Blog Post

Rooftop units are easy to mis-size when the design conversation stops at the breaker. The nameplate may show 43 A MCA and 60 A MOCP, the panel schedule may show a 60 A feeder, and the roof plan may hide a 180 ft raceway path through the building. That feeder can be code-compliant for overcurrent protection and still deliver weak voltage during compressor start, condenser fan cycling, or hot-weather operation.

TL;DR

Use RTU nameplate MCA for conductor ampacity and a conservative voltage-drop current.
MOCP protects HVAC equipment starting needs; it is not the running load.
Check 3-phase voltage drop before conduit, disconnects, and conductors are bought.
For long 208 V runs, voltage drop often controls before ampacity does.

An RTU feeder is the branch circuit or feeder supplying a packaged rooftop HVAC unit from a panelboard, switchboard, or distribution equipment. MCA, or minimum circuit ampacity, is the nameplate current value used to size conductors for HVAC equipment under NEC Article 440. MOCP, or maximum overcurrent protection, is the largest permitted fuse or breaker for short-circuit and ground-fault protection. For international projects, IEC 60364-5-52 frames voltage drop as part of cable selection along with installation method, grouping, temperature, conductor material, and load type.

For rooftop units, the most common field mistake is calculating voltage drop from the breaker handle instead of reading MCA and MOCP separately. A 60 amp MOCP does not mean the unit normally draws 60 amps.
— Hommer Zhao, Technical Director

Read the RTU Nameplate Before Opening the Calculator

NEC 440 changes the normal branch-circuit mindset. A general-purpose 60 A circuit usually leads you straight to conductor ampacity for 60 A, but an air-conditioning unit has compressor motors, fan motors, internal overload protection, and manufacturer-tested protection values. The unit nameplate is the starting point. If it says MCA 43 A and MOCP 60 A, conductors are sized from at least 43 A ampacity after adjustment and correction, while the overcurrent device can be 60 A when permitted by the nameplate and NEC 440.22.

Voltage drop is a performance check layered on top of that code selection. NEC 215.2(A)(1) Informational Note No. 2 points designers toward 3% maximum voltage drop on feeders and 5% total feeder plus branch circuit. NEC 210.19(A)(1) Informational Note No. 4 gives the same 3% branch and 5% total design guidance for branch circuits. These are informational notes rather than hard pass/fail rules, but they are widely used by engineers, inspectors, facility owners, and plan reviewers as a professional design target.

Use the voltage drop calculator to model the run, the wire size calculator to cross-check conductor ampacity, and the three-phase voltage drop guide when power factor or conductor impedance matters.

Worked Example 1: 480 V, 3-Phase RTU at 230 Feet

A commercial building has a 25 ton, 480 V, 3-phase rooftop unit. The nameplate reads MCA 52 A and MOCP 70 A. The panel is on the second floor electrical room, the raceway climbs to the roof, crosses a pipe rack, and reaches the fused disconnect after 230 ft one-way. The engineer wants a feeder voltage-drop target under 2% because the site already measures 472 V at the distribution panel during summer peak.

1 AWG copper resistance check

Voltage drop = 1.732 x I x R x D / 1000

Voltage drop = 1.732 x 52 A x 0.126 ohm/kft x 230 ft / 1000

Voltage drop = 2.61 V, or 0.54% of 480 V

Ampacity must still be checked using the conductor type, terminal temperature rating, ambient temperature, rooftop correction rules where applicable, and number of current-carrying conductors in the raceway. But for voltage drop, 1 AWG copper is comfortably inside the 2% target in this example. If the same unit were fed at 208 V, the same absolute voltage loss would be a much larger percentage, so low-voltage RTU feeders need closer attention.

On 480 volt rooftop equipment, voltage drop is often modest. On 208 volt equipment at the same ampacity and distance, the percent drop is more than double, so a 180 foot run can become a real starting-voltage problem.
— Hommer Zhao, Technical Director

Worked Example 2: 208 V RTU Where Voltage Drop Controls

Now take a smaller urban building with a 208 V, 3-phase packaged unit. The nameplate shows MCA 48 A and MOCP 60 A. The one-way run is 180 ft from an electrical closet to a roof disconnect. A quick ampacity check may make 6 AWG copper look attractive, but voltage drop tells a different story.

Conductor	Approx. ohm/kft	Drop at 48 A, 180 ft	Percent at 208 V	Design call
8 AWG Cu	0.628	9.40 V	4.5%	Usually too high for an RTU feeder
6 AWG Cu	0.395	5.91 V	2.8%	Close; verify starting voltage
4 AWG Cu	0.249	3.73 V	1.8%	Strong choice for long 208 V runs
3 AWG Cu	0.197	2.95 V	1.4%	Useful when source voltage is low
2 AWG Al	0.321	4.80 V	2.3%	May work with listed AL terminals

The table shows why a one-line panel schedule is not enough. If the service voltage sags to 203 V on hot afternoons, a 6 AWG feeder losing another 5.9 V leaves roughly 197 V at the disconnect before compressor starting is considered. A 4 AWG copper feeder gives more margin, fewer nuisance service calls, and a cleaner permit explanation.

Field Scenario: Same Unit, Different Roof Path

On a retrofit review for a 12.5 ton 208 V RTU, we measured 206.4 V line-to-line at the panel with the building cooling load already running. The original drawing assumed a 115 ft feeder, but the actual route through the ceiling space and roof curb was 168 ft one-way. Using the nameplate MCA of 41.6 A, the installed 8 AWG copper feeder calculated near 3.7% voltage drop. With the measured source voltage, the unit could see about 198.8 V at steady load and less during starting. The practical fix was not a larger breaker; it was upsizing the feeder to 6 AWG copper and replacing the disconnect lugs to match the conductor range.

This is the kind of first-pass check that belongs in a submittal or service report: measured source voltage, actual route length, RTU nameplate MCA/MOCP, selected conductor, calculated voltage drop, and final delivered voltage.

If an RTU has low-voltage trips, do not start by changing the breaker. Measure line voltage at the panel and at the disconnect under load, then compare the result with the MCA-based voltage-drop calculation.
— Hommer Zhao, Technical Director

Checklist for RTU Feeder Sizing

Record voltage, phase, MCA, MOCP, compressor RLA/LRA when provided, and equipment MCA notes.

Measure or estimate the real one-way raceway length, including vertical roof transitions.

Size conductor ampacity from MCA after terminal, ambient, rooftop, and bundling corrections.

Select the breaker or fuse from MOCP and NEC 440, not from voltage-drop upsizing.

Calculate 3-phase voltage drop at MCA or expected running load.

Document source voltage if the building is already near the equipment tolerance limit.

FAQ: RTU Voltage Drop and Wire Sizing

Should RTU voltage drop be calculated from MCA or MOCP?

For a rooftop unit, start the voltage-drop check with the expected running load and use the nameplate MCA as a conservative design current. MOCP from NEC 440.22 is for short-circuit and ground-fault protection, not the normal load current.

What voltage drop target should I use for a commercial RTU feeder?

A common target is 3% for the feeder or branch circuit and 5% total feeder plus branch circuit, matching NEC 215.2(A)(1) Informational Note No. 2 and NEC 210.19(A)(1) Informational Note No. 4. Many HVAC designers hold 480 V RTU feeders near 2% when motor starting is sensitive.

How do I calculate voltage drop on a 480 V 3-phase RTU?

Use the 3-phase formula with 1.732 x current x distance x conductor impedance. For a simplified copper resistance check, 1 AWG copper at about 0.126 ohms per 1,000 ft over 230 ft one-way and 52 A is about 2.61 V, or 0.54% on 480 V.

Can a 60 A RTU breaker use conductors smaller than 60 A ampacity?

Sometimes, yes, if the unit nameplate MCA supports it and NEC 440 rules are followed. For example, an RTU with MCA 43 A and MOCP 60 A can often use conductors sized at or above 43 A ampacity while the 60 A MOCP protects compressor starting needs.

When should I upsize RTU feeder conductors for voltage drop?

Upsize when the run length, current, and starting behavior push delivered voltage outside the design target. A 208 V RTU with 48 A MCA and 180 ft one-way can exceed 3% on 6 AWG copper, while 4 AWG may bring the run closer to 2.5%.

Do IEC projects use the same RTU voltage drop limits as NEC projects?

Not exactly. IEC 60364-5-52 treats voltage drop as a design requirement tied to circuit purpose, installation method, and local project rules; 3% for lighting and 5% for other final circuits are common reference values, but local standards can be stricter.

Check the Feeder Before the Roof Work Starts

Before conduit is installed or a rooftop disconnect is ordered, run the MCA, distance, conductor, and voltage-drop numbers together. A few minutes in the calculator can prevent a costly roof pull, a failed startup, or an avoidable service call.

Ask for a voltage-drop review

Rooftop Unit Feeder Voltage Drop: MCA, MOCP, NEC 440, and 3-Phase Wire Sizing