TYPICAL INSTALLATIONS UNDERGROUND IN DUCTS
Single Conductors
Ambient Temperature 20°C Conductor Temperature 90°C Earth RHO 90°C - cm/watt Concrete RHO 85°C - cm/watt Duct RHO 600°C - cm/watt No Sheath Losses (Single point grounding) 5 Inch Duct Surface Limit 45°C (Direct Buried)
* NOTE: Cable surface temperature limit may reduce conductor operating temperature.
Three Conductors Paralleled/Triplexed or One-Three Conductor Cable
TYPICAL INSTALLATIONS IN AIR
Assumptions: Ambient Temperature Indoor 40°C - Outdoor 35°C Conductor Temperature 90°C Surface Emissivity 0.85 No Sheath Losses Additional Outdoor Conditions Surface Absorptivity 0.33 Atmospheric Pressure 1 atm Air Speed 4 ft./sec.
Derating Factors for Cables in Air One Layer of Spaced Cables in a Horizontal Line
5kV thru 35kV TRIPLEXED OR PARALLELED CABLE AMPACITIES SINGLE CONDUCTORS Copper Conductors
Underground in Ducts - Three 1/C Cable Per Duct
Direct Buried Three 1/C Cable Per Circuit
Conductor Size (AWG/ kcmil)
1 Circuit Fig. 4 Load Factor
2 Circuits Fig. 5 Load Factor
4 Circuits Fig. 6 Load Factor
1 Circuit Fig. 9 Load Factor
2 Circuits Fig. 10 Load Factor
50
75
100
Fig. 12 Indoor
Fig. 12 Outdoor
6
101
97
92
91
84
81
73
139
114
135
104
83
130
4
131
125
119
126
117
108
105
93
182
146
175
132
133
171
2
174
166
156
167
154
141
136
120
234
189
152
224
170
179
219
1
199
178
190
160
268
214
172
254
192
153
205
252
1/0
227
215
202
216
181
198
306
242
194
287
173
235
289
2/0
259
245
230
246
226
225
197
351
273
324
244
195
270
332
3/0
295
279
261
280
256
233
223
402
308
247
366
275
220
311
382
4/0
337
317
297
319
291
264
290
253
221
460
349