does a 100A subpanel make a 200A main 100A?

[v7guy]

This shows my true understanding of electrical lol

When you have 200A service at the house and you put a 100A subpanel in the garage do you still have 200A at the house (unless it's being used by the garage) or do you effectively make the house panel 100A?

 

[tfi racing]

The rating is only the size of the overcurrent device(main breaker) and the rating of the panelboard,the electricity itself doesn't know or care where it is going.If you were using 100A in your garage(very unlikely and close to impossible),the house would only have 100A available(also very unlikely and improbable).

 

[Tscott]

The rating of panels is based on safe maximum amperage that the panel can carry. So technically, your standard 200A home panel could operate safely at 200A of current without any negative side effects besides one hell of a power bill. I am not sure if this rating is for a continuous period, or some set time period though it is really not important in this discussion.

In actuality, as stated in the above posts, it would be almost impossible for either your 100A panel or your 200A panel to see the current level they are rated for. The reason for this is what is known in the power industry as "diversity". All this means is that of the dozen or so circuits you have in your home or garage, that at any one time only a hand full will be drawing load. Remember that most loads in your home are intermittent loads. Your AC does not run 24/7, it cycles on and off, as do things like you refrigerator, and your stove. The chance that all these cyclical loads will be on simultaneously is negligible. (Unless you have been without power for more than about 30 minutes, in which case when power is restored, you will likely see almost all of those cyclical loads turn on at once, but this is rare)

So the bottom line is that the sum of the individual circuit breaker ratings in your home can exceed the max rating for the main breaker (i.e. 200A), because they do not all draw load at the same time. So as long as you are not running a lot of heavy draw equipment over long periods of time, you should be just fine.

If your home panel is already a little "overloaded", and you install a 100A feed to the garage, then you may want to be mindful of what you are running and when. If you plan to weld while the wife is making dinner on a cold winter night with the heat running, you may want to switch off the air compressor to be sure you do not trip the 200A main disconnect breaker on the house panel. Worst case scenario, you trip the main breaker and the house goes dark and the wife makes fun of you.


Tom

 

[v7guy]

so the house will still have 200A available even though the garage has a 100A panel assuming everything is turned off in the garage, correct?

I'm more just trying to understand the concept, I realize there is almost no chance I could use all of that.

 

[sberry]

Yes, no power used in the garage means there will be 200 available at the house.

 

[ArthurPE]

technically you should size each panel for its duty factored load...
if the garage would draw 100A continuously (doubt it) the house panel is then 'allowed' to be sized for 100A of load, + the 100A for the garage feed...
but you can always make it 'work' since all circuits are seldom energized at the same time...
ie, cooking, while doing laundry, on a hot/cold day, while running all lights and power tools in the garage

in reality, the combined load should be less than 200A, with no more than 100 to the garage

200A is a good bit of power...unless you have electric heating or large AC load

btw: 200A CB ~ 160A load...everything is usually upsized 25%

 

[Tscott]

so the house will still have 200A available even though the garage has a 100A panel assuming everything is turned off in the garage, correct?

I'm more just trying to understand the concept, I realize there is almost no chance I could use all of that.

Yes this is true.

Try to think of it like water piping. The water pipe that feeds your home is capable of delivering up to, but no more than 200 gallons a minute. So no matter what you do, this pipe will only allow 200 gallons to flow into your home per minute. Then you install a pipe half the size to supply water from your home to your garage. This smaller pipe will supply the garage with up to but no more than 100 gallons per minute. So if you turn off all the valves at the garage (i.e. nothing is turned on) then no water flows from the house to the garage and you still have a 200 gallon per minute capacity at the house. If you turn all the valves on at the garage creating a demand for 100 gallons per minute, then at the house, you will only be able to get 100 gallons per minute to flow out of the valves in your home.

Your panels are rated for a certain amperage. Amps are a measurement of how much electricity is flowing, kind of like gallons for water. If it was an SAT question it would be like Gallons are to water as Amps are to electricity.


Tom

 

[v7guy]

That was a really good analogy, thank you.

 

[ArthurPE]

... If it was an SAT question it would be like Gallons are to water as Amps are to electricity.


Tom

pressure is to water as XXXXXXX is to electricty?

 

[Tscott]

Voltage.......yep, definitely voltage

Are you an engineer aurthurPE?

 

[Falcon67]

Ya, but if you try to draw more than 200 gal/min the pipe doesn't snap shut