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#92373 - 03/15/05 05:06 AM derating
sparkync Offline
Member

Registered: 10/17/02
Posts: 811
Loc: NC
When derating conductors, do you determine the wire size according to the amperage after taking the 125% or before. For example, I have a load of 17.5 amps. If I take # 14 wire amp capacity in the 90 degree column and derate it 70 % I get 17.5 amps. Can I use this calculation or do I have to figure 125% of 17.5 amps. which would be 21.87 amps, which would mean I would have to use # 12 wire?? This is only an example, I would use # 12 wire anyway, but in some of my other calculations, it might make a real difference. I'm figuring out on this job, I'm doing what an electrical engineer should be doing if there had been a set of prints drawn out,like was orignally proposed by the inspectors. Somehow they didn't make the owner get it done like that, so now I'm having to figure it all out Thanks..

I also have a situation that I need to run branch circuits to 7 pieces of machinery. I originally was going to run 1 1/4" conduit and run all in it. Now I realize that if I do that, I will have to derate to 50% which will make me have to run a # 3 wire for one machine that would originally have taken # 6 wire. I guess it would pay me to run separate conduits, at least where I will only have to derate 70% . Any other input on this? thanks again..

[This message has been edited by sparkync (edited 03-15-2005).]

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2014 / 2011 NEC & Related Books and Study Guides
#92374 - 03/15/05 05:48 AM Re: derating
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Sparky, we are missing some key information if you are in need of some answers. Is it a continuous load? Is it a wet, damp or dry location? How many conductors are in the raceway? We will assume 75┬░ terminations. Basically, if the load is continuous you'd take the load times 125% and pick your conductor gauge out of the 75┬░ column. If you are using conductors that have a 90┬░ insulation you'd start your derating from the 90┬░ column. As long as your derating didn't take you below the actual load at 100% you'd be within code. You'd pick your overcurrent protection at 125% for continuous load and round up if you need to do so. Now if we are talking motors that's handled a little differently. Motors are not considered continuous loads.
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George Little

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#92375 - 03/15/05 07:38 AM Re: derating
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
George,
Many sections of the code require that the ampacity of the circuit conductor be 125% of the load current. If this is the case you must have the 125% after any required derating.
Don
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Don(resqcapt19)

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#92376 - 03/15/05 11:56 AM Re: derating
George Little Offline
Member

Registered: 01/18/04
Posts: 1492
Loc: Michigan USA
Don- Conductors are rated for continuous load. Table 210.16 is the wire ampacity at continuous use. See definition of ampacity in Article 100. The issue usually is the terminations. The example I used was pretty generic and there are other factors depending on the load I agree. What examples would you offer that would have us needing 125% after derating??
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George Little

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#92377 - 03/15/05 01:32 PM Re: derating
mustangelectric Offline
Member

Registered: 02/08/04
Posts: 496
Loc: Bentonville, AR
Hi,
You said: "When derating conductors, do you determine the wire size according to the amperage after taking the 125% or before?"

if you have more than three current carrying conductors in a raceway they must be derated..why are you changing wire size? this is not what derrating is all about..

if the wire is #12 and i need to derate that doesnt mean i have to go pull the number 12 out and install a new wire! it means that the wires current carrying capacity is reduced..you would have to limit the CURRENT not the wire size! this is a design issue and so far it looks like a terrible design..you are opening a can of worms by designing this way and it is not the standard! if i have to run 4 circuits i would split it up anyway..you are not gaining anything by using #3! you are making the installation more expensive than is needed. the first rule for design is to do it economically...i can build a highway to the moon but it would be too expensive...not practical..understand?

there is no way you can use #14 for any load over 15A...1st year apprentice stuff..

you said: "I also have a situation that I need to run branch circuits to 7 pieces of machinery. I originally was going to run 1 1/4" conduit and run all in it."

please elaborate on how you plan to run the feed for seven peices of equipment in a single 1-1/4 pipe?

you would be getting outside the box in my opinion if you stuffed all of those circuits in a single run....and what would be the benefit fo doing this?

it is BAD practice to install the way you have been talking about...is this stuff going to be inspected?

here is my suggestion:

throw away ALL that #14 your using...

try not to use a single pipe for more than three circuits.

your life will be a lot simpler and you will have some extra time to do other things beside trying to save a $ on the diff between #14 and #12..what will you do if the vd exceeds 5%?

geo said: "Motors are not considered continuous loads." ...please elaborate for us..is this another broad statement or can you back that up?

this is interesting...

good luck!

regards

greg


[This message has been edited by mustangelectric (edited 03-15-2005).]
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#92378 - 03/15/05 05:30 PM Re: derating
eesac Offline
New Member

Registered: 02/25/05
Posts: 8
Loc: california
I would agree with George that motors are not considered continuous loads. 215-2 indicates feeders to be noncont + 125% cont. However, for motors, circuit conductors can be sized per 430-24, 125% of largest motor + fla of all other motors + other loads. 430-62.A talks about feeder protection which is 430.52 ofr largest motor + sum of other motors. 430-62.B references 430-24. 430-63 also references 430-62. Based on the above, conductors and ocp for motors are sized diferently than other loads, and not sized as continuous loads, at least not in the literal sense that we are used to sizing for continuous loads.

IMO.

(All references are for 2002 NEC)



[This message has been edited by eesac (edited 03-15-2005).]

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#92379 - 03/15/05 05:40 PM Re: derating
mustangelectric Offline
Member

Registered: 02/08/04
Posts: 496
Loc: Bentonville, AR
Hi,
Any motor application SHALL be considered as CONTINUOUS DUTY UNLESS the nature of the apparatus it drives is such that the motor will NOT operate continuosly with load under any condition of use.

Duty, Continuous-Operation at a substantially constant load for an indefinitely long time.

i could go on and on but i think i have made my point.

regards

greg

[This message has been edited by mustangelectric (edited 03-15-2005).]
_________________________
Electricity has no respect for ignorance!

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#92380 - 03/15/05 06:29 PM Re: derating
sparkync Offline
Member

Registered: 10/17/02
Posts: 811
Loc: NC
Greg, sorry to upset you. The loads I'm talking about are not debateable. I cannot change the loads. They are fixed pieces of machinery, with motor loads and fixed amperage.
The machines are 3 phase, and are in a pretty big building. Under the circumstances, it is not fesible to pull a conduit to each piece of machinery from the panel, therefore I am pulling 3 circuits through the appropriate conduit size. That gives me 9 current carrying conductors, which the code says I have to derate to 70%. After derating these conductors, some of the original conductor sizes will not work. I understand what you are talking about, when the load can be regulated; lights, recpts. etc., but in my case, I have no option.
As far as # 14 not being used for anything except 15 amps, you need to look at the previous post about that particular question. I'll not take the time to re run all of that.
Now, if I'm still missing something please let me know ... Steve.....

[This message has been edited by sparkync (edited 03-15-2005).]

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#92381 - 03/15/05 06:38 PM Re: derating
mustangelectric Offline
Member

Registered: 02/08/04
Posts: 496
Loc: Bentonville, AR
Steve,
I am not upset at all...this is lively conversation thats all...dont let my message get lost in the unspoken word..if you could hear my voice you would see that i am just discussing this not trying to be argumentative..

maybe you could use a small cable tray and use tray cable or mc cable?

good luck on the job!

-regards

greg

[This message has been edited by mustangelectric (edited 03-15-2005).]
_________________________
Electricity has no respect for ignorance!

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#92382 - 03/16/05 12:13 PM Re: derating
eesac Offline
New Member

Registered: 02/25/05
Posts: 8
Loc: california
I do not think the continuos duty on the motor is the same as requiring the load to be counted as a continuous load and multiplied by 125%. If that was the case, then the calc method shown in article 430 as mentioned for motor loads would be incorrect. There are calcs for motor loads, and there are calcs for continuos and non-continuos loads that are not motor loads.

Regards.

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