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Voltage Drop Calculations
Ratings of Conductors and Tables to Determine Volt Loss
With larger loads on new installations, it is extremely important to consider volt
loss in mind, otherwise some very unsatisfactory problems are likely to be
encountered.
The actual conductor used must also meet the other sizing requirements such as
full-load current, ambient temperature, number in a raceway, etc.
How to Figure Volt Loss
Multiply distance (length in feet of one wire) by the current (expressed in amps) by the
figure shown in table for the kind of current and the size of wire to be used, by one over
the number of conductors per phase.
Then, put a decimal point in front of the last 6 digits–you have the volt loss to be
expected on that circuit.
Example – 6 AWG copper wire in 180 feet of iron conduit–3 phase,
40 amp load at 80% power factor.
Multiply feet by amperes: 180 x 40 = 7200
Multiply this number by number from table for 6 AWG wire threephase at 80% power factor: 7200 x 745 = 5364000
1
1
Multiply by
5364000 x
= 5364000
#/phase
1
Place decimal point 6 places to left.
This gives volt loss to be expected: 5.364V
(For a 240V circuit the % voltage drop is 5.364 x 100 or 2.23%).
240
These Tables take into consideration reactance on AC circuits as
well as resistance of the wire.
Remember on short runs to check to see that the size and type
of wire indicated has sufficient ampere capacity.
How to Select Size of Wire
Multiply distance (length in feet of one wire) by the current (expressed in
amps), by one over the number of conductors per phase.
Installation in Conduit, Cable or Raceway
NEC® Tables 310.16 through 310.19 give allowable ampacities (currentcarrying capacities) for not more than three conductors in a conduit, cable, or
raceway. Where the number of conductors exceeds three the allowable
ampacity of each conductor must be reduced as shown in the following tables:
Installation in Conduit, Cable or Raceway per 310.15(B)(2)(a)
The Number of
Conductors In One
Conduit, Raceway
Or Cable
4 to 6
7 to 9
10 to 20
21 to 30
31 to 40
41 and over
Percentage of Values
In Tables 310.16 And
310.18
80%
70%
50%
45%
40%
35%
Conditions Causing Higher Volt Loss
The voltage loss is increased when a conductor is operated at a higher
temperature because the resistance increases.
If type RH, RHW, THW, or THWN wire (75°C wire) is loaded to near its full
rating, or if room temperature is 15°C higher than normal, add the following
percentages to get the volt loss.
Conditions Causing Higher Volt Loss
Direct
Single Or Three Phase–Power Factor
Wire Size
Current
100%
90%
80%
70% 60%
14 to 4 AWG
5.0%
5.0%
4.8% 4.7% 4.7% 4.6%
2 to 3/0 AWG
5.0%
5.0%
4.2% 3.8% 3 5% 3.3%
4/0 AWG to 500 kcmil
5.0%
5.0%
3.1% 2.6% 2.4% 2.0%
600 kcmil to 1000 kcmil 5.0%
5.0%
2.5% 2.2% 1.6% 1.3%
If type RHH, THHN or XHHW wire (90°C. wire) is loaded to near its full rating
or if room temperature is 30°C higher than normal, add twice the above percentages to get the volt loss.
Divide that figure into the permissible volt loss multiplied by 1,000,000.
Look under the column applying to the type of current and power factor for the
figure nearest, but not above your result – you have the size of wire needed.
Example – Copper in 180 feet of steel conduit–3 phase, 40 amp
Ioad at 80% power factor–Volt loss from local code equals 5.5 volts.
Multiply feet by amperes by 1
180 x 40 x 1 = 7200.
#/phase
1
Divide permissible volt loss multiplied by 1,000,000 by this
number:
5.5 x 1,000,000 = 764.
7200
Select number from Table, three-phase at 80% power factor, that is nearest
but not greater than 764. This number is 745 which indicates the size of wire
needed: 6 AWG.
Line-to-Neutral
For line to neutral voltage drop on a 3 phase system, divide the three phase
value by 1.73. For line to neutral voltage drop on a single phase system,
divide single phase value by 2.
Room Temperature Affects Ratings
The ampacities (carrying capacities) of conductors are based on a room
temperature of 86°F or 30°C. If room temperature is higher, the ampacities
are reduced by using the following multipliers; (for 0-2000 volt, insulated
conductors not more than 3 conductors in raceway or direct buried, Table
310.16).
Room Temperature Affects Ratings
Room
Temperature
°C
31-35
36-40
41-45
46-50
51-55
56-60
61-70
71-80
TW
°F
87-95
96-104
105-113
114-122
123-131
132-140
141-158
159-176
Ampacity Multiplier
THW, THWN
THHN, XHHW*
(60°C Wire)
(75°C Wire)
.91
.94
.82
.88
.71
.82
.58
.75
.41
.67
–
.58
–
.33
–
–
(90°C Wire)
.96
.91
.87
.82
.76
.71
.58
.41
Open Wiring
The volt loss for open wiring installations depends on the separation between
conductors. The volt loss is approximately equal to that for conductors in nonmagnetic conduit. 310.15 offers a method to calculate conductor ampacity.
©2005 Cooper Bussmann
199
Voltage Drop Calculations
Copper Conductors — Ratings & Volt Loss†
Conduit
Steel
Conduit
NonMagnetic
Conduit
(Lead
Covered
Cables or
Installation
in Fibre or
Other
NonMagnetic
Conduit,
Etc.)
Wire
Size
14
12
10
8
6
4
3
2
1
0
00
000
0000
250
300
350
400
500
600
750
1000
14
12
10
8
6
4
3
2
1
0
00
000
0000
250
300
350
400
500
600
750
1000
Ampacity
Type
T, TW
(60°C
Wire)
20*
25*
30
40
55
70
85
95
110
125
145
165
195
215
240
260
280
320
335
400
455
20*
25*
30
40
55
70
85
95
110
125
145
165
195
215
240
260
280
320
335
400
455
Type
RH,
THWN,
RHW,
THW
(75°C
Wire)
20*
25*
35*
50
65
85
100
115
130
150
175
200
230
255
285
310
335
380
420
475
545
20*
25*
35*
50
65
85
100
115
130
150
175
200
230
255
285
310
335
380
420
475
545
Type
RHH,
THHN,
XHHW
(90°C
Wire)
25*
30*
40*
55
75
95
110
130
150
170
195
225
260
290
320
350
380
430
475
535
615
25*
30*
40*
55
75
95
110
130
150
170
195
225
260
290
320
350
380
430
475
535
615
Direct
Current
6140
3860
2420
1528
982
616
490
388
308
244
193
153
122
103
86
73
64
52
43
34
26
6140
3464
2420
1528
982
616
470
388
308
244
193
153
122
103
86
73
64
52
43
34
26
Volt Loss (See explanation prior page.)
Three-Phase
(60 Cycle, Lagging Power Factor.)
100%
90%
80%
70%
60%
Single-Phase
(60 Cycle, Lagging Power Factor.)
100%
90%
80%
70%
60%
5369
3464
2078
1350
848
536
433
346
277
207
173
136
109
93
77
67
60
50
43
36
31
5369
3464
2078
1350
848
536
433
329
259
207
173
133
107
90
76
65
57
46
39
32
25
6200
4000
2400
1560
980
620
500
400
320
240
200
158
126
108
90
78
70
58
50
42
36
6200
4000
2400
1560
980
620
500
380
300
240
200
154
124
104
88
76
66
54
46
38
30
3836
2508
1540
1040
690
468
394
331
283
232
206
178
157
148
135
126
120
111
106
102
95
3812
2486
1520
1019
669
448
375
300
253
214
188
159
140
128
118
109
103
94
90
83
77
4887
3169
1918
1264
812
528
434
354
292
228
196
162
136
123
108
98
91
81
75
68
62
4876
3158
1908
1255
802
519
425
330
268
220
188
151
127
112
99
89
81
71
65
58
51
4371
2841
1728
1148
745
491
407
336
280
223
194
163
140
128
115
106
99
90
84
78
72
4355
2827
1714
1134
731
479
395
310
255
212
183
150
128
114
103
94
87
77
72
65
59
3848
2508
1532
1026
673
450
376
312
264
213
188
160
139
129
117
109
103
94
89
84
78
3830
2491
1516
1010
657
435
361
286
238
199
174
145
125
113
104
95
89
80
76
70
63
3322
2172
1334
900
597
405
341
286
245
200
178
154
136
128
117
109
104
96
92
88
82
3301
2153
1316
882
579
388
324
259
219
185
163
138
121
110
102
94
89
82
77
72
66
5643
3659
2214
1460
937
610
501
409
337
263
227
187
157
142
125
113
105
94
86
79
72
5630
3647
2203
1449
926
599
490
381
310
254
217
175
147
129
114
103
94
82
75
67
59
5047
3281
1995
1326
860
568
470
388
324
258
224
188
162
148
133
122
114
104
97
91
84
5029
3264
1980
1310
845
553
456
358
295
244
211
173
148
132
119
108
100
90
83
76
68
4444
2897
1769
1184
777
519
434
361
305
246
217
184
161
149
135
126
118
109
103
97
90
4422
2877
1751
1166
758
502
417
330
275
230
201
167
145
131
120
110
103
93
87
80
73
* The overcurrent protection for conductor types marked with an (*) shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15
amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have
been applied.
† Figures are L-L for both single-phase and three-phase. Three-phase figures are average for the three-phase.
200
©2005 Cooper Bussmann
Voltage Drop Calculations
Aluminum Conductors — Ratings & Volt Loss†
Conduit
Steel
Conduit
NonMagnetic
Conduit
(Lead
Covered
Cables or
Installation
in Fibre or
Other
NonMagnetic
Conduit,
Etc.)
Wire
Size
12
10
8
6
4
3
2
1
0
00
000
0000
250
300
350
400
500
600
750
1000
12
10
8
6
4
3
2
1
0
00
000
0000
250
300
350
400
500
600
750
1000
Ampacity
Type
T, TW
(60°C
Wire)
20*
25
30
40
55
65
75
85
100
115
130
150
170
190
210
225
260
285
320
375
20*
25
30
40
55
65
75
85
100
115
130
150
170
190
210
225
260
285
320
375
Type
RH,
THWN,
RHW,
THW
(75°C
Wire)
20*
30*
40
50
65
75
90
100
120
135
155
180
205
230
250
270
310
340
385
445
20*
30*
40
50
65
75
90
100
120
135
155
180
205
230
250
270
310
340
385
445
Type
RHH,
THHN,
XHHW
(90°C
Wire)
25*
35*
45
60
75
85
100
115
135
150
175
205
230
255
280
305
350
385
435
500
25*
35*
45
60
75
85
100
115
135
150
175
205
230
255
280
305
350
385
435
500
Direct
Current
6360
4000
2520
1616
1016
796
638
506
402
318
259
200
169
141
121
106
85
71
56
42
6360
4000
2520
1616
1016
796
638
506
402
318
252
200
169
141
121
106
85
71
56
42
Volt Loss (See explanation two pages prior.)
Three-Phase
(60 Cycle, Lagging Power Factor.)
100%
90%
80%
70%
60%
Single-Phase
(60 Cycle, Lagging Power Factor.)
100%
90%
80%
70%
60%
5542
3464
2251
1402
883
692
554
433
346
277
225
173
148
124
109
95
77
65
53
43
5542
3464
2251
1402
883
692
554
433
346
277
225
173
147
122
105
93
74
62
50
39
6400
4000
2600
1620
1020
800
640
500
400
320
260
200
172
144
126
110
90
76
62
50
6400
4000
2600
1620
1020
800
640
500
400
320
260
200
170
142
122
108
86
72
58
46
3948
2500
1663
1074
708
574
475
391
328
278
239
201
186
168
155
144
130
122
114
103
3926
2480
1643
1053
668
555
456
373
310
260
223
185
167
150
137
128
114
105
95
86
5039
3165
2075
1310
840
668
541
432
353
290
241
194
173
150
135
122
106
95
84
73
5029
3155
2065
1301
831
659
532
424
344
281
234
186
163
141
125
114
96
85
73
63
4504
2836
1868
1188
769
615
502
405
334
277
234
191
173
152
139
127
112
102
92
82
4490
2823
1855
1175
756
603
490
394
322
266
223
181
160
140
125
116
100
90
79
70
3963
2502
1656
1061
692
557
458
373
310
260
221
184
168
150
138
127
113
105
96
87
3946
2486
1640
1045
677
543
443
360
296
247
209
171
153
136
123
114
100
91
82
73
3419
2165
1441
930
613
497
411
338
284
241
207
174
161
145
134
125
113
106
98
89
3400
2147
1423
912
596
480
394
323
268
225
193
160
145
130
118
111
98
91
82
75
5819
3654
2396
1513
970
771
625
499
407
335
279
224
200
174
156
141
122
110
97
85
5807
3643
2385
1502
959
760
615
490
398
325
270
215
188
163
144
132
111
98
85
73
5201
3275
2158
1372
888
710
580
468
386
320
270
221
200
176
160
146
129
118
107
95
5184
3260
2142
1357
873
696
566
455
372
307
258
209
185
162
145
134
115
104
92
81
4577
2889
1912
1225
799
644
529
431
358
301
256
212
194
173
159
146
131
121
111
100
4557
2871
1894
1206
782
627
512
415
342
285
241
198
177
157
142
132
115
106
94
85
* The overcurrent protection for conductor types marked with an (*) shall not exceed 15 amperes for 14 AWG, 20 amperes for 12 AWG, and 30 amperes for 10 AWG copper; or 15
amperes for 12 AWG and 25 amperes for 10 AWG aluminum and copper-clad aluminum after any correction factors for ambient temperature and number of conductors have been
applied.
† Figures are L-L for both single-phase and three-phase. Three-phase figures are average for the three-phase.
©2005 Cooper Bussmann
201
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