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Electrical Data
Electrical Service Sizing
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) and MOP (Maximum Overcurrent Protection).
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.)
Load Definitions
Current of the largest motor (compressor or fan motor)
LOAD 1
Sum of the currents of all remaining motors
LOAD 2
LOAD 3
Current of electric heaters
Any other load rated at 1 amp or more
LOAD 4
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.
Set 2: Rooftop units with Electric Heat
Single Source Power units (460V and 575V)
To arrive at the correct MCA and MOP values for these
units, two sets of calculations must be performed. First
calculate the MCA and MOP values as if the unit was in
cooling mode (use the equations given in Set 1). Then
calculate the MCA and MOP values as if the unit were in
heating mode as follows. (Keep in mind when determining
LOADS that the compressors and condenser fan motors
don't run while the unit is in heating mode).
For units using heaters less than 50 kW:
MCA = 1.25 x (LOAD1 + LOAD2 + LOAD4) + (1.25 x
LOAD3)
RT-SVX063G-EN
For units using heaters equal to or greater than 50 kW:
MCA = 1.25 x (LOAD1 + LOAD2 + LOAD4) + (1.0 x
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. If the
selected MOP value is less than 125 percent of the current
rating of the electric heat load, select the next higher
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.
Dual Source Power units (200–230V with Electric Heat)
These units will have two circuit values shown on the
nameplate. The first circuit value will be the refrigeration
(cooling mode) values calculated per Set 1. The second set
of circuit values shown on the nameplate will be for the
electric heating circuit as follows.
For units using heaters less than 50 kW:
MCA = (1.25 x LOAD3)
For units using heaters equal to or greater than 50 kW:
MCA = (1.0 x LOAD3)
MOP = (1.25 x LOAD3)
Select a fuse rating for the electric heating circuit that is
equal to the MOP value obtained in the equation above. If
the MOP value does not equal a standard fuse size as
listed in NEC 240-6, select the next lower standard fuse
rating (see note below for exception). If the selected MOP
value is less than 125 percent of the current rating of the
electric heat load, select the next higher standard fuse
rating.
Note: If the available MOP option is less than the MCA
obtained in the equation above, then reselect the
lowest standard maximum fuse size which is equal
to, or larger, than the MCA, provided the reselected
fuse size does not exceed 800 amps.
Installation
31
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