Installation
RT-SVX35H-EN 47
Power Wire Sizing and Electrical Service
Sizing 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 Protection), and RDE
(Recommended Dual Element fuse size) for WXHE
(Extended Casing) u
nits, WLHE and WSHE (Cooling with
We
t Heat) units, and WFHE (Cooling with Gas Heat) units
Use Calculation #2 for WEHE (Cooling with electric Heat)
units.
Load Definitions:
Calculation #1 - WXHE, WLHE, WSHE, and
WFHE Units.
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.
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 select
ed MOP
value, th
en select the RDE value to equal the MOP
value.
Calculation #2 - Rooftop units with Electric
Heat WEHE Casings 2 through 9
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 th
e equations
g
iven 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:
MC
A = 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.
The selected RDE value will be the larger of the cooling
mode RDE value or the heating mo
de RDE value calculated
a
bove. 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.
Power Wire Sizing and Protection Device
Equations
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
Table 12. Electrical service sizing data — motors — 20-
130 tons
Motor HP Supply/Exhaust/Return Fan Motor
3 11.0 8.6 4.3 3.8
5 15.2 13.2 6.6 5.4
7½ 21.5 18.8 9.4 7.5
10 29.0 25.2 12.6 10.1
15 40.7 37.8 18.9 15.1
20 56.1 49.4 24.7 19.6
25 70.5 62.0 31.0 24.5
30 85.5 73.2 36.6 29.2
40
(a)
(a) 40 & 50 Hp motor available as standard in 460 & 575 volt only.
— — 47.6 39.0
50
(a)
— — 60.5 48.0
LOAD 1 = Current of the largest motor
LOAD 2 = Sum of the currents of all remaining motors
LOAD 3 = Current of electric heaters
LOAD 4 = Control power transformer
20 - 40 Ton Units, add 3 FL Amps for wire sizing formula
50 - 75 Ton Units, add 6 FL Amps for wire sizing formula
= and any other load rated at 1 Amp or more
MCA = (1.25 x LOAD 1) + LOAD 2 + LOAD 4
MOP = (2.25 x LOAD 1) + LOAD 2 + LOAD 4
RDE = (1.5 x LOAD1) + LOAD2 + LOAD4
RDE = (1.5 x LOAD 1) + LOAD 2 + LOAD 3 + LOAD 4
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