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Old 04-04-2008, 07:00 PM   #1
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How can I determine service amperage?


I've been wandering around the Internet at large for a couple of hours and haven't found a satisfactory answer, so I'm gonna see if anyone here can help me out!

I am trying to determine the electrical service amperage of my panel. The large main disconnect breaker is labeled as a 150, however, most everything I've read says that my panel should be 100 or 200 amps.

I have 3 line service (from the looks of it, anyways): 1 neutral (white), and 2 hots (black) that look to each feed one of the hot bars in my panel. The breaker identifies itself as a 2pole 150. In fact, here, this might be easier:






So, my question really is this:

Is there a control that the utility company can put on a service line besides the size of the main breaker? Do I in effect have 150amp service? Or do I have 100amp service and just have a 150amp breaker?

I've also read something saying that each hot bar can provide the full service level; does that in effect mean that I have amperage twice the rating of that main breaker? This seems unlikely to me, but there's a lot of confusing information out there.

I've spent the better part of the day mapping out the electrical circuits in my house, that was certainly an adventure. I don't know that I'll ever understand why the top outlet of a receptacle in the kitchen was on the 20A circuit that feeds my washer at the complete opposite end of the house, while the bottom outlet of the same receptacle was on a separate 20A circuit with only 1 other outlet in my dining room. 20A seems a bit much for only 1.5 receptacles, even if one is in the kitchen! :p

I also have an unused 30A breaker in my panel, it's labeled AC, but...well, there's no wires coming out of it. There's also ANOTHER 30A labeled AC, so I'm assuming someone didn't know what was going on. I have a "dead" (no wires) plug next to my dryer, and am assuming that 30A used to serve an electric dryer instead of the gas one I have there now. Can I remove the 30A breaker? I'm looking to put in a couple of 15A to run a recessed lighting project in the basement and may as well remove the 30 and replace with the 15's instead of punching new holes if that doesn't present any problems.

Thanks for any and all help, hope the pictures help with any answers!

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Old 04-04-2008, 07:22 PM   #2
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How can I determine service amperage?


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Originally Posted by dawho1 View Post
The large main disconnect breaker is labeled as a 150, however, most everything I've read says that my panel should be 100 or 200 amps.
What makes you say this??? In most cases the number on the main breaker IS the service size. It would not be typical to have a mismatched service such as larger wire on a smaller panel or vice-versa.

From the looks of it that cable may in fact be a bit large for a typical 150A service. They look like 2/0 copper to me.
Can you tell us what is says on the larger service wires or cable?


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Originally Posted by dawho1 View Post
I've also read something saying that each hot bar can provide the full service level; does that in effect mean that I have amperage twice the rating of that main breaker?
Yes, you get 150 amps from each leg of the service. It is NOT a correct statement to say that you have 300 amps at 120v.
What you have is a 150A @ 240v service. Since services like this also have a neutral wire it is correctly called a 120/240v service.



Sure you can replace that 2-pole 30 with single 15A breakers. No problem at all in doing so.

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Old 04-04-2008, 07:54 PM   #3
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How can I determine service amperage?


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Originally Posted by Speedy Petey View Post
What makes you say this??? In most cases the number on the main breaker IS the service size. It would not be typical to have a mismatched service such as larger wire on a smaller panel or vice-versa.
Only mentioned it because all I could find online said that "you probably have 100amp service unless you have an old house and it's a 60amp fuse service, or you upgraded to a 200amp service" I assumed that the 150 was my service size, but thought I'd be better off asking someone who is more knowledgable than I!

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Originally Posted by Speedy Petey View Post
From the looks of it that cable may in fact be a bit large for a typical 150A service. They look like 2/0 copper to me.
Can you tell us what is says on the larger service wires or cable?
Synthinol 902 THW, 2/0 AWG AL 600V is all I can make out without trying to touch or move the cables, so this is where I politely say that I've gone about as far as I'm going!

I assume the AL means it's aluminum wire, and it certainly doesn't look like copper where it attaches. the "outside" wires are sort of light gold tinged, like maybe they had some antioxidant on them, or have corroded, or something, but the "internal" wires are very bright silver. (by "inside" and "outside" i simply mean that on the single cable, the outer wires are a different color than the inner group of wires, that makes sense, right?) There does not see to be any evidence of "powdery" corrosion though, so maybe the outer strands of the cable are simply a different color for some reason.

Thanks for the quick response and knowledgable reply!
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Old 04-04-2008, 08:26 PM   #4
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How can I determine service amperage?


You have copper clad aluminum service wires and 2/0AL is appropriate for 150A.

You will see a lot of what you are describing as far as arm chair electricians go. "You might have this if that" and "Maybe this due to the age of the equipment".

It is hard to judge things sometimes without pics and accurate information.
You gave both so determining what you have was easy.
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Old 04-05-2008, 12:07 PM   #5
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How can I determine service amperage?


This is indeed a 150 amp service. It's perfectly OK to remove the 30 amp 2 pole breaker and install two 15 amp single pole ones.

I would suggest turning the main (150 amp) breaker off before removing the 30 amp breaker. When the main is off, anything below it will be dead. The big wires on top of it are still hot. If you're adding more wires through the top of the panel, be careful not to touch the top of the main.

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Old 04-06-2008, 10:28 AM   #6
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Yep, thanks for the help guys. I have removed the 30A, installed the 2 new 15A. My only other question is whether I'm ok connecting the grounds for the 2 new 15A circuits I ran (12/2 NM) to the neutral bus bar; there does not appear to be a ground bar in my panel. I've examined the other runs in the box, and there are not any grounds running out and into the conduit, leading me to assume that either the conduit itself is working as the ground for the rest of my circuits, or that the ground wires are all clad in white insulation, which I doubt.
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Old 04-06-2008, 10:42 AM   #7
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How can I determine service amperage?


In the main panel the grounds and neutrals go on the same bar(s). You can double up on grounds in the same hole but neutrals are one per hole.

If you ran #12 WHY are you using 15A breakers???
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Old 04-06-2008, 11:07 AM   #8
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How can I determine service amperage?


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Originally Posted by Speedy Petey View Post
In the main panel the grounds and neutrals go on the same bar(s). You can double up on grounds in the same hole but neutrals are one per hole.

If you ran #12 WHY are you using 15A breakers???
Mostly because I'm a relative electrical noob,

Well, I guess my primary reason was that my entire basement (save my furnace and my washer/dryer) is run off a single 15A. I'm installing recessed lighting and wanted to give it it's own circuit so that:
1) I'm not plunged into absolute darkness if anything blows the circuit
2) I can turn off the breaker for the basement if I have to work on it and still have the lights on, and
3) I'm still not sure if I'm going to go CFL or incandescent, and the difference in required power for ~15 cans is quite substantial.
I also have a home office that has more stuff in it than is probably typical...4 computers, multiple monitors, multiple networking devices and a bunch of random stuff. I'd like to be able to flip breakers in other areas and not have to worry about shutting all of that down first also. Would be handy to have that on a separate circuit, which is what I plan to do with that 2nd 15A circuit; theoretically the computers alone could pull about 3200 watts, although it's very unlikely that they would ALL be pulling their maximum load for any length of time, much less all at the SAME time.

Anyways, now you know what the circuits are for, but here's the answer to your actual question: All of the receptacles downstairs are clearly labeled as 15A. I didn't want to run the risk of having a problem with a receptacle and the breaker not tripping because it was a 20A. I honestly don't know what the actual risk realistically would be, but...there you have it. I just ran the 12/2 because I figured I was better off going with that for future-proofing reasons.

Should I not be using 12/2, or should I be replacing all switches/receptacles in my basement with 20A and then upgrading the breaker?
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Old 04-06-2008, 12:22 PM   #9
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How can I determine service amperage?


If it's #12 wire, I'd use 20 amp breakers. It's completely legal, codewise, to run #12's and 20 amp breakers to 15 amp recptacles. In fact, it's a very common practice.

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Old 04-06-2008, 01:14 PM   #10
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How can I determine service amperage?


Yup, NO reason at all to not use 20s. The future is now.
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Old 04-06-2008, 07:03 PM   #11
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How can I determine service amperage?


I run #12 on 20's for most circuits. Now days bedrooms or basements get converted to home offices, sewing rooms, media rooms etc. You never know what future power requirements might be. It's cheaper to do it now than have to go back and redo it later.
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Old 04-06-2008, 09:48 PM   #12
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I run #12 on 20's for most circuits. Now days bedrooms or basements get converted to home offices, sewing rooms, media rooms etc. You never know what future power requirements might be. It's cheaper to do it now than have to go back and redo it later.
Just my opinion, but I only run #12 if I'm being paid for it, except where required of course...
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Old 04-07-2008, 10:47 AM   #13
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How can I determine service amperage?


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Originally Posted by Randell Tarin View Post
I run #12 on 20's for most circuits. Now days bedrooms or basements get converted to home offices, sewing rooms, media rooms etc. You never know what future power requirements might be. It's cheaper to do it now than have to go back and redo it later.
Not to start up this debate again, but I know of nobody that can sew and utilize 20 amps at the same time, except maybe in a sweatshop. In the OP's case, it seems he's going to need a 250' roll of # 12 and a couple rolls of #14. I would therefore wire the lighting with #14, use the #12 where required, and use up the #12 on the furthest receptacle circuit.

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Old 04-07-2008, 11:10 AM   #14
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How can I determine service amperage?


I live in Texas! Because everything is bigger here, our sewing machines run on 220v and are water cooled.

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