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Figure 3-15 Power Wire Sizing and Protection Device Equations
To correctly size the main power wiring for the unit, use the appropriate calculation(s) listed below. Read the load definitions
that follow and use Calculation #1 for determining the MCA (Minimum Circuit Ampacity), MOP (Maximum Over current Pro-
tection), and RDE (Recommended Dual Element fuse size) for SAH_ (Cooling Only) units, SXH_ (Extended Casing) units,
SLH_ and SSH_ (Cooling with Wet Heat) units, and SFH_ (Cooling with Gas Heat) units Use Calculation #2 for SEH_ (Cool-
ing 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 = CURRENT OF ELECTRIC HEATERS
LOAD 4 = ANY OTHER LOAD RATED AT 1 AMP OR MORE
CONTROL POWER TRANSFORMER FOR ALL MODES
- 20 - 40 Ton Units, add 3 FL Amps
- 50 - 75 Ton Units, add 6 FL Amps
- 90 - 130 Ton Units, Add 8 FL Amps
CRANKCASE HEATERS FOR HEATING MODE 460/575V ONLY
- 20 - 30 Ton Units, Add 1 Amp
- 40 - 60 Ton Units, Add 2 Amps
- 70 - 105 Ton Units, Add 3 Amps
- 115 - 130 Ton Units, Add 4 Amps
Calculation #1 - SAH_, SXH_, SLH_, SSH_, and SFH_ Units.
MCA = (1.25 x LOAD 1) + LOAD 2 + LOAD 4 MOP = (2.25 x LOAD 1) + LOAD 2 + LOAD 4
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.
Calculation #2 - Rooftop units with Electric Heat (SEH_ 20 - 130 Tons)
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 operating in the cooling mode (use the equations given in Calculation #1
above). Then calculate the MCA, MOP, and RDE values as if the unit was operating in the heating mode as follows. (Keep in
mind when determining LOADS that the compressors do not operate while the unit is in the heating mode).
For units using heaters less than 50 kw: MCA = 1.25 x (LOAD 1 + LOAD 2 + LOAD 4) + (1.25 x LOAD 3)
For units using heaters equal to or greater than 50 kw: MCA = 1.25 x (LOAD 1 + LOAD 2 + LOAD 4) + LOAD 3
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 LOAD 1) + LOAD 2 + LOAD 3 + LOAD 4
The selected 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.
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