derating and the 90* colm

[dcl34769]

please settle an argument i am having. when derating wires for more than 3 current carrying conductors, 75*c rated equipment, is it permissable to use the 90*c coloum or do you have to use the 75*c coloum? if 90* is allowed please give code reference if any exist. thanks in advance.

 

[wolfman56]

Re: derating and the 90* colm

Think of it as two different items.
1. The conductor in raceway.
2. The termination
Seperate issues. Look at it this way. At a given ampacity, any conductor will heat up. The temprature that you will allow the conductor to rise up to cannot be more than items 1, and 2, will allow.
The termination has nothing to do with heat build up in the raceway due to wire fill.
The answer for you're argument is that you do all ampacity calc's, volt drop, derating, etc. using the insulation's temprature column. Then check to make sure that you're result doesn't exceed the termination's rating.

 

[dereckbc]

Re: derating and the 90* colm

Sure you can use the 90 degree insulation column for more than 3 current carrying conductors for de-rating purposes. The process is straight forward and simple:

1. Select a conductor size from the 90 degree insulation column table 310.16.

2. If applicable, apply ambient temperature correction factor from 310-16.

3. Apply adjustment factor for more than 3 current carrying conductors from table 310.15(B)(2)(a).

4. Check that the corrected number is equal or less than that of the value listed for the 75 degree column in table 310.16.

5. If 4 is OK you are done, if not, use a larger cable and start from step 1.

Example; determine the required conductor size with 4-current carrying conductors in a raceway, using THHN insulated cable with 60-amp breakers @ 27 degree C ambient temperature.

1. Look up the current rating of a #6 AWG in table 310.16 90 degree column. = 75 amps

2. Apply temperature correction. This example, none.

3. Apply adjustment factor for 4 current carrying conductors from table 310.15(B)(2)(a). .8 x 75 = 60 amps.

4. Check to see if 60-amps is equal or less than the value listed for 75 degree column for a 6 AWG conductor, 60 is less than or equal to 65

5. OK. Done

 

[charlie b]

Re: derating and the 90* colm

I concur with both previous replies. The code reference that permits this approach is 110.14(C).

 

[dereckbc]

Re: derating and the 90* colm

Originally posted by charlie b:
I concur with both previous replies. The code reference that permits this approach is 110.14(C).

Thanks Charlie, I should have added that.

 

[dcl34769]

Re: derating and the 90* colm

thank you to all that replied.

 

[itasca_mn]

Re: derating and the 90* colm

dereckbc et al,

You can eliminate a lot of going back and forth and guessing if you divide the correction factors into the required ampacity, instead of arbitrarilly picking a conductor from the table, only to find that after your calculation it is too small and you must go back and re-calculate as you state in step 5 of your explaination.

Here is how I size my conductors.

Using your 60 amp/4 current-carrying conductor example:

Step 1: Compute the load at 125% of continuous, plus 100% of non-continuous load. (In this case all non-continuous). 60 amps.

Step 2: Check equipment ratings as per 110.14c. If you are certain that all the equipment is 75 degree rated, choose a conductor rated 60 amps from the 75 degree column. If not certain, choose the conductor from the 60 degree column. Write that conductor down. A #6 is selected from the 75 degree column for 60 amps. A #4 from the 60 degree column.

If there are no adjustment or temperature factors involved, the correct conductor size has been selected. If adjustment or correction factors are involved, go on to steps 3 and 4. Steps 3 and 4 are applied to the actual load (not 125% of continuous plus 100% non-continuous).

Step 3: Adjacent conductor adjustment. Merely divide the .8 adjacent conductor correction factor thats required for 4 current-carrying conductors in a raceway into 60 amps. 60 divided by .8 equals 75 amps.

Step 4: Ambient temperature correction. No (1.00) ambient temperature correction is required for your example.

You now only have to select a 75 amp rated conductor in the column that matches the temperature rating of the wire that you are using. For conductors in the 90 degree column, a #6 is rated 75 amps. If you were using 75 degree rated wire, #4 would be required for 75 amps.

Compare the conductor selected in steps 3 and 4 with the one selected in steps 1 and 2. The larger of the two is the correct size.