How many amps is my service?

[Jones]

Hello, all,

I'm getting ready to have an electrician put some 220V circuits in my
detached barn workshop. (I only have 110V right now.) I'm hoping I have
enough power running from the main panel in the house out to the
sub-panel in the barn to do what I want.

When my electrician came out to check and make sure I won't need
another/heavier wire between the house and the barn, he opened up the
sub-panel in the barn and said it looked like I have 100A service at
the subpanel in the barn.

(He said that he wanted to check at the main breaker panel in the house
to make sure 100A service was going to the barn, but he forgot to do
so.)

I just took a look in the main panel in the house, and it looks like
there is a dual pole breaker feeding the barn (I think that's the
proper term -- it's like two breakers side-by-side with the switches
connected) and each switch of this double-pole breaker is labeled 50A.

It's a Square-D panel, if that helps.

Can anyone tell me from the information I've provided what the amperage
going to the barn likely is? (Right now I have only 110V at the barn, I
believe, so please express the amperage with that in mind.) I'm hoping
I have enough capacity to have a 220V 30A circuit for a compressor and
a 220V 20A circuit for a saw (though they would never both be used
simultaneously).

Thanks for any replies.

Jones.

 

[[email protected]]

|> I just took a look in the main panel in the house, and it looks like
|> there is a dual pole breaker feeding the barn (I think that's the
|> proper term -- it's like two breakers side-by-side with the switches
|> connected) and each switch of this double-pole breaker is labeled 50A.
|
| That's only 50 amp service, not 100 amps.

However, for 120 volt loads only, it can aggregate as much as 100 amps.
Half of them would be on one pole, and half on the other. This assumes
the feed to the barn is 3-wire plus ground. It is possibly it is only
a 2-wire feed, and only providing one of the poles the breaker can do.

 

[Jones]

The real limiting factor is the wire size going to the barn. If it is
2ga copper. The other issue is the main breaker size. If you have the
capacity in the main panel and big enough wire he can swap out the 50a
breaker.

OK, thanks for the replies.

I'm assuming (and hoping) that since he seemed to think I already had
100A service to the barn, that the wire from the house to the barn is
heavy enough.

Here's hoping he can just swap out the 50A dual-pole breaker for a 100A
dual pole (or whatever is needed)...

 

[Jones]

That's only 50 amp service, not 100 amps.

Yeah, but at what voltage? As I said in the original post, I'm using
110V as the reference point...

 

[[email protected]]

On Mon, 11 Feb 2008 20:02:06 -0500 [email protected] wrote:

| You will probably be fine with 50a at 240v unless you have several
| people working at one time. It is hard for one person to use more than
| 30 amps at a time.

Imagine running a 9600 watt heater in the barn. Now imagine running
equipment that produces that much heat just from operation.

Sometimes I wonder why we need 200 amp services to a home. Well, maybe
we need 100 amps to run air conditioners to remove the heat produced by
using 100 amps for other things.

 

[[email protected]]

| [email protected] wrote:
|> On Tue, 12 Feb 2008 00:00:17 -0600, "Ben Miller" <Ben@somewhere>
|> wrote:
|>> The protective device determines the rating. It is a 50A service,
|>> regardless of whether it is 240V 3-wire or 120V 2-wire.
|>> The difference is the maximum wattage available, not the maximum
|>> amps.
|>
|> Ben, the only reason I believe he really has 240 out there is the 2
|> pole breaker. He probably just has 120v receptacles at this point.
|
| I agree that is probably what he has. Phil seemed to be adding 50+50 to get
| to 100A, however. I was just making the point that 50A is all he has,
| assuming the conductors are properly sized of course!

Right, I was adding 50+50. If his loads are all 120 volt, and can be split
loaded well enough, then he has 100 amps worth of 120 volts, assuming he has
both poles fed in. Now that he wants 240 volt stuff, he does have to look
at it as 50 amps and needs to be sure he has both poles fed.

 

[Jones]

Your 100A subpanel does not mean you have 100A service at the barn unless
you have wire sized for 100A and is protected by a 100A breaker at the main
panel.



Looks like you have 50A, 120/240V, single phase service to the barn assuming
the conductors are correctly sized. You should have 2 hot, a neutral and a
ground to the subpanel.



50A, 120/240V service, but your electrician need to verify the conductor
size. If your compressor is not interlocked with the table saw, how would
you insure it won't run at the same time? Do you turn the compressor off
before you run the saw? In any case, for a 50A service, its ok to have it
run at the same time since typically neither of those loads are continuous.
However, watch out for inrush current as it might trip the breaker when both
starts at the same time. (Hard to say if it trips without doing a
coordination study.) Presuming your electrician is correct regarding the
100A service at the barn, you could than replace the 50A with a 100A breaker
at the main panel. I would just leave it at 50A for now if your not adding
addition loads, as it offers better protection than 100A.

Your electrician should be able to tell you if the main panel is able to
take the additional load (and some future loads, whatever that may be)
without tripping the main panel, bringing down you house as well as your
barn. He should also be able to perform a voltage drop calculation from the
main to the subpanel to assure you that neither the compressor or table saw
motors would burn out. The voltage drop should include future loads just to
be on the safe side.

Frank (any all other repliers),

Thank you very much for all of your thought, debate and replies.

I didn't mean to stir the pot, but I do appreciate all the information!

I'm hoping my electrician will come to install the circuits on
Saturday, and if he does, I will try to supply an after-action report
to fill you all in on what the final disposition was.

Again, many thanks.

Jones.