AirCompDX

3-wire start/stop with seal-in

120 VAC control from a control transformer (H1/H2 – X1/X2)

All wiring guides

3-wire start/stop with seal-in

The base of every magnetic starter control circuit: momentary stop in series, momentary start in parallel with the starter's own auxiliary contact.

Across-the-line startersReciprocating compressorsAny NEMA size 00–5 starter
X1120 V hotSTOPNCSTARTNOOL95–96 NCM coilA1 / A2M aux13–14 NOX2neutral
Step 1 of 7
Step 1

Control power in

From
Transformer X1
To
Stop pushbutton terminal 1

Conductor: 16 AWG MTW, wire #1

Land the fused secondary hot on the input side of the stop button. Fuse X1 per the transformer nameplate — typically a 1–2 A control fuse.

Verify before moving on
  • 120 V from X1 to X2 with the disconnect closed
  • 0 V from X2 to ground on a grounded-secondary transformer

Aux contacts & feedback

  • Auxiliary block numbering: 13–14 = NO, 21–22 = NC, 31–32 = NC, 43–44 = NO on IEC starters. NEMA starters usually mark them 'a' (NO) and 'b' (NC).
  • Use a spare NO aux for a run light or a run signal to the controller; use a spare NC aux for an 'off' indicator or an interlock.
  • One aux contact should be reserved for the machine controller's run-feedback input so it can flag a contactor that failed to pull in.

Before you land a wire

  • Lock out and verify zero energy before landing any wire — control transformers stay hot from a separately fused source on many packages.
  • Never bypass the overload contact to 'test' the starter.

Reference only. Terminal numbers vary by manufacturer — confirm against the actual device drawing and follow NEC/NFPA 79 and your site's electrical safety program.