FAXA-SVX01B-EN
37
electrical
requirements
Installation
Power Wire Sizing and
Protection Device Equations
Each type of unit has its own set of
calculations for MCA (minimum circuit
ampacity) and MOP (maximum
overcurrent protection. For units with
multiple modes of operation, you must
consider each individual field-wired
hazardous voltage circuit. Use the load
that provides the highest value in the
applicable circuit. To correctly size the
unit’s electrical service wiring, read the
load definitions that follow and then find
the appropriate set of calculations
based on your unit type.
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 = control power transformer
= any other load rated at one amp or
more (gas heat, factory-wired
convenience outlet)
Single-Source Power
Set #1: Cooling only units and cooling
with gas heat, single source power
MCA = (1.25 x load 1) + load 2 + load 4
MOP = (2.25 x load 1) + load 2 + load 4
Select an overcurrent protective device
rating equal to the MOP value. If the
MOP value does not equal a standard
device size as listed in NEC 240-6, select
the next lower standard device rating. If
the selected MOP is less than the MCA,
then reselect the lowest standard
maximum overcurrent protective
device size that is equal to or larger than
the MCA, provided the reselected
device size does not exceed 800 amps.
Disconnect Switch Sizing (DSS) – if field
provided and installed
If the unit has a factory-installed, non-
fused disconnect switch, this sizing
equation is not necessary. If the
disconnect is field-provided and
installed, use the following equation to
determine its rating:
DSS = 1.15 x (load 1 + load 2 + load 4)
Select a disconnect switch (DSS) size
equal to or larger than the DSS value
calculated.
Set #2: Units with electric heat, single
source power, non-concurrent load
To arrive at the correct MCA and MOP
values, you must perform two sets of
calculations: one for cooling only (set #1)
and one for electric heat only (set #2).
Calculate the MCA and MOP values as if
the unit were in the cooling mode (use
the equations given in set #1). Then
repeat the calculations for MCA and
MOP for the unit in the heating mode
using set #2. When determining loads,
keep in mind that the compressors and
condenser fans do not run while the unit
is in the heating mode.
For units using heaters:
• less than 50 kW,
use the formula, MCA = 1.25 x (load 1
+ load 2 + load 4) + (1.25 x load 3)
• equal to or greater than 50 kW,
use the formula, MCA = 1.25 x (load 1
+ load 2 + load 4) + load 3
Select the nameplate MCA value that is
the larger of the cooling (set #1) or the
heating mode MCA calculated above.
MOP = (2.25 x load 1) + load 2 + load 3 +
load 4
Select an overcurrent protective device
rating equal to the MOP value. If the
MOP value does not equal a standard
device size as listed in NEC 240-6, select
the next lower standard device rating. If
the next lower standard rating is less
than 125% of the current rating of the
electric heater load, select the next
higher standard device rating. If the
selected MOP is less than the MCA, then
reselect the lowest standard device size
which is equal to or larger than the
MCA, provided the reselected device
size does not exceed 800 amps.
Select the MOP value that is the larger
of the cooling (set #1) mode MOP value
or the heating MOP value calculated
above.
Disconnect Switch Sizing (DSS) – if field
provided and installed
If the unit is ordered with a factory-
installed non-fused disconnect switch,
this sizing equation is not necessary. If
the disconnect is to be field-provided
and installed, use the following equation
to determine its rating:
DSS = 1.15 x (load 1 + load 2 + load 3 +
load 4)
Select the DSS value that is the larger of
the cooling (set #1) mode DSS or the
heating mode DSS value calculated
above. Select a disconnect switch size
equal to or greater than the DSS value
calculated.
Set #3: Units with electric heat, single
source power, concurrent load
Concurrent load = two compressors and
first stage electric heat
Primary Heat Reheat Selection
You may enable primary heat reheat to
take advantage of the primary heat
source as supplemental reheat for the
condenser reheat. Although in most
cases, the MCA and MOP values are
unaffected by the concurrent loads of
compressors, condenser fans, and first
stage of electric heat, there are a few
circumstances where the MCA and MOP
are increased when this sequence is
employed.
You must perform three sets of calcula-
tions to obtain the correct MCA and MOP
values: one for cooling (set #1) only, one
for electric heat (set #2) only, and one for
when the unit is in the cooling mode with
compressors and electric heat on
concurrently (set #3).
For size 031 and 040 units, compressors
A (1K1) and B (1K2) can run concurrently
with
1
/
3
of three-stage electric heat or
1
/
7
of seven-stage electric heat.
For size 051 and 066 size units, compres-
sors A (1K1) and D (1K4) can run
concurrently with
1
/
3
of three-stage
electric heat or
1
/
7
of seven-stage electric
heat.
For units using heaters less than 50 kW
:
MCA = (1.25 x load 1) + load 2 + load 4 +
(1.25 x load 3)
For unit using heaters equal to or greater
than 50 kW:
MCA = (1.25 X load 1) + load 2 + load 3 +
load 4
Select the nameplate MCA value that is
the largest of the three sets of MCA
calculations.
MOP = (2.25 x load 1) + load 2 + load 3 +
load 4
Select an overcurrent protective device