Installation
RT-SVX24J-EN
79
Power Wire Sizing and Protection Devices
To correctly size electrical service wiring for a unit, find the
appropriate calculations listed below. Each type of unit has
its own set of calculations for MCA (Minimum Circuit
Ampacity), MOP (Maximum Overcurrent Protection), and
RDE (Recommended Dual Element fuse size). Read the
load definitions that follow and then find the appropriate
set of calculations based on unit type.
Note:
Set 1 is for cooling only and cooling with gas heat
units, and set 2 is for cooling with electric heat
units.
Load Definitions: (To determine load values, see the
Electrical Service Sizing Data Tables on the following
page.)
LOAD1 = CURRENT OF THE LARGEST MOTOR
(COMPRESSOR OR FAN MOTOR)
LOAD2 = SUM OF THE CURRENTS OF ALL REMAINING
MOTORS
LOAD3 = CURRENT OF ELECTRIC HEATERS
LOAD4 = ANY OTHER LOAD RATED AT 1 AMP OR MORE
Set 1. Cooling Only Rooftop Units and Cooling
with Gas Heat Rooftop Units
MCA = (1.25 x LOAD1) + LOAD2 + LOAD4
MOP = (2.25 x LOAD1) + LOAD2 + LOAD4
Select a fuse rating equal to the MOP value. If the MOP
value does not equal a standard fuse size as listed in NEC
240-6, select the next lower standard fuse rating.
Note:
If selected MOP is less than the MCA, then select
the lowest standard maximum fuse size which is
equal to or larger than the MCA, provided the
selected fuse size does not exceed 800 amps.
RDE = (1.5 x LOAD1) + LOAD2 + LOAD4
Select a fuse rating equal to the RDE value. If the RDE value
does not equal a standard fuse size as listed in NEC 240-6,
select the next higher standard fuse rating.
Note:
If the selected RDE is greater than the selected MOP
value, then select the RDE value to equal the MOP
value.
Set 2. Rooftop units with Electric Heat
To arrive at the correct MCA, MOP, and RDE values for
these units, two sets of calculations must be performed.
First calculate the MCA, MOP, and RDE values as if the unit
was in cooling mode (use the equations given in Set 1).
Then calculate the MCA, MOP, and RDE values as if the unit
were in the heating mode as follows.
(Keep in mind when determining LOADS that the
compressors don't run while the unit is in the heating
mode).
MCA = 1.25 x (LOAD1 + LOAD2 + LOAD4) + LOAD3
The nameplate MCA value will be the larger of the cooling
mode MCA value or the heating mode MCA value
calculated above.
MOP = (2.25 x LOAD1) + LOAD2 + LOAD3 + LOAD4
The selection MOP value will be the larger of the cooling
mode MOP value or the heating mode MOP value
calculated above.
Select a fuse rating equal to the MOP value. If the MOP
value does not equal a standard fuse size as listed in NEC
240-6, select the next lower standard fuse rating.
Note:
If selected MOP is less than the MCA, then select
the lowest standard maximum fuse size which is
equal to or larger than the MCA, provided the
selected fuse size does not exceed 800 amps.
RDE = (1.5 x LOAD1) + LOAD2 + LOAD3 + LOAD4
The selection RDE value will be the larger of the cooling
mode RDE value or the heating mode RDE value calculated
above.
Select a fuse rating equal to the RDE value. If the RDE value
does not equal a standard fuse size as listed in NEC 240-6,
select the next higher standard fuse rating.
Notes:
•
If the selected RDE is greater than the selected MOP
value, then select the RDE value to equal the MOP
value.
•
On 90 to 162 ton rooftops, the selected MOP value is
stamped in the MOP field on the nameplate.
Table 22. Electrical service sizing data
Nom Tons
Compressor
Nominal Voltage
460 V
575 V
380 V
Size
Number
per Unit
Capacity kW (ea.)
RLA
(ea.)
LRA
(ea.)
RLA
(ea.)
LRA
(ea.)
RLA
(ea.)
LRA
(ea.)
60 Hz
50 Hz
Std. Coil High Cap Std. Coil High Cap
90/100
240
4
21.3
21.6
17.6
17.9
34.1
215
27.3
175
34.0
215
105/118
240
2
24.2
24.5
20.2
20.3
34.1
215
27.3
175
34.0
215
300
2
25.0
25.3
20.9
21.0
44.7
260
35.8
210
44.6
260
120/128
300
4
26.9
27.8
22.4
23.0
44.7
260
35.8
210
44.6
260
130/140
300
2
28.5
28.8
23.8
23.8
44.7
260
35.8
210
44.6
260
380
2
33.7
34.2
28.2
28.2
52.1
320
41.1
235
52.0
320
150/162
380
4
34.7
--
28.7
--
52.1
320
41.1
235
52.0
320
