83
RT-PRC010-EN
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
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 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 = (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 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 240-6,
select the next lower standard fuse
rating. NOTE: If selected MOP is less than
the MCA, 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 = (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. NOTE: If the selected RDE is
greater than the selected MOP value,
then reselect the RDE value to equal the
MOP value.
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 240-6,
select the next lower standard fuse rating
(see note below for exception).
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 240-6,
select the next higher standard fuse
rating. NOTE: If the selected RDE is
greater than the selected MOP value,
then reselect the RDE value to equal the
MOP value.
GENERAL NOTES:
On 20 to 130 ton rooftops, the selected
MOP value is stamped in the MOP field
on the nameplate.
