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®
Model MSX Make-Up Air
Installation of Electric Heater
(optional)
WARNING
Electrical Shock Hazard! Disconnect all power
sources before doing any work on the unit.
Effect of Low Voltage on Wattage and
British Thermal Unit (BTU)
The heating elements may be used on voltages
lower than the design voltage of the heater, however,
the wattage and BTU output will be reduced to the
percentages listed in the table below.
1
For 7-24 conductors in raceway or cable, reduce allowable
heater line currents to 87½% of those shown above.
2
Based on 30°C. (86°F.) ambient temperature, for higher
ambient temperature see National Electric Code table
310-16 and 310-18 Note 13.
3
Based on 80% of ratings in the National Electric Code
table 310-16 for 75°C insulation.
De-rated Wattage For Low Voltage
Heater
Voltage
Line
Voltage
% of
Heater
Wattage
and BTU
Heater
Voltage
Line
Voltage
% of
Heater
Wattage
and BTU
480 460 92 208 200 92
440 84 190 83
277 265 92 120 115 92
254 84 110 84
240 230 92
220 84
208 75
200 69
Supply Wire Size
(Not more than six conductors in Single Conduit
1
)
AWG or
MCM
Max. Heater
Line Current
2
AWG or
MCM
Max. Heater
Line Current
2
Copper
3
Copper
3
14 12 0000 184
12 16 250 204
10 24 300 228
8 36 350 248
6 52 400 268
4 68 500 304
3 80 600 336
2 92 700 368
1 104 750 380
0 120 800 392
00 140 900 416
000 160 1000 436
General:
The requirements and practices described below
are based on the National Electric Code (NEC) and
The Space Heating Standard of the Underwriters
Laboratories Inc. (UL). Although UL requirements
are uniform throughout the country, local electrical
codes may deviate from the National Electrical Code.
Therefore, local inspection authorities should be
consulted regarding local requirements.
Electrical Wiring Instructions:
1. Use the wiring diagram supplied with the heater
as a guide in correlating field wiring with the
heater internal wiring.
2. All field wiring to the heater must meet the
requirements of the National Electric Code and
any other applicable local or state codes.
3. Wiring to the heater must be rated for 75°C
(167°F) minimum.
4. The fan is interlocked by the factory to the
control circuit so the electric heater will not
operate unless the fan is on.
5. If heater does not have a built-in disconnect
switch or main circuit breaker, install a remote
disconnect (furnished by others) in accordance
with the National Electric Code, Article 424-65.
Calculation of Line Currents (amps):
Single Phase Current =
watts
volts
EXAMPLE: 5 kW, 208 volt
5000 watts
= 24 amps
208 volts
Three Phase Current =
watts
volts x 1.73
EXAMPLE: 14.4 kW, 208 volt
14400 watts
=
14400
= 40 amps
208 volts x 1.73 360
Sizing of Supply Conductors
The required minimum size of supply conductors
is marked at the field wiring terminals within the
heater control box or reference the Supply Wire Size
table included in this section. The wire gauges are
calculated for 125% of the heater line current as
required by the National Electric Code, Article 424-3(b)
based on conductor insulation rated for 75°C (167°F).