24 V Transformer Overload on a Rooftop Unit
A 5-ton rooftop unit with a 75 VA, 208/230V primary, 24V secondary control transformer has intermittent compressor contactor drop-out. The 24V loads are: control board 8 VA, thermostat 3 VA, compressor contactor coil 10 VA sealed (35 VA inrush), economizer actuator 15 VA, two zone damper actuators at 12 VA each, and a new 24V condensate pump rated 20 VA. The transformer is in a 140°F return-air plenum. With all loads energized, you measure 21.5V at the transformer secondary and 19.8V at the farthest zone damper. The control wire is 18 AWG, run 150 ft from the unit to the zone panel. The contactor chatter stops when the condensate pump is unplugged. The transformer has a built-in 3 A secondary fuse that has not blown. The unit nameplate lists a 75 VA transformer as standard. The installing contractor wants to replace the contactor because it is 'weak.' You need to determine whether the transformer, wire size, or a load is the problem.
1. Calculate total connected VA. Is the 75 VA transformer adequate? Include inrush.
Read the worked answer
Sealed connected load: board 8 + thermostat 3 + contactor 10 + economizer 15 + two dampers 24 + pump 20 = 80 VA. Add a 25% margin for continuous loading and transformer heating: 80 x 1.25 = 100 VA. The 75 VA transformer is undersized even before inrush. Inrush is worse: contactor 35 VA plus economizer actuator and damper inrush can exceed the transformer's short-term capacity. The 3 A fuse does not protect against thermal overload because 80 VA at 24 V is 3.33 A, above the fuse rating, but the fuse may not blow immediately. The 140°F plenum raises transformer temperature and reduces its service life.
2. Interpret the voltage readings and wire size. What is the allowable drop, and what does the 19.8 V at the damper indicate?
Read the worked answer
Nominal secondary is 24 V. 21.5 V is 2.5 V low (about 10.4% below nominal), and 19.8 V is 4.2 V low (17.5% below nominal). Most 24 V contactors are rated to pull in at 85% of coil rating (about 20.4 V), so 19.8 V explains the chatter. The 18 AWG, 150 ft run has significant resistance: roughly 300 ft of conductor at about 0.0064 ohm/ft gives about 1.9 ohm loop resistance; at 3.3 A that is about 6.3 V drop, which matches the low readings. Voltage drop should generally be kept under 5% for control circuits, and the farthest load should remain above the contactor's minimum pull-in voltage with all loads energized.
3. Recommend a remedy and sequence. What should not be replaced?
Read the worked answer
Do not replace the contactor first; it is responding to low voltage. Steps: (1) replace the 75 VA transformer with a 100 VA or 150 VA transformer with adequate VA and temperature rating; (2) move the condensate pump to a relay or a separate control circuit so its inrush does not load the main transformer; (3) replace the 18 AWG control run with 16 AWG or relocate the zone panel or transformer closer to the loads; (4) verify 24 V at the transformer and at the farthest damper with all loads energized; (5) check the economizer actuator and damper for binding, which can increase VA draw. After correction, cycle the compressor contactor repeatedly and confirm no chatter. Document the VA calculation and voltage readings.