508133G01 / 31-5000715
Page 13 of 25
Issue 2140
Electrical - Circuit Sizing and Wire Routing
In the U.S.A., wiring must conform with current local codes
and the current National Electric Code (NEC). In Canada,
wiring must conform with current local codes and the
current Canadian Electrical Code (CEC).
Refer to the furnace or air handler installation instructions
for additional wiring application diagrams and refer to unit
nameplate for minimum circuit ampacity and maximum
overcurrent protection size.
24VAC Transformer
Use the transformer provided with the furnace or air handler
for low-voltage control power (24VAC - 40 VA minimum).
Thermostat Control and Low Voltage
Control Wiring
Conventional 24VAC Thermostat Control
The NS18HMA variable capacity unit may be installed
using a conventional 24VAC two-stage cooling or single-
stage cooling thermostat.
The NS18HMA unit will provide full variable capacity
operation when installed with a conventional 24VAC two
stage heat pump or single-stage heat pump thermostat.
The NS18HMA outdoor control has advanced control
algorithms, which provide true variable speed capacity
operation by modulating the compressor speed to achieve
the target suction pressure set point in cooling mode, and
liquid pressure set point in heating mode.
When utilizing a two-stage conventional 24VAC thermostat,
six wires are required to control the outdoor unit (R, C, W1,
O, Y1 and Y2). Refer to the NS18HMA
field wiring diagram
for a conventional 24VAC two-stage thermostat.
When utilizing a single conventional 24VAC thermostat,
five wires are required to control the outdoor unit (R, C, W1,
O, and Y1) and Y1 is jumpered to Y2 in the outdoor unit.
Note that the published performance data is based upon
the use of a two-stage thermostat. Refer to the NS18HMA
field wiring diagram for a conventional 24VAC single-stage
thermostat.
NS18HMA Low Voltage Control Wiring
Connections
The NS18HMA variable capacity units are provided with
(2) RAST 6-Pin connections for connecting the field low
voltage control wiring to the NS18HMA harnesses in the low
voltage control make-up box. One RAST 6-pin connector is
labeled with terminals TST, DF, R, I+, I- and C. The second
RAST 6-pin connector is labeled with terminals DS, O, Y1,
Y2, L and W.
Electrical Shock Hazard!
Can cause injury or death. Unit must be
properly grounded in accordance with
national and local codes.
Line voltage is present at all components
when unit is not in operation on units
with single-pole contactors. Disconnect
all remote electric power supplies before
opening access panel. Unit may have
multiple power supplies.
WARNING
Fire Hazard. Use of aluminum wire with this product
may result in a fire, causing property damage, severe
injury or death. Use copper wire only with this product.
WARNING
Failure to use properly sized wiring and circuit breaker
may result in property damage. Size wiring and circuit
breaker(s) per Technical Specifications and unit rating
plate.
WARNING
ELECTROSTATIC DISCHARGE (ESD)
Precautions and Procedures
Electrostatic discharge can affect electronic components.
Take care during unit installation and service to protect
the unit’s electronic controls. Precautions will help to
avoid control exposure to electrostatic discharge by
putting the unit, the control and the technician at the
same electrostatic potential. Touch hand and all tools
on an unpainted unit surface before performing any
service procedure to neutralize electrostatic charge.
WARNING
