What it is

Almost every house you work in is fed single-phase, split-phase power: two hot legs and a neutral. Across the two hots you get 240V to run the heavy stuff — compressor, condenser fan, heat strips. From either hot to neutral you get 120V for the little stuff — control transformer primary, condensate pump, a service receptacle. And the low-voltage brains of the system run on 24V, stepped down by a transformer. Understanding how those three voltages relate is what lets you walk up to a dead unit and know where to put your leads.

How it works

The utility transformer gives you a center-tapped secondary. The center tap is your neutral (and it's bonded to ground at the panel). The two ends are your two hot legs, L1 and L2. Each leg measures 120V to neutral. The two legs are 180 degrees out of phase with each other, so when you measure leg-to-leg you get the two 120V halves adding up to 240V. That's the "split phase" idea — one phase, split into two halves around a neutral.

This is different from three-phase commercial power, where you've got three legs and the math gives you 208V between legs on a wye system. On straight residential you only ever see the two legs.

The 24V control circuit comes off a transformer. The primary side takes line voltage — usually 120V from one hot to neutral, sometimes 240V across both legs depending on how it's wired — and the secondary steps it down to 24V. That 24V is the safe, low-energy voltage your thermostat switches around to pull in contactors, energize gas valves, and call the equipment on. The high-current loads never run through the thermostat; the thermostat just closes a 24V circuit that pulls in a relay or contactor, and the contactor does the heavy switching.

In the field

On a no-cool, your first three readings tell the story:

  • L1 to L2 at the disconnect: should be ~240V. No voltage here means the problem is upstream — breaker, disconnect, or service.
  • L1 to neutral and L2 to neutral: each should be ~120V. If one leg reads 120V and the other reads 0V, you've lost a leg, and a single-phasing condition will keep the contactor coil happy on 24V while the compressor sits there humming and tripping on overload.
  • R to C at the board: should be ~24V. This proves the transformer is alive and the control circuit is intact.

When the equipment is dead but the control voltage is present, the problem is on the load side. When the control voltage is missing, you're chasing the transformer, its fuse, or a shorted thermostat wire.

Normal values & targets

  • Leg to leg: 240V nominal, ~228–252V real-world depending on utility loading.
  • Leg to neutral: 120V nominal each leg, ~114–126V typical.
  • Commercial 208V single-phase off a three-phase wye: ~208V leg to leg, ~120V to neutral.
  • Control voltage: 24V nominal, healthy ~22–28V under load. Sagging much below 22V under a call points to an overloaded or weak transformer, or too many devices on the circuit.
  • A common residential control transformer is rated 40 VA, which at 24V supplies about 1.6 amps before it's maxed.

Common faults & what they mean

  • 0V leg to leg, 120V on each leg to neutral → you actually have both legs; recheck your leads. (Both legs present always gives 240 across them.)
  • 120V on one leg, 0V on the other → lost leg / single-phasing; compressor hums and trips, contactor still pulls in.
  • 240V present but 0V on R-to-C → blown transformer, blown low-voltage fuse, or open transformer primary.
  • 24V present but drops to near zero the instant you call cooling → shorted thermostat wire or a shorted/locked coil dragging the transformer down.
  • Control voltage reads 24V open-circuit but collapses under load → undersized or failing transformer.

Tech tips & gotchas

A blown low-voltage fuse or a tripped breaker on the transformer almost always means a shorted 24V wire somewhere — a thermostat cable pinched against the cabinet or a rubbed-through wire at the contactor. Replacing the fuse without finding the short just blows the new fuse. Disconnect the thermostat wires and meter the field side for a short to ground before you re-fuse.

Don't trust the breaker label to tell you which leg is which. Meter it. A unit can lose a single leg from a corroded lug or a failed breaker pole while the panel looks normal.

If you see exactly 208V where you expected 240V, you're probably on a commercial three-phase service tapped single-phase, and that lower voltage changes your heat strip output and your amp-draw math.

Safety / code notes

Disconnecting means and working space are governed by NEC Article 110, and the dedicated equipment disconnect within sight of the unit is required per NEC Article 440 for air-conditioning equipment. The 24V Class 2 control circuit is low-energy but still energized — treat a "low voltage" short as a real troubleshooting target, not a nuisance. Always verify the disconnect actually opens both legs before working on line-voltage components.