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Decoder System Design

Cable Design Formulas

Wire Path Length Formula

Lw =  2 x Vd x 1,000
Rw x Iw

Lw = Wire path length in feet or meters (1 pair wires)
Vd = Allowed voltage drop
Iw = Maximum operation current on wire path
Rw = Wire path resistance in ohm/1,000 ft or ohm/km

Vd (Voltage Drop)

Vd = Output – Minimum operation voltage

Vd = (1.4 × 24V) – 20V

Vd ≈ 14V

Iw (Current In Wire Path)

Iw is the sum of all decoders‘ currents on the wire path and the sum of simultaneous open solenoids.

One decoder uses approximately 3.5 mA (with solenoid off)
One solenoid uses 45 mA (Hunter solenoid marked 250 mA with default Power Factor 2)

Iw = Maximum operation current on wire path

Nd = Number of decoders on wire path

Ns = Number of simultaneously open solenoids (Maximum 14 per ACC controller (12 valves + 2 P/MV))

Iw = (Nd × 0.005) + (Ns × 0.0035)

Rw (Loop Resistance)

Rw = Loop resistance in ohm/1,000 ft or ohm/km

This resistance varies by cable area and the actual values need to be verified by cable manufacturer. The resistance refers to the out and back resistance, or that of both conductors in the two-wire path treated as a single continuous run.

 
Cable (1 pair) Rw
ohm/1000 ft (330 m)
Rw
ohm/km
Comment
#14-2  5.04 16.56 IDWIRE1
2.0 mm-2 ~ 10.98 Metric Wire Gauge
#12-2  3.18 10.42 IDWIRE2
2.5 mm-2 ~ 7.02 Metric Wire Gauge
#10-2  2 6.55 American Wire Gauge

Wire connectors are not taken into account. Good and properly made connections add very little resistance, but a safety margin is needed because resistance may increase (up to 25%) in the connectors when aging.

Examples

All 101 (ACC controller with 99 stations plus 2 Pump/Master Valves assigned to decoders) decoders on a single wire path with the maximum 14 solenoids active (6 programs with 2 solenoids per station plus 2 Pump/Master Valves) on one IDWIRE2 path.

English Metric
Lw =  2 x 14 x 1,000  = 6,267 ft OR Lw =  2 x 14 x 1,000  = 1,913 m
3.18 ((101 x 0.005) + (20 x 0.045)) 10.9 ((101 x 0.005) + (20 x 0.045))

1. 80 decoders with 5 solenoids active on one IDWIRE1 path.

English Metric
Lw =  2 x 14 x 1,000  = 8,889 ft OR Lw =  2 x 14 x 1,000  = 2,705 m
5.21 ((80 x 0.005) + (5 x 0.045)) 17.1 ((80 x 0.005) + (5 x 0.045))

Conditions

  • The decoders and active solenoids are evenly spread over the path
  • Good wire connectors are used
  • Decoders are set up for powerfactor 2 (default)

Design Suggestions

Reduce maximum length by 25% to compensate for wire connections and different types of solenoids and aging. 

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