Instructions / Assembly
Table Of Contents
AT20, AT40, AT72D, AT87, AT88 AT TRANSFORMERS
3 69-1641EF—02
Mounting the AT72D Transformer.
Mount the transformer to meet the application. Use one 
of the methods illustrated. The transformer can be 
mounted in any position.
Foot Mounting.
1. Discard mounting plate.
2. Use screws or bolts through slots in mounting feet 
to fasten transformer to mounting surface (see 
Fig. 2).
Plate Mounting.
The mounting plate allows the transformer to be 
mounted on a 4 in. by 4 in. square or 4 in. octagonal 
junction box.
1. Remove the large center knockout in the mounting 
plate (Fig. 6).
2. Take transformer in one hand and mounting plate 
in the other hand. Keyhole slots on mounting plate 
should be up.
3. Place large knockout in mounting plate over pri-
mary leadwires and end bell of transformer. The 
small tab on the bottom center of the plate fits into 
the slot in the transformer mounting feet (Fig. 7). 
4. Insert the mounting screw through the holes as 
illustrated.
5. Secure the plate to the transformer. Do not 
overtighten.
Mounting plate to transformer (Fig. 4).
The plate can be mounted to the transformer in one of 
two positions:
a. at the clamp on primary end bell (transformer 
all above plate);
b. at the laminations (transformer 3/4 above 
plate. To mount the plate at clamp on primary 
end bell (transformer all above plate):
1. Take mounting plate in one hand. Keep the keyhole 
slots up and the raised portion of the large center 
knockout toward you.
2. Insert the primary leadwires through the center 
hole in the plate.
3. Fit the hole in the plate over the clamp attached to 
the end bell. Clamp screw must be turned almost 
completely out.
4. With the plate in place over the clamp, tighten the 
screw securely against the rim of the round hold 
See Fig. 5. Avoid damaging the leadwires with the 
screwdriver.
To mount the plate at the laminations (transformer 3/4 
above the mounting plate:
1. Remove the large center knockout in the mounting 
plate (see Fig. 6).
2. Take the transformer in one hand. Clamp on end 
bell should face you.
3. Take the mounting plate in the other hand. Keyhole 
slots should be up.
4. Place large knockout in mounting plate over 
primary leadwires and end bell. Small tab at the 
bottom of the center of the plate fits into the 
transformer mounting foot (Fig. 7). Insert mounting 
screw through holes as shown in Fig. 8.
5. Secure plate to transformer. Do not overtighten 
screw.
Mounting Transformer and Plate to 
Electrical Box
The mounting plate can be mounted to 4 in. x 4 in. boxes 
with regular and irregular spaced mounting holes, to 2 in 
by 4 in. boxes and to 4 in. octagonal boxes. See Fig. 9.
1.  Use the screws supplied with the electrical box. 
2. Place them through the proper mounting holes in 
the plate and secure the transformer and plate to 
the box.
3.  Punch out appropriate knockouts for plate 
mounting holes, if necessary.
NOTE: Transformer feet should always be outside of 
the junction box.
Clamp Mounting Using Junction Box 
Knockout
The transformer can also be clamp mounted using a 
junction box knockout, if desired. This mounting option 
does not require the use of the mounting plate. To mount 
the transformer within the knockout:
1. Insert the primary leadwires and clamp and screw 
on transformer end bell through suitable 1/2 in. 
(13 mm) knockout in junction box. Clamp screw 
must be turned almost completely out in order to 
get clamp through knockout.
2. Tighten clamp screw securely against rim of 
knockout. Avoid damaging the leadwires with the 
screwdriver.
Fig. 4. Plate may be mounted to transformer in one of 
two positions (AT72D shown).
PLATE MOUNTED AT CLAMP ON END BELL
(ALL ABOVE PLATE MOUNTING)
PLATE MOUNTED AT THE LAMINATIONS
(3/4 ABOVE PLATE MOUNTING)
END BELL
MOUNTING PLATE
RAISED PORTION OF LARGE CENTER KNOCKOUT
CLAMP
SCREW
AND
CLAMP
MOUNTING SCREW
MOUNTING PLATE
SCREW
TERMINALS
FOR
SECONDARY
CONNECTIONS
SCREW
TERMINALS
FOR
SECONDARY
CONNECTIONS
LEADWIRES
FOR PRIMARY
CONNECTIONS
LEADWIRES
FOR PRIMARY
CONNECTIONS
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