Line voltage is present at the switch terminals even with the lever in OFF. Lock out the disconnect, bleed the tank to zero, and verify absent voltage before removing the cover. Never set cut-out above the tank's rated working pressure or the safety relief valve setting.
Confirm the switch model and factory range
Read the label inside the cover — for example 9013FHG12J52M (95–125 psi, 1Φ, with unloader) or 9013FHG42J59M (140–175 psi). The adjustment screws can only move within the switch's rated range; you cannot make a 95–125 psi switch a 145–175 psi switch. Never set cut-out above the tank's rated working pressure or above the relief valve setting.
Lock out power and bleed the tank to zero
Unplug the compressor or open and lock the disconnect. Open the tank drain and bleed to 0 psi. Verify absence of voltage at the switch line terminals with a meter (live–dead–live). Capacitors on single-phase motors hold a charge; discharge before probing.
Identify the two adjusters under the cover
The large center nut/spring is the RANGE (cut-in) adjuster: it moves cut-in and cut-out together. The smaller offset nut is the DIFFERENTIAL adjuster: it changes cut-out only, widening or narrowing the gap above cut-in. On single-adjustment models (9013FSG/9013GHG style) only the range nut is present — the differential is fixed.
Set cut-in with the range nut
Turn the range nut clockwise to raise cut-in, counterclockwise to lower it. On most 9013FHG switches one full turn moves the setting roughly 2–4 psi; make quarter-turn changes and retest. Typical shop setting: 90–100 psi cut-in.
Set cut-out with the differential nut
Turn the differential nut clockwise to increase the gap (higher cut-out), counterclockwise to decrease it. Keep at least 20 psi differential — a narrow band short-cycles the motor and cooks the starter contacts. Typical shop setting: 90 psi cut-in / 120 psi cut-out.
Verify with two full cycles
Restore power, close the cover, and run the compressor through two complete cycles with a known-good gauge. Record the actual cut-in and cut-out. Re-bleed and readjust in small increments until both match target. Confirm cut-out stays at least 10–15 psi below the safety relief valve setting.
Confirm the unloader bleeds
On models with the -M suffix (unloader), you should hear a 1–2 second hiss from the small tube at the switch within a couple seconds of cut-out. No hiss means a plugged unloader or a check valve stuck open — the motor will hum, stall, or trip the breaker on the next start.
120 V single-phase, 2-pole switch
Line hot to L1, line neutral to L2 (or to the neutral lug if the switch has one). Motor hot from T1, motor neutral from T2. Bond ground to the green screw on the switch base — never leave the compressor frame unbonded.
240 V single-phase, 2-pole switch
Both legs are hot: L1 and L2 from the breaker, T1 and T2 out to the motor. The switch breaks both legs. Ground to the green screw. A 120 V-only switch must not be used to break a single leg of a 240 V circuit.
3-phase (9013 as pilot only)
Do not run 3-phase motor current through a 9013 line-voltage switch above its rating. Wire the switch as a pilot device in the starter control circuit: switch contacts in series with the coil, so the switch makes and breaks only the contactor coil while the starter carries motor current.
Auto / Off lever (9013F with lever)
The lever is a manual disconnect for the switch contacts only, not a lockout. Line terminals stay energized with the lever in OFF. Always lock the upstream disconnect for service.
LINE side (from breaker)
L1 L2
| |
[ 9013 contacts ] ← range nut = cut-in
| | diff nut = cut-out
T1 T2
LOAD side (to motor)
GRN screw ── equipment ground
1/4" tube ── unloader to pump head (M models)Which screw raises cut-out only on a Square D 9013?
The small differential nut. Clockwise widens the differential and raises cut-out while leaving cut-in where it is. The large range nut moves cut-in and cut-out together.
What is a normal cut-in and cut-out for a shop compressor?
90 psi cut-in and 120 psi cut-out is the common default on a 95–125 psi switch. Keep at least 20 psi of differential to avoid short-cycling, and keep cut-out below the tank rating and relief valve setting.
Can I set a Square D switch higher than its label range?
No. The range printed on the label is the mechanical limit of the spring. Forcing the nut past it gives inconsistent trip points and can hold the contacts closed. Use a switch rated for the pressure you need, such as a 140–175 psi model.
Why does my compressor hum and trip after I adjusted the switch?
Almost always an unloader problem, not the adjustment. Head pressure is still on the pump at restart. Confirm the unloader tube vents after cut-out and that the tank check valve is not leaking back.
What does the M suffix mean on 9013FHG12J52M?
M indicates an integral unloader valve for the pump head bleed line. Switches without it are used where an external unloader or a continuous-run setup handles head relief.