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#792 - 03/16/01 06:49 AM ampacity question
Boscodog Offline
Member

Registered: 02/27/01
Posts: 13
Loc: Massachusetts
OK, this is probably gonna be a dumb question. If a residential panel is rated at 100 amps that means that each hot leg in the panel provides 100 amps, and therefore the panel actually provides a total of 200 amps to the house, right? wrong? So many questions, so little time.

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#793 - 03/16/01 02:35 PM Re: ampacity question
sparky Offline
Member

Registered: 10/18/00
Posts: 5545
hmmmm, really it's not so dumb a Q, considering some of the threads going on in the "Electrical Theory" area.

each leg IS a #2, not a #8 or #6....


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#794 - 03/16/01 05:50 PM Re: ampacity question
sparky66wv Offline
Member

Registered: 11/17/00
Posts: 2339
Loc: West Virginia
I realize that I may be considered the token "village idiot" in this forum, but this seems rather simple to me.

100 Amps requires #2 Aluminum wire. 277V, 240V, 208V, 120V, doesn't matter.

Each leg in a 120/240V 100A service provides 12,000 Volt-Amps of power. Both legs provide 24,000 Volt-Amps of power...either 100A @120V or 100A @240V. All of the "doubled" power comes from the voltage, not the current. The current remains 100A.

GE Panels (200A) use 4 100A single pole breakers, 2 100A paired in parrallel to get 200A, but still 200A per leg...

Make sense?

[This message has been edited by sparky66wv (edited 03-16-2001).]
_________________________
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Residential/Commercial Inspector
5 Star Inspections
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#795 - 03/16/01 05:51 PM Re: ampacity question
Boscodog Offline
Member

Registered: 02/27/01
Posts: 13
Loc: Massachusetts
hehe, I saw some of those theory posts. You gotsta put your beanie cap on before going in there.

ok, not to beat a horse to death, but I just want to make sure I'm clear on this ...

a 100 amp service drop means there will be 200 amps of capacity to use, a 200 amp drop means there will be 400 amps to use, etc.

Yes?

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#796 - 03/16/01 05:55 PM Re: ampacity question
Boscodog Offline
Member

Registered: 02/27/01
Posts: 13
Loc: Massachusetts
Hi Sparky66,
I was composing my post while you were posting yours. I'm still trying to digest yours. Burp!

Thanks for the responses guys. I'm still wet behind the ears, but getting drier day by day.

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#797 - 03/16/01 05:59 PM Re: ampacity question
sparky66wv Offline
Member

Registered: 11/17/00
Posts: 2339
Loc: West Virginia
Use Volt-Amps in your service calculations and you won't get confused. Think about it in terms of volt-amps and it makes more sense.
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Residential/Commercial Inspector
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#798 - 03/16/01 06:03 PM Re: ampacity question
Boscodog Offline
Member

Registered: 02/27/01
Posts: 13
Loc: Massachusetts
Oh, and hey ... pass that village idiot cap over to me! I'll do right by it!

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#799 - 03/16/01 06:06 PM Re: ampacity question
Boscodog Offline
Member

Registered: 02/27/01
Posts: 13
Loc: Massachusetts
We've gotta stop posting like this. People are gonna start talking.

OK, Roger that! Makes sense.

Thanks again guys.

[This message has been edited by Boscodog (edited 03-16-2001).]

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#800 - 03/17/01 02:09 AM Re: ampacity question
Scott35 Offline

Broom Pusher and
Member

Registered: 10/19/00
Posts: 2724
Loc: Anaheim, CA. USA
You guys are never going to take the Village Idiot Cap away from Me, so don't even try to!!
I also hold the title of "Anesteticly Induced Messages", which suppliments the Village Idiot Achievement Award.

Now on the serious side, '66 gave a good suggestion as to using Volt-Amps [or KVA] to figure a system's rating [or service's rating].

To answer Boscodog's original question in as few words possible [oh, how I've said that so many times before ]:

For a simple 100 amp, 120/240 volt 1 phase 3 wire service, the panel would be able to draw a maximum of 100 amperes per Ungrounded Conductor [hot].
That loosely translates into two [2] 100 amp hots at 120 volts [line to neutral], or a total of 100 amps at 240 volts [line to line].

It's more proper to say that the panel / service is rated at 100 amps, because that's the highest load that can be drawn on one "hot" conductor, or through one pole of the main breaker, at one time.

It will, as you asked, have a combined total of 200 amps when using Line to Neutral [120 volt] loads only - so it's not really totally incorrect to say that, just not completely and technically correct.

That alone must be a mountain of jibberish to understand

As Sparky mentioned in the first reply, the ampacity [size] of the wire will be the primary thing to determine maximum current.
This plus the main breaker's rating will determine the maximum amperage a panel can draw [continuously and without LCL], provided it's not more than the panel's designed for [like using a 150 amp breaker for the main on a 100 amp panel with 100 amp bus bars].

Can't really use the combined totals of the branch circuit breakers to do this.

Also, as '66 mentioned, the typical 120/240 VAC 1 phase 3 wire service will have a total Volt-Amp capacity of 12,000 VA [12.0 KVA] per Line to Neutral load [hot to neutral], for a combined total of 24,000 VA [24.0 KVA]. The 12.0 KVA is 120 volts times 100 amps.

Consequently, the total Volt-Amp capacity for the same panel, using Line to Line loads [240 volts, or both "hots"] is also 24,000 Volt-Amps [24.0 KVA].
Multiply 240 volts times 100 amps, which equals 24,000 VA [24.0 KVA].

Please pardon the vague and slang terms I have used in this message. I'm trying to make it easier to comprehend [any luck ?? ].

Other than "twilight Zone Stuff", what did you think of the topics and threads in the Theory discussion area?

Let us know!!

Scott SET
_________________________
Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!

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#801 - 03/17/01 11:21 AM Re: ampacity question
Boscodog Offline
Member

Registered: 02/27/01
Posts: 13
Loc: Massachusetts
Thanks Scott and Sparkies. I think I'm finally totally clear on this now. As I mentioned in my very first post on this board a week or 2 ago I'm not a sparky myself, just a homeowner who has known how to do basic wiring for some time now, but now I want to know more than the basics. I actually think wiring is fun! I fantasize sometimes about scrapping this desk job of mine, becoming an apprentice, and roping some homes. Yeeee dawgy!! Pretty bizarre, huh?

Scott, as long as you've got the time and energy, keep posting the theory stuff. They work way better than my Sominex!

JUST KIDDING!!

Seriously, although I don't understand everything you write, I do pick out pieces here and there. I greatly appreciate the effort that you, and everyone else, puts into this board. It's a fun place to come to. (Can you tell I don't have a life? )

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