code calculations
Industrial Calculations:
Feeder Loads – Part 2
By James Stallcup, Jr., NEC and OSHA Consultant
I
n this month’s column, we continue our discussion of calculating feeder loads for an industrial
facility. We have shown you how to calculate
the loads for feeders No. 1, No. 2, and No. 3. Now
we’ll look at calculating the loads for feeders No. 4,
No. 5, and No. 6. Then, we’ll size the conductors for
each feeder based upon these calculated loads.
Use the electrical one-line diagram (Fig. 1 at
right) and the following tables for all calculations.
Feeder No. 4. This load is a “given,” and it’s 15A.
You don’t always need to calculate everything!
Fig. 1. A side-by-side comparison of sizing conductors
based on different terminal ratings.
Fig. 1. THHN (90°C) conductors enable you to use the
higher ampacity when adjusting conductor ampacity.
Feeder No. 5. Based on the following loads, compute what size conductors are required to supply the
motors on the secondary of the transformer per Part
B to Art. 220.
Step 1: Calculate total amps for all motor loads
per Sec. 430-24.
10, 10 hp motors at 14A (ea.)510314A5140A
12, 15 hp motors at 21A (ea.)512321A5252A
15, 5 hp motors at 7.6A (ea.)51537.6A5114A
16, 3 hp motors at 4.8A (ea.)51634.8A576.8A
Subtotal5582.8A
Step 1a: Add to this total 25% of the highest
rated motor in the group (Sec. 430-24).
21A325%55.25A
Total load5582.8A15.25A5588.05A
Step 2: Determine how many No. 3/0 THHN cu.
conductors you must connect in parallel between
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May 2001 • EC&M • www.ecmweb.com
the transformer and the panel per Sec. 310-4 and
Table 310-16.
No. 3/0 cu.5200A
No. of conductors5Total Ampacity4Ampacity
of one conductor
No. of conductors5588.05A4200A52.94,
which we round to 3.
Solution: The parallel connection requires three
No. 3/0 THHN cu. conductors per phase.
Step 3: Compute what size OCPD (CB) is required to protect the secondary conductors.
Step 3a: Size OCPD based on the largest motor
load per Secs. 430-62(a) and 430-52(c)(1), and
Table 430-152. Remember, our largest motor load
was 21A. Assuming we have an inverse time breaker,
the maximum rating or setting of the breaker must
be 250% of that 21A (Table 430-152).
21A3250%552.5A
Step 3b: Select appropriate size OCPD per Sec.
430-52(c)(1) and Table 430-152. Remember, the
breaker cannot exceed 52.5A, as calculated above.
So, we select a 50A breaker.
Step 3c: Size OCPD for tapped conductors per
Secs. 430-28(1) and 430-62(a). Motor loads, in
code calculations
addition to the largest load, are:
10314A5140A
12321A5252A221A5231A
1537.6A5114A
1634.8A576.8A
Add these loads to get a subtotal of 561.8A. Add
in largest OCPD we calculated earlier at 50A.
Total load5611.8A.
Solution: Select OCPD for the secondary conductors per Secs. 430-62(a) and 240-6(a). The
breaker rating cannot exceed the total load. This
load of 611.8A requires the use of a 600A OCPD.
Step 4: Compute what size copper grounding
electrode conductor (GEC), bonding jumper (BJ)
and grounded conductor (GC) are required to
ground the secondary to building steel.
Step 4a: Size the GEC per Sec. 250-30(a)(2),
Table 250-66 and Table 8, Ch. 9.
No. 3/05167,800 CM 3 3 conductors in parallel5503,400 CM (Table 8, Ch. 9.)
kcmil5503,400 CM41,0005503.4
Solution: A 503.4 kcmil conductor requires a
No. 1/0 cu. conductor. (Table 250-66).
Step 5: Size the BJ per Secs. 250-30(a)(1) and
250-102(c), and Table 25066.
Solution: A 503.4 kcmil
conductor requires a No. 1/0
cu. conductor.
Step 6: Size the GC per
Secs. 250-24(b)(2) and 3104, and Table 250-66.
Solution: A 503.4 kcmil
conductor requires a No. 1/0
cu. conductor.
Feeder No. 6. Based on the
following loads, compute the
load for feeder No. 6 per Part
B to Art. 220.
Step 1: Calculate total amps
for loads per Sec. 430-24 and
430-25.
2 heavy load machines at
12.0A (ea.)5 23 12.0A
524.0A
7 large motors:
1320.0A520.0A
1326.0A526.0A
1315.0A515.0A
1325.0A525.0A
3350.0A5150.0A
1 distribution panel at
50.0A51350.0A550.0A
Motor control center (5 motors)
1327.9A527.9A
1320.9A520.9A
1318.5A518.5A
237.6A515.2A
Total Load5392.5A
Step 1a: Add to this total 25% of the highest rated motor
in the group.
50.0A325%512.5A
Solution: Total load for this feeder5405A