ECN Forum Forums Code Related Discussion NEC & other Code issues derating

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.

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

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).]

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

[This message has been edited by George Little (edited 03-16-2005).]

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).]

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).]

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.

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.

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.
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).]

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