Electrical Data
Electrical Service Sizing
To correctly size electrical service wiring for your 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 your unit type.
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 DataTables on the following page.)
LOAD1 = CURRENT OFTHE LARGEST MOTOR (COMPRESSOR OR FAN MOTOR)
LOAD2 = SUM OFTHE 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
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
then reselect the RDE value to equal the MOP value.
Set 2. Rooftop units with Electric Heat
a. Single Source Power (380V, 415V, 460V, and 575V)
To arrive at the correct MCA, MOP, and RDE values for these units, you must perform two sets of calculations. 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).
For units using heaters less than 50 kw.
MCA = 1.25 x (LOAD1 + LOAD2 + LOAD4) + (1.25 x LOAD3)
For units using heaters equal to or greater than 50 kw.
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
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
b. Dual Source Power units (200V and 230V)
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.
MCA = (1.25 x LOAD3) MOP = (1.25 x LOAD3)
Select a fuse rating for the electric heating circuit that’s 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
NOTE: If selected MOP 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.
RDE = LOAD3
Select a fuse rating for the electric heating circuit that’s equal to the RDE value. If the RDE value does not equal a standard fuse size as listed in NEC
GENERAL NOTES:
On 20 to 130 ton rooftops, the selected MOP value is stamped in the MOP field on the nameplate.
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