ECN Forum
Posted By: sparkync derating - 03/15/05 01:06 PM
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 [Linked Image] 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).]
Posted By: George Little Re: derating - 03/15/05 01:48 PM
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.
Posted By: resqcapt19 Re: derating - 03/15/05 03:38 PM
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
Posted By: George Little Re: derating - 03/15/05 07:56 PM
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??
Posted By: mustangelectric Re: derating - 03/15/05 09:32 PM
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).]
Posted By: eesac Re: derating - 03/16/05 01:30 AM
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).]
Posted By: mustangelectric Re: derating - 03/16/05 01:40 AM
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).]
Posted By: sparkync Re: derating - 03/16/05 02:29 AM
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).]
Posted By: mustangelectric Re: derating - 03/16/05 02:38 AM
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).]
Posted By: eesac Re: derating - 03/16/05 08:13 PM
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.
Posted By: DiverDan Re: derating - 03/16/05 11:19 PM
One other factor to consider is derating for ambient temperature considerations. Remember that some of the NEC tables are set at 86ºF, which is not a realistic ambient temperature in the USA. There is software available that takes all these factors inclusive of voltage drop to IEEE 141 standards and computes the required conductor size quicker than you can pickup a pencil.
Posted By: resqcapt19 Re: derating - 03/17/05 12:02 AM
George,
Any branch circuit that is required by code to have an ampacity of 125% of the load would have to have conductors with an ampacity of at least 125% of the load. Motor cicuits, some appliance circuits,and any circuit that serves a continuous load.
Don
Posted By: DiverDan Re: derating - 03/17/05 12:18 AM
Hi resqcapt19,

As a point of correction, not all continuous loads require upsizing the conductors 125%.

Bacially, only motor type loads require the 125% upsize. This is to allow a significant conductor size for the motor device's inrush of current when it is started, as well as the ability to offset the loss of current and added impedance (resistance) generated from the motor device's power factor. I might also add that it is a very good idea to upsize non-linear load (UPS or any device that converts AC power to DC) conductor sizes as well.


[This message has been edited by DiverDan (edited 03-16-2005).]
Posted By: George Little Re: derating - 03/17/05 01:06 AM
Don, I've always taught that we pick the conductors and the overcurrent device of a continuous load based on 125%. If we have a combinatin of non continuous and continuous load it's a matter of adding together the non-continuous load at 100% and the continuous at 125%. given the total we pick an overcurrent device to be not less than that number. Now if there are any ampacity adjustments for raceway fill or ambient tempetature we apply these adjustments to the conductors and as long as the result did not get below the actual connected load at 100%, we were within code. We need to also check our adjusted ampacity of the conductor to see if there was any conflict with the overcurrent device which needs to be at least the size of the continuous loat at 125% plus the non-continuous a100%. If the conductor's adjusted ampacity were below the rating of the overcurrent device we are usually permitted to round up under most circumstances.

Code references avaible- to lazy to look them up [Linked Image]

[This message has been edited by George Little (edited 03-16-2005).]
Posted By: DiverDan Re: derating - 03/17/05 03:11 AM
Hi George Little,

I am assuming Don is Dan...

quote:I've always taught that we pick the conductors and the overcurrent device of a continuous load based on 125%.

That's not quite right. You upsize only the conductor 125%. The overload (not over current) device is sized to either 115, 125 or 140% depending on the device's service factor and temperature rise per somewhere in the 430 section. The over current protection is usally sized to 300% of the motor device's FLA as it is intended to protect against AFI.

Continuous and non-continuous load computations are only for the OCPD size and have nothing to do with sizing the conductors.

(opinion: I agree with George Little that the conductors should be upsized along with the OCPD. If you allow a larger current pathway, you should adjust the conductors to deal with the increased pathway...but that is only my opinion and not NEC's.)

Once again, there is software available that computes these values and quotes the applicable tables and articules faster and more accuratly than it took me to misspell the words in this reply.

[This message has been edited by DiverDan (edited 03-16-2005).]
Posted By: mustangelectric Re: derating - 03/17/05 03:24 AM
Hi,
Well said there new guy..diver..welcome aboard..this is a great place for a guy like you!

what about locked rotor?

-regards

greg

[This message has been edited by mustangelectric (edited 03-16-2005).]
Posted By: George Little Re: derating - 03/17/05 04:11 AM
Actually Dan, I was responding to Don and my speel had to do with other than motors. Don seemed to indicate that the need for 125% capacity for continuous load applied after derating and I read that the 125% applies for continuous load prior to derating. See 210.19(A)(1) for conductors. As for the overcurrent device, it is required to be 125% of continuous load period. 210.20(A). There seems to be two subjects in this thread- one is motors and the other one for continuous loads.
Posted By: DiverDan Re: derating - 03/17/05 05:36 AM
You're right George Little...my mistake! I was looking at the User Names and not the signitures. As I didn't see any Dons (until now) I thought you had typoed my name. Sorry again.
Posted By: resqcapt19 Re: derating - 03/17/05 04:45 PM
George,
Any time the code requires that the branch circuit have an ampacity of 125% of the load the conductor must have this ampacity after any required adjustment or correction factors are applied. These factors change the ampacity of the conductor. Here is an example of the type of rule that I am talking about.
Quote
440.32 Single Motor-Compressor.
Branch-circuit conductors supplying a single motor-compressor shall have an ampacity not less than 125 percent of either the motor-compressor rated-load current or the branch-circuit selection current, whichever is greater.
Any time the rule is worded like this, the branch circuit conductor ampacity after derating must be at least 125% of the load current.

edited code reference to one that makes my point

[This message has been edited by resqcapt19 (edited 03-17-2005).]
Posted By: resqcapt19 Re: derating - 03/17/05 06:14 PM
Quote
If the conductor's adjusted ampacity were below the rating of the overcurrent device we are usually permitted to round up under most circumstances.
Where ever the code requires a conductor with a minimum ampacity, that is the ampacity after all adjustment and/or correction factors are applied. The rule in 240.4(B) does not change the ampacity of the conductor, it only permits you to use an OCPD that has higher rating than the conductor's ampacity.
Don
Posted By: George Little Re: derating - 03/17/05 07:37 PM
I'm not 100 per cent convinced Don, I read the words in 210.19(A)(1) and they say "the branch circuit conductors shall have an ampacity not less than the load to be served." It goes on to say the rule applies before the application of any adjustment or correction factors"and this rule applies to everything in Chapter 1-4
How about this example:
8000va water heater
1.25 x 8000 = 10000va
10000va ÷ 240 = 41.5 (42a)
Assume 75° terminations and THHN wire
45a overcurrent protection
# 8 awg conductor
Raceway has 6 conductors in ambient of 86°F
Adjusted ampacity 55 x .8 = 44a
44a conductor will carry the load of 33.3 a. and we are allowed to round up to 45a overcurrent device.
Rating of circuit is still 45a. per 210.3

Okay Don, I've got my kevlar vest on and my PPE gear, so let the sparks fly [Linked Image]
Posted By: resqcapt19 Re: derating - 03/17/05 08:45 PM
George,
The rounding of of the overcurrent protection does not change the ampacity of the conductor. Any place that the code calls for a conductor of a minimum ampacity, that must be the ampacity of the conductor under the conditons where that conductor is installed. Remember that 310.15(B) tells us that a conductors ampacity shall be as specified in the ampacity tables as modified by (1) through (6). The temperature correction factors are part of the ampacity tables and the adjustment factor for more than 3 conductors in a raceway or cable is 310.15(B)(2).
This is an ongoing problem with the use of the code. In the 2005 code they changed the wording to say that you cannot use the provisions of 240.4(B) for tap conductors. This was not a rule change, but so many code users were missing the fact that 240.4(B) does not change the conductor ampacity. The tap rules are like the rule that I cited in 440.32. They require a minimum conductor ampacity. Some additional sections that require a minimum conductor ampacity are 424.3(B), 424.82, 426.4, 427.4, 430.22, 645.5, 669.5, 680.9. In all of these cases the rule requires a branch circuit conductor ampacity of 125% of the nameplate current. In all of these cases that conductor would have to have that minimum ampacity after the application of the adjustment and/or correction factors.
If the code section that covers a installation does not require a minimum conductor ampacity, then you are correct and the provisions of 210.19(A)(1) apply. The don't apply where other code sections set a minimum conductor ampacity.
Don
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