Industrial Calculations: Feeder Loads.
Industrial Calculations:
Feeder Loads.
Over the last three months in this column,
we’ve discussed how to calculate various
loads in an industrial facility. We’ve
looked at lighting loads, receptacle loads, appliance loads, motor loads, compressor loads, and heating or AC loads. Now we’ll show you how to use a one-line diagram to calculate feeder loads.
When computing the feeder load for an existing electrical system—or when designing an entirely new system—an up-to-date electrical one-line diagram is
important to the success of your calculations. (Fig. 1).
Fig. 1. The above is a one-line diagram depicting typical electrical equipment in an industrial plant.
As we work through our calculations, you’ll see just how important the one-line diagrams are: The diagram supplies the crucial information at the beginning of each example.
Now, using a one-line diagram and the information supplied in the following tables, let’s try to calculate the load for a system with three feeders.
Feeder No. 1. Based on the following loads, compute the amps and size the conductors (three per phase) and overcurrent protection device (OCPD)
Step 1: Calculate total amps for all loads per
Secs. 215-2(a), 220-4(b), and 440-34.
2 ballast at 0.38A (ea.) =
60×2×0.38A×125%=57.0A
HID 1000W at 0.86A (ea.)=
42 × 0.86A × 125% = 45.15A
Large equipment at 21.0A (ea.) =
62 × 1.0A × 125% = 157.5A
Process units at 34.0A (ea.) =
10 × 34.0A × 125% = 425.0A
code calculations
By James Stallcup, Jr., NECA and OSHA Consultant
Total load = 684.65A
Rounded up to 685A per Sec. 220-2(b).
Step 2: Size conductors for feeder No. 1 by
paralleling conductors (three per phase), per
Sec. 310-4.
Ampacity of one conductor (A) =
Total Ampacity ÷ No. of conductors
A = 685A ÷ 3
A = 229A.
Step 3: Select conductors for feeder No. 1 per
Table 310-16 (use 758C column).
229A requires No. 4/0 cu.
No. 4/0 THHN cu. = 230A.
Step 4: Select total ampacity of conductors per
Sec. 310-4.
No. 4/0 cu. = 230A × 3 = 690A
690A > 685A.
copper conductors, paralleled three times (once
per phase).
Now let’s size the OCPD:
Size OCPD based upon load per Secs. 240-3(b)
and 240-6(a).
685A or 690A requires a 700A OCPD.
Feeder No. 2. Based on the following loads, compute what size 90°C, medium-voltage copper conductors are required and how many mains are permitted to supply power to the switchgear per Part B
to Art. 220 and Art. 215:
Step 1: Calculate total amps for all loads per Secs.
215-2(a), 230-202, 230-208(b), and 430-24.
2 heavy equipment at 2.0A (ea.) =
2 × 2.0A × 125% = 5.0A
2 large motors at 1.5A (ea.) =
2 × 1.5A × 125% = 3.75A
1 distribution center at 15.0A =
1 × 15.0A × 125% = 18.75A
Largest motor load at 1.5A (ea.) =
1.5A × 25% = 0.375A
Total load = 27.875A.
Step 2: Size conductors for feeder No. 2 per Fig.
310-60, Detail 3, and Table 310-77.
27.875A requires No. 6 cu. conductor (the mini-
mum size for 15kV).
Step 3: Find number of mains per Secs. 225-33(a)
and 225-34(a).
The number of mains per minute required is six.
Solution: Use No. 6 copper and six mains for
underground feeder No. 2.
Feeder No. 3. Based on the following loads, what
size OCPD and 90°C, medium-voltage conductors are required per Part B to Art. 220:
Step 1: Calculate total amps for all loads per Secs.
215-2(a), 220-3(b)(8)(b), 220-3(b)(9) and
Table 220-13.
150 fluorescent at 0.86A (ea.) =
150 × 0.86A × 125% = 161.25A
50 recess fixtures at 1.25A (ea.) =
50 × 1.25A × 125% = 78.125A
40 150W bulbs at 1.25A (ea.) =
40 × 1.25A × 125% = 62.5A
200 receptacles at 1.5A (ea.) =
200 × 180W = 36,000W
First 10,000W × 100% = 10,000VA
Next 26,000W × 50% = 13,000VA
Total load =,23,000VA
I = 23,000VA / 360V = 63.9A
10 10-ft. multioutlet assemblies at 1.5A per ft4
102 × 10 ft × 1.5A = 150.0A
10 isolation receptacles at 1.5A (ea.) =
10 × 1.5A × 125% = 18.75A
Total load = 534.525A.
Now find the minimum size OCPD for primary
side protection of the transformer.
Step 1: Size the transformer based on secondary
loads (based on 125% of load).
kVA = I × V × 1.732 = 1000
kVA = 534.525 × 208V × 1.732 = 1000
kVA = 192.566
Step 2: Select the correct size transformer (based
on 125% of load).
192.428kVA requires a 225kVA transformer.
Step 3: Find FLA of transformer
I = (kVA × 1000) ÷ (V × 1.732)
I = (225kVA × 1000) ÷ (13,800V. × 1.732)
I = 59.4A.
Step 4: Size OCPD for primary per Sec. 240-6(a)
and Table 450-3(a).
9.4A × 600% = 56.4A
56.4A requires a 50A OCPD.
Now, size the 908C medium-voltage copper con-
ductors to supply the primary side of the transformer.
Step 1: Select appropriate conductors based on
OCPD size, Sec. 110-40 and Table 310-77.
50A OCPD requires a No. 6 cu conductor.
Note: All continuous loads are computed
at 125%.
ductors to supply the primary side of the transformer.
Step 1: Select appropriate conductors based on
OCPD size, Sec. 110-40 and Table 310-77.
50A OCPD requires a No. 6 cu conductor.
Note: All continuous loads are computed
at 125%.
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