# 3/0 on 200 amp service? Why?



## chicken steve (Mar 22, 2011)

So 200A /1.73= 115A for ea 3ph leg?

T310.15(b)(6) {formerly 310.16} sezs i can use #2 cu @75F

right?

:whistling2:


~CS~


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## wildleg (Apr 12, 2009)

Dkaskie said:


> Why are people putting 3/0 cu on a 200 amp single phase 3 wire service. They keep quoting 310.15 using 90 degree rating for copper as being 200 amp. I am questioning if they even understand what happens inside an electrical panel.
> 
> Let me explain. Each ungrounded conductor of the service carries 100 amp a piece of the 200 amp rated service. Meaning....... each conductor does not have to be rated at 200 amps. This is why table 310.15(b)(6) exists, to keep "people" from trying to tell you to do something you don't need to.
> 
> Sorry. This is just driving me crazy.


unfortunately, you have some misconceptions about how an electrical panel is rated. I suggest you read the code book or take some courses. Your statement is 90% wrong.

*If the calculated load is 200A for a 200A panel, each leg must be able to carry 200A.*

read that again until you understand it.

for residential single phase, you can go less than the 3/0 per code rules, but not for the reasons you stated.


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## Dkaskie (Dec 18, 2014)

I have never seen anywhere in the code on any course where it says that each leg is required to carry 200 amps on each leg. I also never said anything about 3 phase


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## chicken steve (Mar 22, 2011)

I can see this may be a long thread.....

~CS~


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## Ultrafault (Dec 16, 2012)

What would make you think think each leg only carries 50% of the rated current?


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## wildleg (Apr 12, 2009)

Dkaskie said:


> I have never seen anywhere in the code on any course where it says that each leg is required to carry 200 amps on each leg. I also never said anything about 3 phase


start at article 220 and do a load calculation.

then go to article 230, and start to figure out your service. let's say it's overhead:
*230.23 Size and Rating.
(A) General. Conductors shall have sufficient ampacity to
carry the current for the load as calculated in accordance
with Article 220 and shall have adequate mechanical
strength.*

now, where precisely, in between your load calc in 220, and your service conductor rating in 230.23, does it say you can divide the load in half ?

It doesn't.

If you don't want to believe me, then consider this: 

If what you say were true, why would the conductors have to be 2/0 ? you can get 100 amps out of #1 or #2 next size up at 60 degrees or 1/0 AL? Explain why 200 amp panels would ever need 2/0 ?


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## chicken steve (Mar 22, 2011)

I sense a theory/code clash here....

~CS~


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## Dennis Alwon (May 9, 2009)

Take 2 sp 20 amp breakers and put them together with a handle tie. Now what amperage is each phase capable of handling without tripping. 10????? I don't think so.. LOL


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## wildleg (Apr 12, 2009)

chicken steve said:


> I sense a theory/code clash here....
> 
> ~CS~


the allowed reductions for single phase, and neutral reductions take into account the safety factor and theory IMO Steve. 

The OP has a misunderstanding of the rating of an enclosure IMO.

when you work out the watts, like most engineers do in the plans on the panel schedules, all is fairly clear.

theory be damned, we don't install by theory, we are required to install per code.


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## hardworkingstiff (Jan 22, 2007)

If you have a 3000 watt heater that operates at 240V, what size circuit would you install?

I would do the math like this,
3000/240= 12.5A on each leg of the 240V circuit.

12.5*1.25 = 15.63A

So, I would run a 20A circuit to this heater.

Now, according to what you posted (at least the way I read it), you would call that a 40A circuit. It's not a 40A circuit, it's a 20A circuit.


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## chicken steve (Mar 22, 2011)

wildleg said:


> theory be damned, we don't install by theory, we are required to install per code.


i find one w/o the other a rather obvious detriment , no matter what i do, electrical work, or playing music.

My library follows suit, but hey, that's me.....:laughing:~CS~


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## BSK3720 (Mar 29, 2014)

Dkaskie said:


> Why are people putting 3/0 cu on a 200 amp single phase 3 wire service. They keep quoting 310.15 using 90 degree rating for copper as being 200 amp. I am questioning if they even understand what happens inside an electrical panel.
> 
> Let me explain. Each ungrounded conductor of the service carries 100 amp a piece of the 200 amp rated service. Meaning....... each conductor does not have to be rated at 200 amps. This is why table 310.15(b)(6) exists, to keep "people" from trying to tell you to do something you don't need to.
> 
> Sorry. This is just driving me crazy.


Wow!


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## Dkaskie (Dec 18, 2014)

Okay. Okay. I'm wrong. I get your point. Total misunderstanding on my part. 

Don't shoot me.


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## 8V71 (Dec 23, 2011)

> Let me explain. Each ungrounded conductor of the service carries 100 amp a piece of the 200 amp rated service. Meaning....... each conductor does not have to be rated at 200 amps.


Start here:

Current is equal in all parts of a series circuit. So if you had a 240 volt 200 amp load both conductors would be carrying 200 amps.


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## AK_sparky (Aug 13, 2013)

Dkaskie said:


> Okay. Okay. I'm wrong. I get your point. Total misunderstanding on my part.
> 
> Don't shoot me.


I think your question was fair. Some things get heated pretty quick around here.
I suggest putting a post in the intro section telling us a bit about you and where you are at in your apprenticeship, experience, etc. Without doing that, usually people respond assuming you are a full-blown-know-everything-already-done-1000-homes electrician and that you should know the answer to this.


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## ponyboy (Nov 18, 2012)

AK_sparky said:


> I think your question was fair. Some things get heated pretty quick around here.



What no they don't. I can't believe you said that. Go to hell.


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## 480sparky (Sep 20, 2007)

Perhaps they use 3/0 because_ they have enough on hand to do the job_, as opposed to spending time & money buying more 2/0.

Jez sayin.


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## Black Dog (Oct 16, 2011)

Dkaskie said:


> Okay. Okay. I'm wrong. I get your point. Total misunderstanding on my part.
> 
> Don't shoot me.


Welcome to the forum:thumbsup:

Don't let these guys bust your chops


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## bkmichael65 (Mar 25, 2013)

Dkaskie said:


> Okay. Okay. I'm wrong. I get your point. Total misunderstanding on my part.
> 
> Don't shoot me.


That's not how you do it! You're supposed to make several thousand posts explaining how you're right and everybody else is stupid. Search grounding and bonding threads for examples.


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## 8V71 (Dec 23, 2011)

bkmichael65 said:


> Search grounding and bonding threads for examples.


oops....now you did it. :laughing:

ground F5.....grounding F5.....grounded F5...rod F5...rods F5....egc F5...gec F5...bond F5...bonding F5...bonded F5...ufer F5...


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## cabletie (Feb 12, 2011)

Perhaps they use 3/0 because they have enough on hand to do the job, as opposed to spending time & money buying more 2/0.



Jez sayin.


Perhaps they use 3/0 because because 2/0 is only good for 175 amps. It was never answered if it was residential


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## aftershockews (Dec 22, 2012)

cabletie said:


> Perhaps they use 3/0 because they have enough on hand to do the job, as opposed to spending time & money buying more 2/0.
> 
> 
> 
> ...


Really?

Look at Table 310.15(B)(6)


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## cabletie (Feb 12, 2011)

Just edited the post. It was never stated that it was residential or not


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## cabletie (Feb 12, 2011)

Re-checked 2/0 = 175A at 75c


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## acro (May 3, 2011)

I just had this conversation/argument with my father-in-law. He was positive that a 200A service only had 100A on each leg. He is retired GTE. He should stick to telco.


He also said that all the breakers on one side of the panel was one leg, and the other was on the other side of the panel.:blink:

Stubborn, he is.


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## dronai (Apr 11, 2011)

I love when my customers start adding up all the breakers in the panel to do a load calculation


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## Meadow (Jan 14, 2011)

bkmichael65 said:


> That's not how you do it! You're supposed to make several thousand posts explaining how you're right and everybody else is stupid. Search grounding and bonding threads for examples.


:laughing: Don't even get me started :no:


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## Meadow (Jan 14, 2011)

acro said:


> I just had this conversation/argument with my father-in-law. He was positive that a 200A service only had 100A on each leg. He is retired GTE. He should stick to telco.
> 
> 
> He also said that all the breakers on one side of the panel was one leg, and the other was on the other side of the panel.:blink:
> ...


As long as he isn't posting on DIY chatroom he is ok :laughing:


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## j johnson (Jul 20, 2009)

voltage drop!


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## Dennis Alwon (May 9, 2009)

Since the OP ruined this thread by deleting his original post and changing his profile, I will close this thread.


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