Rooftop Feeder Ampacity and a Panel in a Narrow Corridor
A 208Y/120-volt, three-phase feeder supplies a rooftop packaged unit and two exhaust fans of a restaurant. It leaves a 400-ampere distribution panel in a first-floor mechanical room, runs 85 feet in 2-inch EMT across an unshaded black EPDM roof, then drops to a fusible disconnect at the unit. The run contains three phase conductors and one neutral (a 120-volt control transformer makes the neutral a current-carrying conductor), plus a green equipment grounding conductor. The calculated load is 130 amperes continuous and 20 amperes noncontinuous at 208 volts. All terminations are rated 75 degrees C and the conductors are THWN-2. Summer design ambient is 100 degrees F at grade, and an infrared survey reads 145 degrees F on the conduit surface at 2 p.m. Separately, the contractor wants to mount a second 42-circuit panel on the opposite wall of a 42-inch-wide corridor. A 4-inch condensate line crosses the face of the proposed panel location at 5 feet 6 inches above the finished floor.
1. Size the feeder conductors, showing the ambient correction, the adjustment for the number of current-carrying conductors, and the termination limitation. State the equipment grounding conductor size.
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Required ampacity (215.2(A)(1)). 125% of continuous plus noncontinuous = (1.25 x 130) + 20 = 182.5 A.
Ambient correction. Use the code method rather than the surface reading: the design ambient plus the rooftop temperature adder for a 2-inch conduit run on a roof (the adder table is at 310.15(B)(3)(c) in the 2017 and 2020 editions; table numbering shifts between editions, so cite by content to the AHJ). For 1-inch through 2-inch trade size the adder is roughly 40 degrees F, giving about 140 degrees F, or 60 degrees C. At 60 degrees C the 90 degrees C correction factor is 0.71. A designer using the 145 degrees F surface reading would land between 60 and 65 degrees C; using 0.71 is not conservative against 145 degrees F, so document the choice and consider 0.65.
Adjustment for conductors. Four current-carrying conductors (three phases plus the neutral) = 80%.
Combined factor = 0.80 x 0.71 = 0.568.
Selection. Required 90 degrees C ampacity = 182.5 / 0.568 = 321 A. Table 310.16: 300 kcmil copper at 90 degrees C is 320 A, which derates to 181.8 A - just under 182.5 A, so it fails by less than an ampere and must not be rounded up. 350 kcmil copper (350 A at 90 degrees C) derates to 198.8 A, which exceeds 182.5 A.
Termination limit (110.14(C)). The 350 kcmil final ampacity must also be checked against the 75 degrees C column (310 A) and against the load: 182.5 A is well under 310 A, so the conductor is acceptable.
Equipment grounding conductor. 250.122 with a 200-ampere feeder overcurrent device gives 6 AWG copper as the minimum. Because the phase conductors were increased from the minimum size that had sufficient ampacity (3/0 copper) to 350 kcmil, 250.122(B) requires the wire-type EGC to be increased proportionally by circular mil area; 6 AWG increased by the ratio of 350 kcmil to 167.8 kcmil lands at roughly 2 AWG copper. This is the item most often missed on a derated feeder.
2. Evaluate the proposed panel location: 42 inches of corridor, with a 4-inch condensate line crossing the face at 5 feet 6 inches. What does 110.26 require and what must change?
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Depth (110.26(A)(1)). For 151 to 600 volts, Condition 1 requires 36 inches of clear depth where the equipment faces no live or grounded parts; Condition 2 requires 42 inches where the equipment faces grounded parts (a masonry wall, a metal enclosure, metal conduit); Condition 3 requires 48 inches where live parts face each other. In a corridor, the opposite wall is usually a grounded surface, so 42 inches is the applicable figure, and the proposed corridor is exactly 42 inches - no margin.
The pipe destroys that margin. The working space is measured from the face of the equipment, and the 4-inch condensate line protruding from the opposite wall at 5 feet 6 inches above the floor reduces the clear distance to about 38 inches. It also sits inside the 0-to-6-foot-6-inch height envelope required by 110.26(A)(3). Either the line is relocated or the panel moves; a 42-inch corridor cannot absorb a 4-inch obstruction.
Dedicated equipment space (110.26(E)). Indoor panelboards require the space equal to the width and depth of the panel extending from the floor to 6 feet above the top of the enclosure (or the structural ceiling, whichever is lower) to be dedicated to the electrical installation. No piping, duct, or other foreign equipment may be located there. A condensate line routed above or across the panel is a violation independent of the working-space issue, and the leak risk makes it a bad idea even where dimensions technically allow.
Width and height. 110.26(A)(2) requires 30 inches wide or the width of the equipment, whichever is greater, centered logically on the equipment; a 24-inch panel needs 30 inches, which the corridor can provide. Height is 6 feet 6 inches (110.26(A)(3)), and 110.26(B) prohibits using the space for storage - a storage corridor is a common red tag.
Facing panels. If a second panel is installed directly opposite, the two working spaces overlap and, on most AHJ interpretations, cannot both be satisfied in a 42-inch corridor. Offset the panels so their footprints do not overlap, or widen the corridor. Get the layout approved by the subcode official before the framing is closed up, because the fix is structural.