Rooftop Replacement: Reusing a Branch Circuit That No Longer Fits
A 208/230 V single-phase packaged rooftop unit is replacing an aging 3-ton unit on a small office building. The new nameplate reads: minimum circuit ampacity 32.4 A, maximum overcurrent protection 50 A, compressor rated load amps 18.6 A, outdoor fan 1.5 A, indoor blower 2.0 A. The existing branch circuit is 10 AWG copper NM-B on a 30 A breaker, run about 45 ft inside the building from the panel. The new unit sits 110 ft from the panel along the only practical conduit route, so the installer proposes to reuse the existing 10 AWG NM-B inside the building, splice it to 10 AWG THHN in liquidtight flexible conduit for the outdoor portion, and keep the 30 A breaker because “the compressor only draws 18 amps.” A used 60 A non-fused disconnect is on the truck, and the existing disconnect is mounted on a wall 55 ft away around a corner, out of sight of the unit.
1. Evaluate the conductor and overcurrent plan against the nameplate. What is acceptable and what must change?
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Conductors. Article 440 requires branch-circuit conductors with an ampacity of at least the nameplate minimum circuit ampacity - 32.4 A here. The existing 10 AWG NM-B is limited to the 60°C column, 30 A, and NM is capped at 60°C by its own article regardless of terminal rating. 30 A is below 32.4 A, so the NM-B cannot be reused. Replace the interior run with 8 AWG NM-B (40 A at 60°C) or 8 AWG THHN in conduit. The proposed splice is also flawed for a second reason: NM-B is not permitted in wet or outdoor locations, so the outdoor portion must be a wet-location conductor such as THWN in liquidtight, with the splice made in a listed, accessible enclosure - not a floating set of wire nuts above the roof.
Overcurrent. The 30 A breaker is undersized for starting and will nuisance-trip; it also does not match the equipment listing. The branch-circuit short-circuit and ground-fault device may be sized up to the nameplate maximum of 50 A, and motor-compressor circuits are specifically permitted to use an overcurrent device above the conductor's ampacity. Install a 50 A breaker.
The 60 A non-fused disconnect is actually acceptable - a disconnect rated higher than the minimum is fine - provided it is rated for the voltage and is installed where it belongs. The wrong part here is not the disconnect rating; it is the location.
2. The installer wants to leave the disconnect where it is. What does the code require, and what will you approve?
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For a hermetic refrigerant motor-compressor, the disconnecting means must be within sight of and readily accessible from the equipment it controls. A switch 55 ft away around a corner is neither. The intent is that a technician servicing the unit can see the open disconnect and be certain the unit cannot be energized by someone else - the same reason lockout is impossible to do properly from a distance.
What I will approve: a new disconnect mounted on the unit or on the curb within sight of the service panel, sized at not less than 115% of the rated-load current of the largest motor plus the sum of the other loads. Here that is well under 30 A, so a 30 A or 60 A non-fused disconnect is fine electrically; the used 60 A unit can be used if it is in good condition, rated for the voltage, and listed for outdoor/wet locations. Verify the enclosure is actually rated for the location - a rusted indoor-rated box on a roof is a defect, not a bargain.
Do not forget:
- A servicing receptacle within 25 ft of the equipment, on the roof, as required for maintenance.
- Working clearance at the disconnect per the working-space rules - 30 in. wide, 36 in. deep, 6½ ft high, unobstructed.
- An equipment grounding conductor run with the circuit and bonded to the unit frame and any metallic conduit; no reliance on the flex connector for grounding.
- A permanent label on the disconnect identifying the unit it serves.
Put the relocation in the estimate as part of the replacement scope. Reusing the old location is a code violation the inspector will catch and the customer will blame you for.
3. Run the voltage-drop check and decide the final conductor size. Show the reasoning the customer can follow.
Read the worked answer
Use the single-phase formula Vd = 2 × K × I × L ÷ CM, with K ≈ 12.9 for copper, I = 32.4 A and L = 110 ft one way.
- 10 AWG (CM = 10,380): Vd = 2 × 12.9 × 32.4 × 110 ÷ 10,380 ≈ 8.9 V, about 3.9% at 230 V.
- 8 AWG (CM = 16,510): Vd = 2 × 12.9 × 32.4 × 110 ÷ 16,510 ≈ 5.6 V, about 2.4% at 230 V.
The code does not set a voltage-drop limit for branch circuits; it is an informational recommendation in the 3% branch-circuit / 5% total range. But this is a compressor with high inrush, and running 3.9% at full load means considerably more sag on start, contactor chatter, and a compressor that runs hotter and draws more current for its whole life.
Decision: 8 AWG copper for the full 110 ft, in a wiring method legal for each portion of the route - NM-B inside the conditioned space, THWN in conduit outdoors, spliced in a listed junction box. 8 AWG satisfies the 32.4 A minimum ampacity, holds drop near 2.4%, and allows a 50 A breaker without any conductor/overcurrent conflict under the motor-compressor rules.
What to tell the customer: the existing wire is too small for the new unit's nameplate and too long a run for good performance; reusing it would mean a nuisance-tripping breaker and a shortened compressor life. Quote the wire upgrade as part of the job, and put the ampacity and voltage-drop numbers on the proposal so the price does not look arbitrary.