157
type cutout. It is wired into the control circuit which operates the
magnetic disconnecting contactors (the same contactors that also
switch on and off the various steps of the coil). The secondary
manual reset cutout is a bi-metal disk-type cutout. This secondary
thermal cutout is load carrying and is installed in each heater sub-
circuit. The primary and secondary overtemperature protection
systems are independent of each other. The secondary system is
designed to protect against possible failure of the primary system
to deenergize the heater.
Subcircuits in the heaters are designed in compliance with para-
graph 424-22 of the NEC. The coil is subdivided into circuits that
draw no more than 48 amps each and is fused for at least 125% of
the circuit rating.
NOTE: Airflow tube is to be positioned so that the airflow switch
is actuated by a minimum negative pressure of 0.07 in. wg. In the
event that the minimum 0.07 in. wg cannot be achieved, extend
the high pressure tube to the next upstream bulkhead to the fan
discharge for draw-thru electric heat or extend the low pressure
tube to the negative side of the fan for blow-thru electric heat.
Refer to Table 40 for heater electrical data including VAV electric
heaters.
Table 39 — Field Wiring for Incoming Conductors
Sized for 125% of Heater Load
LEGEND
*Values are based on Table 310-16 of the NEC for 75 C insulated cop-
per wire. Not more than 3 conductors in a raceway.
NOTES:
1. Be sure to consider length of wiring run and possible voltage drops
when sizing wires.
2. Field power wiring — Heaters are furnished with a terminal block
sized for incoming copper conductors with 75 C insulation rated to
carry at least 125% of the heater load. However, conductors can
be sized to carry 100% of the heater load if the heater is rated at
50 kW or more, and the heater is controlled by a cycling device
such as a multi-stage thermostat, step controller, or SCR (silicon
control rectifier) power controller. Terminal blocks and knockouts
are sized to handle either 100% or 125% conductors.
WIRE SIZE
(AW
G OR
KCMIL)
LOAD AMPS* WIRE SIZE
(AWG OR
KCM
IL)
LOAD AMPS*
COPPER COPPER
14 12 2/0 140
12 16 3/0 160
10 24 4/0 184
8 40 250 204
6 52 300 228
4 68 350 248
3 80 400 268
2 92 500 304
1 104 600 336
1/0 120 700 368
AWG — American Wire Gage
kcmil — Thousand Circular Mils
NEC — National Electrical Code
NOTE: All wiring must be copper and must conform to the NEC (National Electrical Code).
Fig. 137 — Electrical Heater Wiring Schematic (Typical)
a39-1721
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