• Keep the sensor leads as short as possible in your application. The additional resistance in long
sensor cables creates an offset between the actual temperature and the displayed temperature.
See Table 1 when you extend sensor leads.
• A99 sensors are not polarity specific. You can connect either lead to the SEN or COM terminals.
• It is best practice to use 22 AWG, stranded, twisted-pair cable with a shield to extend sensor cable
runs.
Table 1: Maximum sensor cable lengths and wire sizes
Wire gauge
1
Maximum sensor cable length
1
16 AWG 500 ft (152 m)
18 AWG 300 ft (91 m)
20 AWG 200 ft (61 m)
22 AWG 125 ft (38 m)
1 The maximum sensor cable lengths have less than 1°F (0.6°C) error between the temperature sensed at the A99 sensor
and the temperature displayed on the LCD.
TB2 Terminal Block and SPDT relay output
The terminals LC, LNO, and LNC on the TB2 terminal block connect to an SPDT dry-contact relay in
the A421 Control (Figure 3). The control does not provide any internal power to the TB2 terminals
or relay contacts. The A421 Control energizes and de-energizes the relay to open and close the
contacts based on the On/Off temperature values.
• Relay De-energized (Off) = LC open to LNO as shown in Figure 3, and the relay status LED is off
• Relay Energized (On) = LC closed to LNO and the relay status LED is on
Figure 4 shows how to wire the A421 Control to use the same power source that powers the
controlled equipment to also power the A421 Control.
Figure 3: TB2 Terminal Block showing connections to the internal SPDT relay
You can also provide an independent power source for the A421 Control on the TB1 terminals and
then wire the TB2 relay terminals to a separate power source to switch and power the controlled
equipment circuit.
A421 Series Electronic Temperature Controls with Off-Cycle Defrost Installation Guide6
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