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Code Calculations

Apr 1, 2000 12:00 PM, By Mike Holt, NEC Consultant

You must size pull boxes and junction boxes, as well as conduit bodies (enclosures), so you don’t damage the conductor insulation. For conductors No. 4 and larger, size pull boxes, junction boxes, and conduit bodies according to the National Electrical Code (NEC) Sec. 370-28.

Straight pull calculation [Sec. 370-28(a)(1)]. A straight pull calculation applies when conductors enter one side of an enclosure and leave through the opposite wall of the enclosure. The minimum distance between these walls must not be less than eight times the trade size of the largest raceway.

Angle pull calculation [Sec. 370-28(a)(2)]. An angle pull calculation applies whenever conductors enter one wall of an enclosure and leave through the adjacent wall (not opposite the wall). The minimum distance (from the wall where the raceway enters to the opposite wall of the enclosure) must not be less than six times the trade diameter of the largest raceway, plus the sum of the diameters of the remaining raceways on the same wall and row. When more than one row exists, you must calculate each row separately. The row with the largest calculation is the minimum angle pull dimension.

U pull calculations [Sec. 370-28(a)(2)]. A U pull calculation applies when the conductors enter and leave an enclosure from the same wall. This distance must not be less than six times the trade diameter of the largest raceway, plus the sum of the diameters of the remaining raceways on the same wall.

Distance between raceways calculation [Sec. 370-28(a)(2)]. After sizing the enclosure, be sure to install raceways so the distance between raceways enclosing the same conductors are not any closer than six times the trade diameter of the larger raceway. Measure this distance from the nearest edge of one raceway to the nearest edge of the other.

Depth of enclosure [Sec. 370-28(a)(2), exception]. When conductors enter an enclosure opposite a removable cover such as the back of a pull box or conduit body the distance from the wall where the raceways enter to the removable cover must not be less than the distances listed in Table 373-6(a); one wire per terminal.

Example No. 1—sizing a pull box. A pull box has one 4-in. raceway on the left side, one 4-in. raceway on the bottom, and one 3-in. raceway on the top and right side.

Q. What is the horizontal dimension of the box?

(a) 28 in.
(b) 21 in.
(c) 24 in.
(d) none of these

A. Sec. 370-28(a)(2) requires you to size the box based on the larger raceway.

Left wall to right wall (angle pull) = 6 x 4 in. = 24 in. Therefore, the answer is (c) 24 in.

Q. What is the vertical dimension of the box?

(a) 18 in.
(b) 21 in.
(c) 24 in.
(d) 14 in.

A. Again, the answer is (c) 24 in. based on the same calculation and reference above.

Determine the minimum distance between the two 4-in. raceways (containing the same conductors) in the same manner as above (i.e. 6 x 4 in. = 24 in.).

Example No. 2—sizing a junction box. A junction box contains two 3-in. raceways on the left side and one 3-in. raceway on the right side. You must pull the conductors from one of the 3-in. raceways on the left wall through the 3-in. raceway on the right wall. You then pull the conductors from the other 3-in. raceway through a 3-in. raceway at the bottom of the pull box.

Q. What is the horizontal dimension “A” of the box?

(a) 18 in.
(b) 21 in.
(c) 24 in.
(d) none of these

A. Sec. 370-28(a)(1) requires you to size the box for the straight pull based on the following calculation:

Left wall to right wall (straight pull) = 8 x 3 in. = 24 in.

Right wall to left wall (straight pull) = 8 x 3 in. = 24 in.

Sec. 370-28(a)(2) requires you to size the box for the angle pull based on the following calculation:

Left wall to right wall (angle pull) = (6 x 3 in.)+ 3 in. = 21 in.

Right to left wall (angle pull) = no calculation.

The largest answer you calculate governs the horizontal dimension of the box. Therefore, the answer is (c) 24 in.

Q. What is the vertical dimension “B” of the box?

(a) 18 in.
(b) 21 in.
(c) 24 in.
(d) none of these

A. In this case, you only need to perform one calculation. Bottom wall to top wall (angle pull) = 6 x 3 in. = 18 in. Therefore, the vertical dimension of the box is (a) 18 in.

Q. What is the minimum distance “C” between the two 3 in. raceways that contain the same conductors?

(a) 18 in.
(b) 21 in.
(c) 24 in.
(d) none of these

A. Determine the minimum distance between the two 3-in. raceways (containing the same conductors) in the same manner as above (i.e. 6x 3 in. = 18 in.).

Q. If the enclosure has two 2-in. conduits that enter the back with No. 4/0 conductors, what is the minimum depth of the enclosure?

(a) 4 in.
(b) 6 in.
(c) 8 in.
(d) 10 in.

A. You determine the depth of the enclosure as per Table 373-6(a). Assuming our installation uses one wire per terminal, the answer is (a) 4 in.

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