What size hole do I need for a 1 1/4 inch EMT connector?

The number you want is 1 3/4 inches. Standard knockout diameters are keyed to conduit trade size, and trade size 1 1/4 uses a 1.75 inch opening.

The full knockout chart

Trade size 1/2 is 0.875 inches. Trade size 3/4 is 1.109 inches, commonly cut with a 1 1/8 inch bit. Trade size 1 is 1.375 inches. Trade size 1 1/4 is 1.75 inches. Trade size 1 1/2 is 2.0 inches. Trade size 2 is 2.5 inches. Trade size 2 1/2 is 3.0 inches and trade size 3 is 3.5 inches.

Note the pattern breaks. The hole is not the conduit trade size, it is the outside diameter of the threaded hub plus clearance, which is why a 1 1/4 connector needs a 1 3/4 inch hole and not a 1 1/4 inch one.

Which tool

A knockout punch labeled for 1 1/4 conduit is the clean answer. Punches are sold by conduit size, not by hole diameter, so you ask for the 1 1/4 die set and it produces the correct 1.75 inch opening. A hydraulic or ratcheting punch leaves a burr free round hole and does not shed chips into the enclosure.

If you are drilling instead, a 1 3/4 inch carbide tipped hole cutter is the right bit for painted steel enclosures, panelboard tubs, and stainless. A bi metal hole saw works in thin gauge sheet metal but dulls fast on a panel can and tends to wander. Either way, tape the arbor pilot area and vacuum the enclosure afterward. Section 110.12 requires the work to be executed in a neat and workmanlike manner, and loose steel chips sitting on a bus is how a panel fails a year later.

What the code says about the hole itself

Section 110.12(A) requires unused openings other than those intended for the operation of equipment to be effectively closed by a means providing protection substantially equivalent to the wall of the equipment. If you cut a 1 3/4 inch hole and then decide to run 1 inch conduit instead, that gap gets a reducing washer set, not tape.

Section 314.17(A) requires openings in boxes and conduit bodies to be adequately closed, and 312.5(A) applies the same to cabinets and cutout boxes. Section 300.18(A) requires raceways to be installed complete between outlet, junction, and splice points before conductors are pulled.

The bonding angle matters too. A concentric or eccentric knockout that remains in the enclosure, or an oversized opening, can break the fault current path through the connector locknut. Section 250.97 requires bonding jumpers or listed fittings such as bonding locknuts and bonding bushings where circuits over 250 volts to ground pass through concentric or eccentric knockouts, and 250.96(A) requires an effective ground fault current path on every metal raceway connection.

Practical notes on the fitting

Use a set screw or compression connector listed for EMT, not a rigid fitting, unless the fitting is dual listed. Section 358.6 requires EMT fittings to be listed. In wet locations use a raintight compression connector marked with a rain drop symbol per 358.42, and land the connector on top of a panel only if it is raintight, per 312.2 and 314.15.

Field takeaway

1 3/4 inch hole for a 1 1/4 inch EMT connector. Use a 1 1/4 conduit size knockout punch if you have one, a 1 3/4 inch carbide cutter if you do not, and close any oversized opening with reducing washers under 110.12(A).