Installation Electrical
RT-SVX34F-EN
27
Electrical Wire Sizing and
Protection Device Equations
To correctly size the main power wiring based on MCA
(Minimum Circuit Ampacity), use the appropriate
equation listed below. Read the definitions that follow and
then use Calculation #1 for determining MCA (Minimum
Circuit Ampacity), MOP (Maximum Over current
Protection), and RDE (Recommended Dual Element fuse
size) for TC (Cooling Only) units and YC (Cooling with Gas
Heat) units. Use Calculation #2 for TE (Cooling with Electric
Heat) units.
Load Definitions:
•
LOAD 1 = CURRENT OF THE LARGEST MOTOR
(Compressor or Fan Motor)
•
LOAD 2 = SUM OF THE CURRENTS OF ALL
REMAINING MOTORS
•
LOAD 3 = FLA (Full Load Amps) OF THE ELECTRIC
HEATER
•
LOAD 4 = ANY OTHER LOAD RATED AT 1 AMP OR
MORE
•
CRANKCASE HEATERS FOR HEATING MODE ONLY:
•
208/230 VOLT
–
27.5 - 35 Ton Units, Add 1 Amp
–
40 - 50 Ton Units, Add 2 Amps
•
460/575 VOLT
–
27.5 - 35 Tons Units, Add 1 Amp
–
40 - 50 Ton Units, Add 1 Amp
Calculation #1 - TC*, YC*-27.5 to 50 Ton Units
MCA = (1.25 x Load 1) + Load 2 + Load 4
MOP = (2.25 x Load 1) + Load 2 + Load 4 (See Note 1)
RDE = (1.5 x Load 1) + Load 2 + Load 4 (See Note 2)
Calculation # 2 - TE*-27.5 to 50 Ton Units
A. Single Source Power (all voltages)
To calculate the correct MCA (Minimum Circuit Ampacity),
MOP (Maximum Over current Protection), and RDE
(Recommended Dual Element fuse size), two (2) sets of
calculations must be performed;
1.
Calculate the MCA, MOP and/or RDE values using the
above equation as if the unit is operating in the cooling
mode.
2. Calculate the MCA, MOP and/or RDE values as if the
unit is operating in the heating mode, as follows:
Note:
When determining loads, the compressors and
condenser fan motors do not operate during the
heating cycle.
Units with less than 50 KW Heaters
MCA = 1.25 x (Load 1 + Load 2 + Load 4) + (1.25 x Load 3)
Units with 50 KW or Larger Heaters
MCA = 1.25 x (Load 1 + Load 2 + Load 4) + Load 3
The MCA value stamped on the nameplate is the largest of
the two calculated values.
MOP = (2.25 x Load 1) + Load 2 + Load 3 + Load 4 (See Note
1)
The MOP value stamped on the nameplate is the largest of
the two calculated values.
RDE = (1.5 x Load 1) + Load 2 + Load 3 + Load 4 (See Note 2)
Note:
Select an over current protection device equal to
the MOP value. If the calculated MOP value does
not equal a standard size protection device listed in
NEC 240-6, select the next lower over current
protection device. If the calculated MOP value is
less than the MCA value, select the lowest over
current protection device which is equal to or larger
than the MCA, providing the selected over current
device does not exceed 800 amps.
Note:
Select a Dual Element Fuse equal to the RDE value.
If the calculated RDE value does not equal a
standard dual element fuse size listed in NEC 240-
6, select the next higher fuse size. If the calculated
RDE value is greater than the MOP value, select a
Dual Element fuse equal to the calculated MOP
(Maximum Over current Protection) value
Table 11.
Electrical service sizing data – electric heat
module (electric heat units only)—50Hz
Models: TE(D,H,F,R) 275 through 500 Electric Heat FLA
Nominal
Unit Size
(Tons)
Nominal
Unit
Voltage
KW Heater (380/415V)
23/27 34/40 45/54 56/67 68/81
23-29
380
34.5
51.1
68.9
85.5
–
415
37.6
55.6
–
–
–
33, 42
380
–
51.1
68.9
85.5
103.4
415
–
55.6
75.1
93.2
112.7
Note:
All FLA in this table are based on heater operating at 380 or 415 volts
as shown above.
Table 12. Electrical service sizing data — crankcase
heaters (heating mode only) — 50Hz
Nominal Unit Size
(Tons)
FLA Add
Unit Voltage
380
415
23 - 29
1
1
33 - 42
1
1
