Rooftop HVAC Unit Wire Sizing Service Guide
// PLAN RTU FEEDERS, BRANCH CIRCUITS, DISCONNECTS, ROOF CONDUIT, AND VOLTAGE DROP FROM THE NAMEPLATE MCA AND MOCP BEFORE THE UNIT IS LIFTED ONTO THE CURB. //
Rooftop HVAC wiring is a nameplate, motor-compressor, ambient-temperature, and distance problem. Start with MCA, MOCP, voltage, phase, full-load current, locked-rotor data, conductor insulation, terminal rating, rooftop raceway temperature, current-carrying conductor count, and one-way route length before selecting copper, aluminum, AWG, or mm2 cable.
Nameplate first
For listed RTUs, MCA sizes the conductors and MOCP sizes the breaker or fuse before field adjustments.
Roof heat matters
A 45 C roof ambient and grouped raceway can reduce usable ampacity under NEC 310 correction and adjustment factors.
Distance matters
A 208V unit at 140 ft one-way may need a larger conductor for starting voltage even when ampacity already passes.
TL;DR
- Use nameplate MCA for conductors and MOCP for overcurrent protection
- Check NEC 440, NEC 310, NEC 430, disconnects, grounding, and terminal temperature
- Apply rooftop ambient correction before trusting a table ampacity
- Target about 3% branch-circuit voltage drop and 5% total feeder plus branch where practical
- For IEC jobs, document IEC 60364-5-52 installation method, grouping, ambient, and voltage-drop basis
Key definitions
Minimum circuit ampacity
Minimum circuit ampacity is the nameplate current used to size HVAC branch-circuit conductors for listed equipment.
Maximum overcurrent protection
Maximum overcurrent protection is the largest breaker or fuse rating allowed by the equipment nameplate.
Voltage drop
Voltage drop is the voltage lost in conductors because current flows through resistance over a known route length.
Sizing workflow
Step 1 - read the nameplate
Record MCA, MOCP, voltage, phase, compressor RLA or rated load amps, fan motor FLA, minimum conductor temperature, and manufacturer notes.
Step 2 - choose the circuit basis
Use MCA for conductor ampacity on listed HVAC equipment. Use MOCP for the breaker or fuse, then verify NEC 440 and any NEC 430 motor rules referenced by the unit.
Step 3 - correct for the roof
Apply NEC 310 ambient correction, conductor-count adjustment, terminal temperature limits, wet-location insulation, sunlight exposure, and conduit fill.
Step 4 - check voltage drop
Enter one-way length to the rooftop disconnect or unit. Long 208V runs often need upsizing sooner than 480V runs with the same MCA.
Step 5 - document the install
Keep the calculator result with nameplate photos, disconnect rating, equipment grounding conductor size, conductor insulation, conduit schedule, and AHJ notes.
Worked examples
208V three-phase RTU
Nameplate MCA 34A, MOCP 50A, copper THWN-2 in roof conduit, 140 ft one-way.
8 AWG copper may satisfy MCA after basic ampacity checks, but 6 AWG is often reviewed to hold voltage drop near 3% and improve compressor starting.
480V RTU feeder
Nameplate MCA 62A, MOCP 80A, 220 ft one-way feeder, rooftop ambient 45 C, three current-carrying conductors.
4 AWG copper may be a starting point before correction factors, while 3 AWG or 2 AWG should be checked when derating and voltage drop are both tight.
400V IEC packaged unit
Rated input 28A, 55 m copper cable on tray with other mechanical loads, installation method reviewed under IEC 60364.
10 mm2 may pass many ampacity checks, while 16 mm2 can reduce drop and heat rise when grouping and Clause 525 voltage drop are included.
HVAC circuit comparison
| Circuit | Sizing basis | Likely conductor check | Voltage-drop risk | Code note |
|---|---|---|---|---|
| Small 240V single-phase condenser | Nameplate MCA and MOCP | 10-8 AWG copper common range | Medium on 75-125 ft runs | NEC 440 and 310 |
| 208V three-phase RTU | MCA for conductors, MOCP for OCPD | 8-4 AWG copper depending MCA and route | High on long low-voltage feeders | NEC 440, 430, 310 |
| 480V rooftop unit | MCA plus correction factors | 6-2 AWG copper depending load | Medium because current is lower | NEC 440 and disconnect rules |
| Roof conduit bundle | Corrected ampacity after heat and grouping | Upsize after NEC 310 factors | Medium because hot copper has higher resistance | NEC 310.15 and Chapter 9 |
| IEC mechanical feeder | Design current and installation method | 6-25 mm2 depending method and length | Medium on 30-80 m routes | IEC 60364-5-52 and IEC 60228 |
Code checkpoints
Use these references as design checkpoints, then confirm the adopted code edition, equipment listing, manufacturer instructions, and local AHJ interpretation.
NEC 440
Covers air-conditioning and refrigerating equipment, including branch-circuit conductors, overcurrent protection, and disconnecting means.
NEC 310
Check conductor ampacity, ambient-temperature correction, current-carrying conductor adjustment, terminal limits, and insulation ratings.
NEC 430
Motor rules may apply where NEC 440 refers to motor-compressor and motor circuit requirements.
NEC 250
Size equipment grounding conductors and verify bonding at disconnects, units, curbs, and metal raceways.
IEC 60364-5-52
Reviews wiring-system selection, current-carrying capacity, grouping, ambient correction, and Clause 525 voltage drop.
IEC 60228
Defines metric conductor classes and nominal cross-sectional areas used on IEC cable schedules.
Field checklist
- Photograph the RTU nameplate before sizing conductors.
- Use MCA for conductor ampacity and MOCP for breaker or fuse selection.
- Apply rooftop ambient correction for raceways exposed to sun and heat.
- Confirm 60 C or 75 C terminal limitations before using higher insulation ampacity.
- Enter one-way route length from panel to disconnect or unit in the voltage-drop tool.
- Verify disconnect rating, working clearance, roof support, wet-location conductors, and equipment grounding conductor size.
- Keep calculations with the permit set, panel schedule, nameplate photo, and startup paperwork.
Rooftop HVAC wire sizing FAQ
Can the breaker be larger than the wire ampacity on HVAC equipment?
Often yes when the nameplate allows it. NEC 440 permits listed HVAC equipment to use MCA for conductors and MOCP for the breaker or fuse, so a 50A breaker with 8 AWG conductors can be valid when the nameplate supports it and derating is satisfied.
Do I size RTU wire from tonnage?
No. A 5 ton and another 5 ton unit can have different MCA values. Use the nameplate MCA, voltage, phase, and manufacturer instructions before using the calculator.
How much voltage drop is acceptable for an RTU?
Many designers target about 3% on the branch circuit and 5% combined feeder plus branch circuit. Compressor starting can justify upsizing on 208V runs even when the code minimum passes.
Does roof conduit temperature require upsizing?
It can. A rooftop raceway at 45 C or higher may need NEC 310 ambient correction, and bundled conductors may need adjustment factors before the conductor is accepted.
Can I use aluminum conductors for rooftop HVAC feeders?
Yes when equipment terminals are listed for aluminum, the conductor is sized correctly, terminations are torqued, and voltage drop is checked. Many small units have copper-only terminals, so verify the label.
How is IEC HVAC cable sizing different?
IEC projects use mm2 conductor areas, installation methods, grouping factors, protective-device coordination, and IEC 60364 voltage-drop limits instead of AWG tables alone.
Check the RTU run before cable is ordered
Use the ampacity, voltage-drop, and motor wire-size tools to compare the code-minimum conductor with the practical size that protects compressor starting and inspection clarity.