How to Wire a Rooftop Unit (RTU)

1. Read the nameplate: MCA and MOP are the whole game

Every packaged RTU carries a nameplate required by NEC 440.4(B) listing voltage, phase, Minimum Circuit Ampacity (MCA), and Maximum Overcurrent Protection (MOP). MCA is the minimum conductor ampacity — the manufacturer already applied the 125% factor to the largest motor-compressor and added the other loads, so you do not multiply it again. MOP is the largest breaker or fuse allowed — it protects the unit's internal components and you may not exceed it, period.

A typical 15-ton 480V unit might read MCA 38 A, MOP 50 A. That means conductors with at least 38 A of ampacity after all corrections, protected by an overcurrent device no larger than 50 A. If the calculated conductor ampacity would let you use a 45 A breaker, you can still go up to the 50 A MOP — the nameplate MOP already accounts for compressor inrush, and undersizing the OCPD is a common cause of nuisance trips on hot-day starts.

Photograph the nameplate and the wiring diagram inside the control panel before you start. If the unit has a factory-powered convenience outlet or a separate control power tap, the nameplate will say so — that changes what you have to disconnect and lock out later.

2. Size the conductors — then apply the Arizona rooftop adder

Start from the MCA and pick a conductor from the 75°C column of Table 310.16, since virtually all modern terminations are rated 75°C per 110.14(C). For MCA 38 A, 8 AWG copper THWN-2 at 50 A looks comfortable. But on a roof you're not done: ambient correction will eat into that number fast.

Design for the real ambient. A Tucson roof deck in July sits well above the 30°C table basis — use the outdoor design temperature plus the sunlight adder where it applies. Under 2023 NEC 310.15(B)(2), raceways or cables in direct sunlight less than 3/4 in. above the roof surface take a 33°C (60°F) adder on top of outdoor ambient. That threshold changed from 7/8 in. in earlier editions, and there's a significant carve-out: XHHW-2 conductors are exempt from the adder entirely, which is why XHHW-2 has become the default rooftop conductor for a lot of Arizona contractors.

Run the numbers both ways: THWN-2 on struts 4 inches off the deck avoids the adder but still needs correction for a 43°C+ ambient; conduit lying on sleepers under 3/4 in. clearance with THWN-2 could see an effective ambient over 75°C, which zeroes out its ampacity. Keep raceways up on proper rooftop supports and this mostly solves itself.

3. Mount the disconnect within sight — and off the service panels

NEC 440.14 requires the disconnecting means within sight of and readily accessible from the unit. Within sight means visible and not more than 50 ft away per the Article 100 definition. The section also prohibits mounting the disconnect on panels designed to allow access to the RTU's serviceable parts — don't screw it to a removable compressor-access panel, because the tech has to take that panel off with the switch still attached and live conductors flexing behind it.

Mount a NEMA 3R switch on the unit's fixed corner post, on the curb, or on adjacent strut. Sizing follows 440.12(A)(1): at least 115% of the unit's rated-load current — in practice, pick the standard switch size at or above the MCA and confirm the switch is HP-rated or listed for the load. Non-fusible is fine when the MOP device is the breaker back at the panel; if the plans call for fuses at the unit, fuse at the MOP value.

Per 110.25 the disconnect must be lockable in the open position with the provision staying in place — standard on modern switches, but check, because the service tech's lockout depends on it.

4. Rough the feeder and build the whip

Run the feeder in EMT or rigid up through the roof, on listed rooftop supports — not wood blocks — with expansion fittings where straight EMT runs will see the roof's full temperature swing. Flash and seal the penetration properly or bring the circuit up inside the curb where the unit design allows it; through-the-curb entry keeps the roof membrane intact and is worth coordinating with the mechanical contractor before the unit sets.

From the disconnect to the unit, use liquidtight flexible metal conduit — short, supported, and with listed fittings. LFMC lengths up to 6 ft are standard practice for the vibration-isolating whip. Include the equipment grounding conductor sized from Table 250.122 based on the OCPD — don't rely on the liquidtight jacket or fittings alone for grounding.

Land the conductors on the unit's power block, torque to the value on the label per 110.14(D), and re-check the factory lugs while you're in there — shipping vibration loosens terminations, and a loose factory lug reads exactly like your problem on the thermal scan later.

5. Install the 210.63 service receptacle — with GFCI

NEC 210.63 requires a 125V, 15 or 20 A receptacle within 25 ft of the equipment, on the same level, for service personnel. On a roof that means a weatherproof receptacle on strut or on the curb near the unit — not three floors down by the panel. It must be on its own path from a general-purpose circuit; it cannot be tapped from the load side of the RTU disconnect, because the tech needs power for tools and lights while the unit is locked out.

GFCI protection is required for this receptacle — outdoor receptacles in non-dwelling occupancies fall squarely under 210.8(B), and the 2023 code ties equipment-servicing receptacles to GFCI protection explicitly. Use a WR-rated GFCI device in an in-use cover, or protect the circuit with a GFCI breaker and standard WR receptacle.

If several units share a roof, one receptacle can serve any unit within its 25 ft radius — lay them out on the plan before the conduit goes up.

6. Wire the control side and the low-voltage interlocks

Most packaged units generate their own 24V control power from an internal transformer, so the field control wiring is thermostat or BAS conductors landing on the unit's terminal strip. Keep line voltage and Class 2 control wiring in separate raceways — 725.136 prohibits Class 2 conductors sharing a raceway with power conductors except under narrow conditions.

Run the control conductors in their own liquidtight whip or a separate small raceway into the unit's low-voltage knockout, never through the power entry. If the job has a smoke detector shutdown, economizer, or CO2 sensor interlock, those land here too — check the mechanical drawings, because a missed duct-detector interlock will fail the fire inspection, not the electrical one.

7. Pre-energization checks and startup

Before energizing: megger the feeder phase-to-phase and phase-to-ground, confirm every termination is torqued and marked, verify the EGC lands on the unit's ground lug, and check that the disconnect's load side is wired to the unit — not backward. Confirm nameplate voltage matches the system; a 208V unit on a 240V high-leg system or a 460V unit on a 480V system is fine, but a 208V unit wired to 480V dies at first contact.

Energize, then check rotation before letting the unit run loaded. Scroll compressors are rotation-sensitive: run backward they move no refrigerant, scream, and cook themselves. Three-phase units usually have a phase monitor that locks out on reversed rotation — if yours trips, swap two phases at the disconnect load side, not inside the unit.

Measure running amps per phase against the nameplate RLA, verify 24V control voltage under load, and log everything: voltages, amps, rotation, torque values. The startup sheet you hand the mechanical contractor is what keeps the warranty callback from becoming your callback.

What's the difference between MCA and RLA?

RLA (rated-load amps) is the compressor motor's individual rating. MCA (minimum circuit ampacity) is the manufacturer's calculated minimum for the whole unit — 125% of the largest motor plus 100% of everything else. You size conductors from MCA, never by adding up RLAs yourself.

Can the breaker be smaller than the MOP?

Yes — MOP is a maximum, not a target. But don't go so small that hot-start inrush nuisance-trips it. Most contractors set the OCPD at the MOP value unless the plans say otherwise, since the nameplate already coordinates it with the unit's internal protection.

Does the whip have to be liquidtight?

It has to be a wet-location wiring method suitable for the vibration and movement — LFMC with listed fittings is the standard answer. Regular FMC isn't listed for wet locations, and NM or SE cable has no business on a commercial roof.

Is XHHW-2 really exempt from the rooftop adder?

Yes — 2023 NEC 310.15(B)(2) exempts XHHW-2 from the 33°C sunlight adder based on industry testing of its thermal performance. You still apply normal ambient correction for the outdoor design temperature, so the exemption doesn't make ampacity math go away — it just removes the brutal 60°F penalty.

Can one receptacle serve multiple rooftop units?

Yes, as long as it's within 25 ft of each unit it serves, on the same level. On a roof with several RTUs, place receptacles so their 25 ft radii cover every unit — usually far cheaper than one receptacle per unit.

The unit came with a factory disconnect. Do I still need mine?

If the factory disconnect is listed as the disconnecting means, is within sight, is readily accessible, and is lockable per 110.25, it can satisfy 440.14. Verify it's an actual disconnect and not just a control-circuit switch — the nameplate and installation manual will say.