Branch Circuit - A.O. Smith DRE Installation And Operation Manual

The branch circuit wire and fuse size should be established
through reference to the current version of the National Electrical
Code or any other locally approved source in conjunction with
the heater amperage rating. Branch circuit wires should have a
temperature rating no less than 75°C. For convenience, portions
of the wire size tables from the code are reproduced here. It is
suggested the electrician size the branch circuit at 125% of the
heater ampere rating and further increase wire size as necessary
to compensate for voltage drop in long runs. Branch circuit voltage
drop should not exceed 3% at the heater.
TABLE 310-16. Allowable Ampacities of Insulated Conductors
Not more than three conductors in raceway, cable, or earth (directly buried), based
Size
60°C 75°C
(140°F) (167°F)
TYPES TYPES
RUW, T FEPW,
TW, UF RH, RHW,
AWG RUH,
THW
MCM THWN,
XHHW,
USE, ZW
COPPER
18 . . . . . . . .
16 . . . . . . . .
14 15 15
12 20 20
10 30 30
8 40 45
6 55 65
4 70 85
3 80 100
2 115
1 130
0 150
00 175
000 200
0000 230
250 255
300 285
350 310
400 335
500 380
Ambient For ambient temperatures over 30°C, multiply the ampacities shown above by the
Temp. °C appropriate correction factor to determine the maximum allowable load current.
31-40 .82 .88
41-50 .58 .75
51-60 . . . . .58
61-70 . . . . .35
71-80 . . . . . . . .
+The load current rating and the overcurrent protection for these conductors 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.
*For dry locations only. See 75°C column for wet locations.

BRANCH CIRCUIT

on ambient temperature of 30°C (86°F)
Temperature Rating of Conductor, See Table 310-13
85°C 90°C 60°C
(185°F) (194°F) (140°F)
TYPES TYPES TYPES
V, MI, TA, TBS, RUW, T
SA, AVB, TW, UF
SIS, +FEP,
+FEPB,
+RHH,
+THHN,
+XHHW*
ALUMINUM OR COPPER-CLAD ALUMINUM
. . . . 21 . . . .
22 22 . . . .
25 25 . . . .
30 30 15
40 40 25
50 50 30
70 70 40
90 90 55
105 105 65
120 120 75
140 140
155 155
185 185
210 210
235 285
270 270
300 300
325 325
360 360
405 405
CORRECTION FACTORS
.90 .91 .82
.80 .82 .58
.67 .71 . . . .
.52 .58 . . . .
.30 .41 . . . .
15
75°C 85°C 90°C
(167°F) (185°F) (194°F)
TYPES TYPES TYPES
RH, RHW, V, MI, TA, TBS,
RUH, SA, AVB,
THW, SIS,
THWN, +RHH,
XHHW, +THHN,
USE +XHHW*
. . . . . . . . . . . .
. . . . . . . . . . . .
. . . . . . . . . . . .
15 25
25 30
40 40
50 55
65 70
75 80
90 95
100 110
120 125
135 145
1155 165
180 185
205 215
230 240
250 260
270 290
310 330
.88 .90
.75 .80
.58 .67
.35 .52
. . . . .30
Size
AWG
MCM
. . . .
. . . .
. . . .
25 12
30 10
40 8
55 6
70 4
80 3
95 2
110 1
125 0
145 00
165 000
185 0000
215 250
240 300
260 350
290 400
330 500
Ambient
Temp. °F
.91 86-104
.82 105-122
.71 123-141
.58 142-158
.41 159-176