11
This is an example of calculating heater amperage for
both types of element operation. From this, the branch
circuit conductor and over-current protection sizing can be
established.
The example is of a three-phase 240-volt unit with two 6
KW elements. The notations are for units field-converted
to single-phase. Check the heater model and rating plate
for actual specifications and substitute those values in the
following:
Non-Simultaneous
(Factory-Wired)
Simultaneous
(Field-Converted)
3000 / 240 = 12.5 amps*
3000/ 240 = 12.5 amps*
12.5 x 1.73 = 21.6 amps
*NOTE: As a single-phase
non-simultaneous unit.
*NOTE: as a single-phase
simultaneous unit the total
is:
12.5 x 2 = 25 amps
The rating of the over-current protection should be
computed on the basis of 125% of the total connected
load amperage. Where the standard rating and settings
do not correspond with this computation, the next higher
standard rating or setting should be selected.
Portion of Table 310-16 (NFPA-70) follows:
Allowable Ampacities of Insulated Copper Conductors
No more than three conductors in Raceway or Cable or
Direct Burial (Based on Ambient Temperature of 86˚F).
These ampacities relate only to conductors described in
Table 310-13 in Code.
For ambient temperatures that over 86˚F, see Correction
Factors (Note 13 in Code).
Size
Temperature Rating of Conductor
(See Table 310-13 in Code)
AMG
MCM
140˚F
167˚F
Types:
RUW, (14-2), T, TW,
UF
Types:
RH, RHW, RUH,
(14-2), THW, THWN,
XHHW, USE
18
---
---
16
---
---
14
15
15
12
20
20
10
30
30
8
40
45
6
55
65
4
70
85
3
80
100
Portion of Table 310-18 follows:
Allowable Ampacities of Insulated Aluminum and Copper -
Clad Aluminum Conductors.
Not more than three conductors in Raceway or Cable
or Direct Burial (based on Ambient Temperature of 86˚F.
These ampacities relate only to conductors described in
Table 310-13 in Code.
For ambient Temperatures over 86˚F, see correction
Factors, Note 13 in Code.
Size
Temperature Rating of Conductor
(See Table 310-13 in Code)
AMG
MCM
140˚F
167˚F
Types:
RUW, (12-2), T, TW,
UF
Types:
RH, RHW, RUH,
(12-2), THW, THWN,
XHHW, USE
12
15
15
10
25
25
8
30
40
6
40
50
4
55
65
3
65
75
2
75
90
1
85
100
OPERATION, NEXT PAGE
ELECTRICAL CONTINUED
WARNING
Before removing any access panels
or servicing the water heater, make
sure the electrical supply to the
water is turned OFF.
Failure to do this could result in
death, serious bodily injury, or
property damage.
TABLE 3
TABLE 5
TABLE 4
