




#19119  12/23/02 06:19 AM
Re: Motor branch circuit calculation

Member
Registered: 03/25/01
Posts: 93
Loc: Laporte, MN, USA

Fla Sparky, You're right on the money!
Here is my break down;
Motor #1: Step 1: 25HP = 34 amps from table 430.150 article 110.14C1a applies, so we know we must use the 60 degree column of table 310.16.
Step 2: 34 amps X 125% for continuous duty = 42.5 amps. A look in table 310.16 in the 60 degree column, shows that a #6 fits the bill. We'll jot it down for now.
Correction factors for adjacent conductors, and ambient temperature correction are a seperate calculation, and are calculated using 100% of the FLA, not the 125%.
Step 3: We need a correction factor of 80% for 6 adjacent current carrying conductors. 34 amps divided by .8 = 42.5 amps.
Step 4: We will be using the THWN2, 90 degree rated wire, therefore we see an ambient temperature correction factor of .87 from the bottom of table 310.16. 42.5 amps divided by .87 = 48.85 amps.
Step 5: Looking in the 90 degree column of 310.16, our selected conductor size for 48.85 amps is a #8. Compare this conductor to the one selected in steps one and two, and the larger gets the nod.
Conductor size for motor #1 is #6 THWN2
A look in table 430.52 shows that we need a dual element TD fuse of no larger than 175% of full load amps. 34 amps X 1.75 = 59.5. Round up to the nearest size gives us a 60 amp fuse.
Motor #2: Step 1: 52 FLA (table 430.150)
Step 2: 125% X 52 = 65 amps for continuous duty. Table 310.16 60 degree column shows that we need a #4 conductor. Jot it down for later comparison.
Step 3: 52 amps divided by .8 for adjacent current carrying conductor correction = 65 amps.
Step 4: 65 amps divided by .87 for ambient temperature correction = 74.7 amps. A trip to the 90 degree column of 310.16 shows a #6 conductor.
Step 5: Comparing the conductor selected in step 5 with the one determined in step 2 gives the nod to the larger #4 conductor.
Conductor size for motor #2 = #4 THWN2.
52 FLA X 1.75 = 91 amps. Round up to the next standard size gives us a 100 amp DETD fuse.
A trip to chapter 9 table 5 shows an area of .0507 sq. inches for a #6 THWN2, X 3 gives us .1521 sq. inches.
A single #4 THWN = .0824 sq. inches, X 3 gives us .2472 sq. inches.
.1521 + .2472 = .3993 sq. inches.
A trip to chapter 9 table 4 shows a 11/4" RMC has a 40% capacity of 0.610, a 1" only 0.355. Our RMC size is 11/4".

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