First, we have taken a chart out of the National Electrical Code, Section 430, Motors and expanded the chart to better define where the various aspects of motors and motor control are located. Next we have developed a chart that defines the rules for sizing wire, overloads, etc. for motors. We follow this with an example of the calculations.
Refer to the above chart as the connection diagram in this example>
Motor 1 - 100 hp 480 VAC 3ph Nameplate Full Load Amps 128 amps, Design B, fused
Motor 2 – 10 hp 480 VAC 3ph Nameplate full Load Amps 19 amps, Design B, MCP,
Heater – 50 KVA 480 VAC 3ph
Maximum ambient temperature – 104 Degrees F
Terminal rating – 75 Degree C
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Motor 1 |
Motor 2 |
Heater |
|
Size |
1 |
100 hp |
10 hp |
50 kva |
|
NEC FLA |
1 |
124 amps |
14 amps |
60 amps |
|
Disconnect Size *FLA |
2&7 |
115% |
115% |
125% |
|
Disconnect Size |
2&7 |
142.6 Amps |
16.1 amps |
75 amps |
|
Nominal size |
2&7 |
200 amp |
30 amp |
100 amp |
|
Branch Circuit Size |
3 |
|
|
|
|
125% |
3 |
155 amps |
17.5 amps |
75 |
|
Wire Size |
3 |
2/0 (175 amps) |
#14 (20 amps) |
#4 (85amps) |
|
Wire Derating |
3 |
.88 |
.88 |
.88 |
|
Derated Size |
3 |
154 amps |
17.6 amps |
74.8 amps |
|
|
3 |
Violation |
OK |
Violation |
|
Revised Wire size |
3 |
3/0 (200 amps) |
|
#3 (100amps) |
|
Derated Size |
3 |
176 amps |
|
88 amps |
|
Nameplate Amps |
4 |
128 amps |
19 amps |
|
|
Overload sizing |
4 |
1.15% |
1.15% |
NA |
|
Overload Size |
4 |
147.2 amps |
21.85 amps |
NA |
|
Starter Size |
5 |
Size 4 |
Size 1 |
100 amp |
|
Branch Circuit Protection |
6 |
175% (max) |
250% (max) |
125% (max) |
|
Protection Size |
6 |
217 amps |
35 amps |
75 amps |
|
Next Largest Size |
6 |
225 amps (max) |
35 amps (max) |
75 amps (max) |
|
Equipment Ground |
9 |
#4 |
#10 |
#8 |
|
Feeder Conductor |
10 |
|
|
|
|
FLA |
10 |
124 Amps |
14 amps |
60 amps |
|
25% of Largest Motor |
10 |
31 amps |
|
|
|
Total Amps |
10 |
(124+14+60+31) 229 amps |
|
|
|
Wire Size |
10 |
4/0 (230 amps) |
|
|
|
Wire Derating |
10 |
.88 |
|
|
|
Derated Size |
10 |
202 amps |
|
|
|
|
10 |
Violation |
|
|
|
Revised Wire size |
10 |
300 kcmil (285 amps) |
|
|
|
Derated Size |
10 |
250 amps |
|
|
|
Feeder Protection |
11 |
|
|
|
|
Largest Motor BCP |
11 |
225 amps |
|
|
|
FLA – other loads |
11 |
|
14 amps |
60 amps |
|
Total Load |
11 |
(225+14+60) 299 amps |
|
|
|
Feeder Protection |
11 |
250 Amps |
|
|
|
Feeder Disconnect |
12 |
|
|
|
|
Full Load Amps |
12 |
124 amps |
14 amps |
60 amps |
|
Total FLA |
12 |
198 amps |
|
|
|
115% of FLA |
12 |
227.7 amps |
|
|
|
Lock Rotor Amps |
12 |
725 amps |
81 amps |
60 amps |
|
Total LRA. |
12 |
866 amps |
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|
|
Equivalent Hp |
12 |
125 hp |
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|
|
Disconnect Size |
12 |
200hp (240amps) |
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