Contactor Chatter on a Long 24V Control Circuit
A 24V AC control circuit operates a contactor coil rated 20 VA. The transformer is a 40 VA Class 2 unit located in a mechanical room, and the contactor is 150 ft away. The installer used 18 AWG stranded thermostat cable, two conductors, with a total loop length of 300 ft. At startup the contactor chatters, and a voltmeter at the coil reads 21.5V while the transformer secondary reads 24.2V. The installer wants to replace the 40 VA transformer with a 75 VA unit to fix the problem. The contactor nameplate lists a pickup voltage of 18V and a dropout of 6V. The installer says the circuit is Class 2, so voltage drop does not matter.
1. Calculate the approximate voltage drop and explain whether the 75 VA transformer will solve the problem.
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Load current = 20 VA / 24 V = 0.833 A. 18 AWG copper is about 6.5 ohms per 1,000 ft, so a 300 ft loop is about 0.3 kft and 1.95 ohms. Drop = 0.833 A x 1.95 ohms = 1.63 V, leaving about 22.6 V at the coil, close to the measured 21.5 V after connections and tolerances. A 75 VA transformer has more capacity but does not reduce the wire resistance. It may hold voltage slightly better under load, but the drop is still set by the 18 AWG cable. It is not the correct fix.
2. What minimum conductor size would keep the drop at or below 5% at 20 VA? Show the calculation.
Read the worked answer
5% of 24 V = 1.2 V. Allowable loop resistance = 1.2 V / 0.833 A = 1.44 ohms. For 300 ft, or 0.3 kft, allowable resistance is 1.44 / 0.3 = 4.8 ohms per 1,000 ft. 18 AWG at about 6.5 ohms per 1,000 ft fails; 16 AWG at about 4.1 ohms per 1,000 ft meets it. Use 16 AWG minimum, and 14 AWG for more margin. Verify terminals, Class 2 listing, and pickup and dropout at minimum primary voltage.
3. Why can a Class 2 circuit still fail in service, and what Article 725 requirements apply?
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Class 2 limits energy for fire and shock but does not guarantee performance. Article 725 requires listed cable, support, separation from power conductors, and staying within Class 2 power limits. The 24V, 40 VA circuit is within limits, but voltage drop, connections, and coil inrush can cause chatter. The installer must follow the manufacturer's instructions and verify the circuit works at minimum supply voltage. Increasing transformer size without fixing wire size is both a performance error and a code compliance error.