Resideo AT40A1121/U Foot Mounted 120 Vac Transformer with 9 in Lead Wires

Item #: HONE-AT40A1121/U
Mfr. part #: AT40A1121/U
UPC: 085267035736

Key specifications

Frequency50 Hz; 60 Hz
Temperature Range-20 F to +105 F
NEMA RatingNEMA type D

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$52.29

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Shipping details

Shipping weight: 1.8 lb
Box size: 4.65 × 4.1 × 3.6 in

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Brand	Resideo
Manufacturer part #	AT40A1121/U
Frequency	50 Hz; 60 Hz
Electrical Ratings	Primary voltage--120 Vac; Secondary voltage--26.5 V.O.C.
Mounting	Channel Frame Mounting allows for Direct, Horizontal, or Vertical Foot mounting.
Dimensions	2 3/32 in. high x 2 5/8 in. wide x 2 3/16 in. deep
UL	UL Component Recognized.
CSA	Certified
Temperature Range (F)	-20 F to +105 F
NEMA Rating	NEMA type D
Electrical Connections (Primary)(in.)	9 in. color coded primary and secondary leadwires
Electrical Connections (Secondary)(in.)	9 in. color coded primary and secondary leadwires
Electrical Ratings, Output	24 Vac at 40 VA
UPC	085267035736

The AT40A1121 is a control transformer from Resideo (Honeywell Home), 40 VA, 120 V primary, foot mount.

Almost every heating and cooling control runs on 24 volts, and the transformer is what makes it. It takes line voltage in and delivers the low-voltage supply that the thermostat, the zone valves, the relays and the gas valve all share.

Size it by VA, not by voltage. VA is the load the transformer can carry, and every device on the control circuit adds to it — a zone valve pulls current while it drives, a relay coil holds current the whole time it is energized, and a Wi-Fi thermostat draws continuously. A transformer sized for the original equipment will not carry the same system after two zones and a smart stat have been added, and the symptom is intermittent: things work alone and fail together.

Mounting matters more than it looks. The transformer has to land where the enclosure has room and where the leads reach, and a foot-mount body will not sit in a knockout that was made for a plate mount.

Leadwires are 9 inches.