# minimum ampacity load question



## heyhi (Jun 25, 2008)

Whats up guys, its me again. I'm studying this book ,commercial wiring, and I'm doing a panel worksheet to calculate circuit loads and component selecting. I have a good understanding on everything except minimum ampacity.This is the book page 46 thru 50 http://books.google.com/books?id=ZT...=X&oi=book_result&resnum=1&ct=result#PPA46,M1

On the example worksheet the first one is

- 9 luminaries
-783 VA
-6.5 amps
-continuous load
-1.25 * 6.5 = 9 amps
-9 amps minimum breaker rating 
- we are gonna use a 20 amp breaker 12 awg wire
-the minimum ampacity is 16 thats column k

The thing is Iwas reading about minimum ampacity and it says on page 49 (If the load Type is C or N, this case its continuous ,the minimum ampacity must be greater then column h,which is ocp selection ampere or the absolute minimum breaker rating, and not less the column I ,which is the breaker rating we are gonna be using which is 20 amps.
But as u can see the minimum ampacity is 16 not greater then column I which is 20

on page 50 number 2 we have 10 120 v fluorescents @ 150 VA each

150*10 = 1500 VA total
1500 v/a divided 120=12.5 amps
I'm using continuous load 
12.5 *1.25=15.6 amps 
15.6 amps ocp selection ampere
20 amp ocp rating
#12 wire
now for the min ampacity should be 21 since it has to be greater then the ocp rating but on the example worksheet above it is also a continuous load type and the min ampacity is less the the ocp rating. some one please explain


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## heyhi (Jun 25, 2008)

wow 90 views and not one of u know the answer


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## Nodoggie (Oct 17, 2008)

heyhi said:


> The thing is Iwas reading about minimum ampacity and it says on page 49 (If the load Type is C or N, this case its continuous ,the minimum ampacity must be greater then column h,which is ocp selection ampere or the absolute minimum breaker rating, and not less the column I ,which is the breaker rating we are gonna be using which is 20 amps.
> But as u can see the minimum ampacity is 16 not greater then column I which is 20
> 
> on page 50 number 2 we have 10 120 v fluorescents @ 150 VA each
> ...


Ok... i will then. i think you are making this harder than it has to be, my friend. I do not have your book in front of me...in fact, nor an NEC book. but it seems pretty simple. So, regarding the second example you end up with 15.6A current draw for the circuit (12.5 x 1.25 for load over 2 continuous operation hours i believe). Your OCP is 20A, not 21...if your book says otherwise it's a misprint. You can't use 15 but you can use the next standard size up...which is 20A. Wire ampacity for #12 is always 20A (not only 310-16, but also regarding "small conductors," which I believe is in Art. 300)...

...So i'm not sure what your question is beyond that.


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## heyhi (Jun 25, 2008)

Id love to get an answer from some one who really knows there calculations. If u click on the link ul see what i mean. The link will show you a panel worksheet.It tells u what the loads are and what calculations to use. 15.6 amps is the load and 20 amps is the breaker,Any one who knows there code will tell u that a conductor ampacity is based on the type of wire and its ampacity is used for penalty factors.12 thhn is 30 amps my friend oh my friend it is 30 amps. so number 12 is not always 20 amps it can only be used with a breaker with a max of 20 amps. you have no clue just like me on what the answer is.and its 3 continuous hours lol stop please ur making me look good and i been into the nec less then 6 months. 

Wait till some of these guys read what u said there really gonna lay into you


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## JohnJ0906 (Jan 22, 2007)

In my opinion, that book needlessly complicates things.

I know the calculations, but I don't understand that book (but I didn't try too hard)


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## ralph (Apr 6, 2008)

heyhi said:


> wow 90 views and not one of u know the answer


I probably could take the time and give you an answer, like alot of the "veiws" on the site, but lets be honest. I saw all that nonsence you had , and I turned and went elsewhere. Thats the stuff I learned in Tech school, and Im not in the mood to deal with it now.


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## Nodoggie (Oct 17, 2008)

yeh. fact is, i didn't look at the link either. I applied real world knowledge to answer the question. and heyhi, FYI, #12 conductors are ALWAYS rated for 20A, never 30A. Irrespective of degree columns in 310-16, or temperature de-rating columns, there is a secton in the codebook that suercedes that in regards to "small conductors." I believe it is in Art. 300.


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## Speedy Petey (Jan 10, 2007)

Nodoggie said:


> ..., FYI, #12 conductors are ALWAYS rated for 20A, never 30A. Irrespective of degree columns in 310-16, or temperature de-rating columns, ...


Always??? 
What about derating and adjustment factors?
What about motor circuits?
What about A/C circuits?


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## Nodoggie (Oct 17, 2008)

I can't speak for anyone else, but I would NEVER use OCP over 20A for 12-guage wire. EVER. NO EXCEPTIONS....even with de-rating. Once again, refer to Section 240.4 (D). (See the asterick next to certain wire sizes in the 310-16 table?) Could an exception to be found to conform with this section? possibly. But a general--and safe---rule would be to stick to 20A max for 12 AWG.


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## Bkessler (Feb 14, 2007)

There are situation where it is legal to install a motor with #12 and have it on a 25 or 30 amp breaker. I have never done it personally other than the math on an electrical exam.


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## Nodoggie (Oct 17, 2008)

Repeat:


Nodoggie said:


> Could an exception to be found to conform with this section? possibly. But a general--and safe---rule would be to stick to 20A max for 12 AWG.


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## Speedy Petey (Jan 10, 2007)

Nodoggie said:


> Repeat:


Are you familiar with Art.'s 430 and 440.



Also, you state: "_#12 conductors are ALWAYS rated for 20A, never 30A._"

What about derating????? So when you derate a #12 conductor you start from 20 amps? 
If you do you are doing yourself a great disservice.

#12 THHN *IS*, most certainly, without question, and clearly, rated at 30 amps.
The ONLY reason we are _normally_ held to 20 amps is 240.4(D).


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## Nodoggie (Oct 17, 2008)

Speedy Petey said:


> The ONLY reason we are _normally_ held to 20 amps is 240.4(D).


 
:blink: good enough reason for me.

EDIT: you could also squeeze 16 #12 wires in a 3/4 conduit....but i wouldn't recommend it. lol


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## Speedy Petey (Jan 10, 2007)

You like to avoid questions you can't answer, huh?


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## nyerinfl (Dec 1, 2007)

Nodoggie said:


> :blink: good enough reason for me.
> 
> EDIT: you could also squeeze 16 #12 wires in a 3/4 conduit....but i wouldn't recommend it. lol


What would you rate those 16 #12s after derating?


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## Nodoggie (Oct 17, 2008)

Speedy Petey said:


> You like to avoid questions you can't answer, huh?


Nope. I just happen to understand that the NEC is the _minimal_ safety standard. Safety is paramount to me. 



nyerinfl said:


> What would you rate those 16 #12s after derating?


You could have 16 wires (per Addendum C), not _conductors_, and you wouldn't even have to de-rate.


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## Speedy Petey (Jan 10, 2007)

Nodoggie said:


> Nope. I just happen to understand that the NEC is the _minimal_ safety standard. Safety is paramount to me.


Nice try at a save, but the shot got by you.
I have a theory that folks who play the "_I always over do things for safety reasons_" card, do so because they do not know the right way.








Nodoggie said:


> You could have 16 wires (per Addendum C), not _conductors_, and you wouldn't even have to de-rate.


Oh really??? Maybe if there were 13 grounds.
Are you familiar with 310.15(B)(2)???

Do you mean, Annex C? 
Also, wires??? I think conductors is the proper term.


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## Nodoggie (Oct 17, 2008)

first of all. Yes. you are right ok? you got me. I meant "Annex C". jeez. :lol:

Second. No, I did not mean "conductors." If you had 16 _conductors_ then you would have to de-rate per 310.15(B)(2). But that would still not affect how many _wires_ you could legally fit in a 3/4" conduit. I think you're confusing yourself. :blink:



Speedy Petey said:


> I have a theory that folks who play the "_I always over do things for safety reasons_" card, do so because they do not know the right way.


You're a Super for Berg aren't you?


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## Speedy Petey (Jan 10, 2007)

I really don't feel like playing this back and forth game all night, but I have one last question.

What are you saying the difference is between wires and conductors?


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## Speedy Petey (Jan 10, 2007)

Nodoggie said:


> You're a Super for Berg aren't you?


WTF???


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## Nodoggie (Oct 17, 2008)

wires are indeed electrical conductors (from a physics perspective). However, a wire is not always a "current-carrying" conductor per NEC definitions. A ground(ing) wire is not a current-carrying conductor under normal circumstances. A ground(ed) wire is. 

A better term for me to use would be "non-current carrying" conductor. that is more precise and aligns with the NEC. But I said "wire" because my arthritis is kicking in. Are you happy now? 

:wallbash: hehe


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## rlc3854 (Dec 30, 2007)

Berg Electric Inc., large EC, super's are paid well with bonues. Some feel the super's don't know their *ss from a hole in the ground but get jobs done and make the company money. My question to nopuppy "What is an IBEW Master"?


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## Nodoggie (Oct 17, 2008)

Usually an I.B.E.W. Master Electrician is a fancy term for a foreman...that is, if he has an actual cert.


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## Speedy Petey (Jan 10, 2007)

rlc3854 said:


> Berg Electric Inc., large EC, super's are paid well with bonues. Some feel the super's don't know their *ss from a hole in the ground but get jobs done and make the company money.


So then puppy asking me: _"You're a Super for Berg aren't you?"_ was quite the insult then huh? 
Being I'm from NY I'm wondering why he thinks I'd know this. 

It's OK. I'll let it go this time.


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## rlc3854 (Dec 30, 2007)

Yea Speedy kind of. I worked in California for a lot of years and never heard of a foreman as a master especially since I have worked as a foreman, general foreman and super there. However when I started in the trade my journeyman always refered to himself as a master but, he was from your neck of the woods.


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## Nodoggie (Oct 17, 2008)

lolol. it's all good, Speedy. :thumbsup:


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## heyhi (Jun 25, 2008)

[ So, regarding the second example you end up with 15.6A current draw for the circuit (12.5 x 1.25 for load over 2 continuous operation hours i beliIeve


I don't think u guys caught that.Some one tell this guy its 3 hrs, on a continuous load.


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## Nodoggie (Oct 17, 2008)

i know it is, heyhi. no need to beat me over the head with it. i just said it wrong is all. (Art 100; 210.19 A). And yes, I did catch that the circuit draw is 15.6. if you go back and read you will see i made that quite clear.


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