Okay lightman or other electricians... set me straight

nadams

I'm having a discussion on another forum, but I don't quite agree with what they're saying. I don't know their qualifications, and I don't want to start an argument over this, so I figured I'd post here where I know there's knowledgeable folks.

I have 100a service to my house. If I load 55amps on one leg, and 50 on the other, will the main breaker trip? Or, does it need to see 100 amps on each leg (or at least over 100 on one or the other).

Basically, the other forum is saying that I could draw 200 amps at 120v, because the main breaker is rated PER LEG. So, as long as my 120v circuits are evenly distributed across the two legs in the panel, I could draw up to 200 amps.

I'm saying that if I draw over 100 amps, period, I'm going to pop the main.

ben62670

Have you ever tripped the mains? Do you have a voltage drop?

obieone

I'm not 100% sure what your asking? Are you trying to add more branch circuits?

Serendipity

If your main breaker says 100, then you have 100A service.

If it says 200, you have 200A service.

Serendipity

ben62670 wrote: »

Have you ever tripped the mains? Do you have a voltage drop?

If your load is not balanced, you will trip the mains. However, with a balanced load there should be no problem.

jimmydep

If you have a 100amp main breaker, the main is a 2 pole breaker, that means you have 100amps on each pole or 200 available amps in the panel. Most codes allow for a total load of 80% of the breaker rating, or 80amp total load on each pole which = 160amp total load for the entire system.



Jim D

obieone

IF either leg see's 100A's, it will trip. Again, what are you trying to do?

nadams

I'm not *trying* to do anything. The discussion came up on the Garage Journal forums whether 200 amp service was really needed for anyone in a residential setting. Obviously, these are guys who might have 200amp service with 100amp subfeed to the garage.

The general consensus was leaning in the direction that 100amps is enough for anyone, regardless of garage. So, I went around with my kill-a-watt and took measurements of some common appliances... all of which could be on at the same time:

Microwave - 12.5a
Heater upstairs (1) - 12a
Washer - 10a (if on fast agitate)
Heater upstairs (2) - 9.75a
Fridge - 2.5a
Computer - 2a idle, laptop, desktop, and 2 LCD monitors. 10a total if printing (laser printer)
Triple flourescent light fixture - .65a

Added together, that's 57.4 amps. So, I said, if I also happened to be running the dryer and cooking on the stove (both electric), I could easily top out 100 amps and trip my main. However, this is the response I got:

"It looks to me that all the devices you list here are 110v. Keep in mind that 57.4amps at 110v draws only 1/2 that from your 220v service, or 28.7 amps. Now, add about 30 more for each of the dryer and range and a 100 amp service would still handle it. It would be rare to turn on every device you own at the same time. We don't calculate service that way. If we did, we all would need 400 amp service, and in reality, 100amp serves us just fine."

Now, keep in mind this is all theoretical at the moment. But, I've been considering upgrading to 200amp service, just to know I have the extra capacity (IE- if I ever go with electric hot water or dedicated electric strip heat upstairs). As it is, my panel is getting full-breaker-wise. I know that's not an indication of the load present, but I like overkill

MillerLiteScott

The house I am building right now has 600 amps an I have been in a few with 800 amps.

Just depends. My house had 100 amps when I bought it but I bumped it up to 200 amps, mostly to handle AC, a dryer, and 60 amp sub panel in a detatched garage so I could run a compressor and table saw at the same time with out tripping a breaker.

Scott

obieone

My recommendation: List all the possible additional loads you MIGHT get in the future, then contact your public utility, and have them audit your service/ panel.
They'll be able to tell you the best course to take

nadams

I'm not so much worried about my own panel in this thread as just using it as an example. Is a 100amp breaker rated at 100amps per 120v leg, or is it 100 amps draw total, no matter what leg?

mmadden28

I was a bit confused at first trying to see the argument--
I think I see now.

While there is a difference between how Amps are calculated with respect to 110V vs. 220V; Amps are Amps/Current is Current. so the one guy saying that the Mains Amps are half is somewhat true and can lead to some discussion which apparently is the case. And since an Amp is an Amp, and I may be wrong but, I am going to say the guy is incorrect with respect to being able to dbl the current used based on 110V measurements. Of course I never really did understand how I can have many many times more breakers ratings than the Mains.

When I buy a 20A breaker for 110V, I expect that its rated for 20A when using it with 110V, If it were a Double 20A for 220V, I expect it to trip at 20A when I'm using it for 220V. Now lets say I used that same 20A DBL breaker but used it for two separate 110V circuits (which I am doing for a split receptable), would the breaker trip at 10A if that were seen on only one of the 'legs'?? I don't think so. It wouldn't trip until 20A on either leg, mainly because in reality it really is just two separate 20A breakers with the switches tied together so they trip in Tandem.

Now I guess the bigger question is for the Main breaker itself--Is it 'wired' the same as a typical Double breaker used within? If it is then I would expect that each leg would be able to handle up to the rated current, meaning that a Main rated at 100A should be able to handle 100A on each leg simultaneously. Two 110V lines come into the main, No? Again I am no authority on the subject matter...Now if the Main is wired differently than a typical Double breaker, then it could be total current for both legs additively.

I can see the 'both sides' view though. But I suspect it all comes down to the Amp rating on the Main and whether its based on 220V or two separate 110V legs.
I'm interested to know what the authoritative answer is on this one.


CAVEAT: I am NOT an electrician! (but I did stay at a Holiday Inn Express last night :rolleyes::o)

jimmydep

jimmydep wrote: »

If you have a 100amp main breaker, the main is a 2 pole breaker, that means you have 100amps on each pole or 200 available amps in the panel. Most codes allow for a total load of 80% of the breaker rating, or 80amp total load on each pole which = 160amp total load for the entire system.



Jim D

This is the answer^^^^^^^
100amps per pole (leg) total 200amps

obieone

I hope this pic help explains how a breaker panel works. The wire on the left is L1, the wire on the right, is L2. The two legs intermingle going down the center, to keep the loads balanced.

disneyjoe7

Sal,

Even next to each other (up and down) they are on different legs.

jimmydep

Lasareath wrote: »

So when you install the breakers you want to install the same amount on each side and not just install them on one side?

Sal, Breakers that are installed across from each other (head to head) are on the same pole (leg), breakers installed next to each other (side by side), pull from different poles. If you use a double pole (240v breaker), you pull power from both 120v legs, which gives you 240v.

All your 240v ciruits will pull equally from both legs. When putting breakers in your panel, keep alternating from the right to left side, this should keep the loads somewhat equal.

Jimmy

danger boy

I checked and my main breaker is 200 amps.. is this standard for today's homes?

obieone

danger boy wrote: »

I checked and my main breaker is 200 amps.. is this standard for today's homes?

Probably based on the avg. home, yes. My house is 1200 sq ft, w/ 200 amps, and is only 4 years old. Same size house 30- 40 years ago probably has 100 amps, because back then, we didn't have all the bells a whistles of modern convenience;)

megasat16

I think 200 amps service lines for residential meters are pretty common nowadays. The older homes has only 100 amps service lines.

mmadden28

So are we all in agreement then that 200A service is actually capable of 400 with 200 per leg? (assuming everything is balanced and also assuming theoretically that you can even pull that much with every possible thing in the house on and pulling max)