Cindy,
Although we ultimately ended up selecting the same size conductor in the end, I'd like to point something out to you.
Derating for adjacent conductor adjustment and ambient temperature correction can be calculated at 100% of the actual load as per 210.19. You can apply these derating factors before calculating the 125% of continuous plus noncontinuous loads. Also, when figuring voltage drop there is no need to use 125% of full load.
Here is the method that I learned and use to calculate wire size:
STEP 1: Compute the load at 125% of continuous load (unless the assembly is rated for 100% operation) plus 100% of noncontinuous load.
STEP 2: Go to table 310.16 and select the conductor from the correct temperature column. The equipment rating dictates the correct temperature column, and is covered in 110.14c.
Having completed steps 1 and 2, write the conductor size down; if their aren't any adjacent conductor or temperature factors involved, you are done. If adjacent conductor or temperature correction factors are involved, go to steps 3 and/or 4.
Steps 3 and 4 are a completely seperate calculation and applied to the actual load, not 125% of continuous load plus the noncontinuous load.
Step 3: Apply when there are more than 3 current carrying conductors in a raceway or cable. Use table 310.15b2a, the adjustment factor table.
Step 4: Apply when the conductors are going to be installed in an elevated temperature such as a boiler room or rooftop. The correction factor table at the bottom of table 310.16 is used to determine the corrected ampacity of the conductor. If using THHN, which is rated at 90 degree C operation, apply steps 3 and 4 at the actual load and select the conductor from the 90 degree column of table 310.16.
Step 5: Compare the conductor selected in steps 1 and 2 with the conductor selected in steps 3 and 4; the larger of the two is the correct size.
Now do your voltage drop calculation, if neccesary, increase the size of the conductor to compensate.
So, step 1: a 40 HP 3 phase motor @ 460 volts = 52 amps X 1.25 = 65 amps.
Step 2: If we go by the book, we cannot assume that all equipment is rated at 75 degee C, and must use the 60 degree column of 310.16 which dictates a #4 copper conductor (110.14c). In reality though, we realize that this equipment is probably rated at 75C, which dictates a #6 copper conductor.
Step 3: Adjacent conductor factor for 9 conductors is 70%. If we divide 52 amps by 70%, we see that our conductor must carry 75 amps. Article 430.22a dictates 125% of 52 amps for continuous duty. 125% X 52 = 65 amps. As you can see, the adjustment for adjacent conductors dictates 75 amps, already more than 125% of full load that is required for continuous duty. A trip back to the 90 degree column of 310.16 we select a #6 copper conductor.
Step 4 does not apply.
Step 5: Comparing the conductor selected from steps 1 and 2 to the one selected in step 3, we can determine that if the equipment temperature rating is known to be 75 degree C or higher, a #6 THWN or better can be used. If the equipment temperature rating cannot be determined, a #4 THWN or better must be used.
Now voltage drop. I like to use the reciprical formula for 3 phase: Cmils needed = 1.73 X K X I X L/desired volts dropped.
1.73 X 12 X 52 amps (no need to calculate at 125%) X 350'/3% of 480V is roughly 14 volts = 26,988 Cmils. #6 = 26,240 Cmils, #4 = 41740 Cmils. The #4 would be required to maintain 3% or less voltage drop.
Matt
[This message has been edited by Matt M (edited 11-16-2002).]
