to a 2 amp load on the neutral, what made the concept clear (and obvious) was that the series connection of 2 equal 120 VAC loads across 240VAC would result in zero amps on the neutral - beautiful!
I guess the old adage "Don't confuse me with the facts - I already have my mind made up" strikes again or " I think I know what I thought you said, but what I heard is not what you think you said" or something like that. Great discussion!

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