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Q.To retrofit light fixtures for old plaster ceilings that will be covered with new drywall, I plan to screw 1/2-inch-deep metal pancake boxes into the lathe of the existing ceilings and hang the new drywall around the boxes. But my electrician wants to cut into the ceiling and install deeper ceiling boxes, which will require additional blocking and more labor. He says that 3 1/2-inch-diameter by 1/2-inch-deep pancake boxes don’t have sufficient fill capacity to make the electrical connection; is he right?

A.Harlan Madsen of South Side Electric, an electrical contractor in Bloomington, Minn., responds: A single 14/2 NM feed with three conductors (hot, neutral, and ground) would require a box with a 6-cubic-inch capacity (2 cubic inches for each conductor), while a 3 1/2-inch pancake box has only a 3.9-cubic-inch volume allowance (2008 NEC 314.16).

A 4-inch-diameter pancake box has a 6-cubic-inch volume allowance, enough for a single light fixture but not enough if you’re using a 12/2 feed or making a splice — and not enough if the fixture has a mounting device that occupies space in the box. The NEC does allow the canopy of the fixture to add to the cubic inch capacity of the box, but the volume must be indicated on the canopy.

Because of these limitations, we install 3 1/2-inch pan boxes only if the fixture specifically requires it, or in cases where there will be transition to wire mold and an extension can be added to the box; I don’t recommend them under any other circumstances.

A final complication is that the National Electrical Code now requires ceiling boxes to be rated to carry 50 pounds (2008 NEC 314.27[A]) — and I doubt that screwing the box to the lathe will meet that requirement. Your electrician is right in calling for additional blocking, and you might as well use bigger boxes, too.