Okay, running conduit on a flat roof to supply power to an AC disconnect is a fairly common task. Here's a breakdown of advice covering materials, methods, and code considerations

1. Planning & Materials:
Conduit Type



Rigid Metal Conduit (RMC)

Very durable and weather-resistant. Best choice for physical protection and long life, especially in harsh environments. It's heavier, more expensive, and requires threading.

Rigid PVC Conduit (Schedule 40 or 80)

Lighter than RMC, resistant to corrosion. Less expensive but can become brittle in extreme cold. Must be protected from physical damage and sunlight (painting is often required). Schedule 80 is thicker and provides better protection.

Electrical Metallic Tubing (EMT)

Thin-walled steel conduit. Easier to bend and cut than RMC, but less robust. Not generally recommended for direct exposure on a flat roof, especially in areas with foot traffic or potential for damage.

Liquidtight Flexible Metal Conduit (LFMC) or Liquidtight Flexible Nonmetallic Conduit (LFNC)

Can be used for short runs to equipment for vibration isolation or awkward connections. Not suitable for long runs across the roof because of sag.

Recommendation

For most flat roof applications,

RMC is the best choice for long life and protection. PVC can be used if budget is a major concern and physical protection is carefully considered. Conduit Supports



Strut (Unistrut)

A common framing system used for supporting conduit, boxes, and other electrical equipment. You can mount strut to the roof and then use strut clamps to secure the conduit. Allows for easy adjustment.

Conduit Clamps/Straps

Choose clamps designed for the conduit type you're using (RMC clamps, PVC straps, etc.).

Roof Penetration/Support Blocks

Avoid direct contact between the conduit and the roof surface. Use approved supports designed to distribute weight, prevent abrasion of the roofing material, and minimize roof penetrations. These are often made of a durable plastic or rubber. Some even have integrated flashing to create a watertight seal.

Concrete Pier Blocks

Can be used as a base for your conduit support system. Elevates the conduit and makes for easy leveling on uneven surfaces.

Recommendation



Strut with appropriate roof supports is generally the best approach for a professional and durable installation. Disconnect Enclosure



NEMA 3R or NEMA 4 Enclosure

Essential for outdoor use. Provides protection against rain, snow, and ice. NEMA 4 offers even greater protection against hose-directed water.

Size

Select an enclosure large enough to accommodate the disconnect switch/breaker and the wire terminations.

Material

Stainless steel, painted steel, or non-metallic enclosures are all options. Consider the environment and desired lifespan.

Wiring



THWN or XHHW conductors

These are commonly used for wet locations and are suitable for conduit installations.

Wire Size

Properly sized according to the AC unit's nameplate information and the applicable electrical codes.

Grounding

Proper grounding is absolutely critical for safety. Ensure a continuous grounding path back to the service panel.

Roofing Sealant



Compatible with Roofing Material

Use a sealant specifically designed for the type of roofing material you have (e.g., EPDM, TPO, asphalt). Incompatible sealants can damage the roof.

UV Resistance

Choose a sealant that is UV-resistant to prevent degradation from sunlight.

Fasteners



Stainless Steel or Hot-Dipped Galvanized

Use corrosion-resistant fasteners for all outdoor connections.

Type

Appropriate screws, bolts, or anchors for attaching the supports to the roof structure.

2. Installation Steps


Safety First

Turn off the power to the existing circuit. Verify that the circuit is de-energized with a voltage tester. Wear appropriate PPE (safety glasses, gloves, etc.).2.

Plan the Route

Carefully plan the conduit route from the power source to the AC unit. Consider the shortest practical path while minimizing bends. Avoid areas where the conduit could be easily damaged.3.

Mounting the Disconnect



Locate a suitable spot

Choose a location near the AC unit that is accessible for maintenance and meets code requirements for working clearances.

Proper Support

Use strut or other suitable supports to mount the disconnect enclosure securely to the roof. Make sure the support is level.

Seal Penetrations

If you need to penetrate the roof to mount the support structure, use flashing and roofing sealant to create a watertight seal around the penetrations.4.

Install Conduit Supports



Spacing

Follow code requirements for conduit support spacing (typically every 10 feet for RMC, more frequent for other types).

Attachment

Secure the supports to the roof using appropriate fasteners and sealant. Avoid damaging the roofing material. If possible, attach to structural members below the roofing.

Elevation

Elevate the conduit above the roof surface to prevent water from pooling around it.5.

Run Conduit



Cutting and Bending

Cut and bend the conduit as needed to follow the planned route. Use proper conduit bending tools.

Connections

Use appropriate fittings (couplings, elbows, etc.) to connect the conduit sections. Tighten all connections securely.

Expansion Fittings

If the conduit run is long, consider using expansion fittings to accommodate thermal expansion and contraction.6.

Pull Wire



Lubricant

Use a wire pulling lubricant to make it easier to pull the wires through the conduit.

Color Coding

Follow proper color coding for the conductors (black for hot, white for neutral, green or bare for ground).

Strain Relief

Leave enough slack in the wires at the disconnect and at the source to allow for connections and maintenance.7.

Connect Wiring



Disconnect

Connect the wires to the disconnect switch/breaker according to the manufacturer's instructions.

Source

Connect the wires at the power source. Ensure all connections are tight and secure.

Grounding

Connect the grounding conductor to the grounding bar in the disconnect enclosure and to the grounding electrode system.8.

Test and Inspect



Continuity Test

Verify the continuity of the grounding conductor.

Insulation Resistance Test

Check the insulation resistance of the conductors to ensure there are no shorts to ground.

Voltage Test

Verify the correct voltage at the disconnect.9.

Final Touches



Labeling

Label the disconnect with its purpose (e.g., "AC Disconnect").

Cleanup

Remove any debris from the roof.

3. Code Considerations (NEC)



Article 300 (Wiring Methods)

General requirements for wiring methods.

Article 310 (Conductors for General Wiring)

Conductor types and ampacity.

Article 314 (Outlet Boxes, Device Boxes, Pull Boxes, and Conduit Bodies)

Requirements for boxes and conduit bodies.

Article 344 (Rigid Metal Conduit)

Specific requirements for RMC.

Article 352 (Rigid PVC Conduit)

Specific requirements for PVC conduit.

Article 430 (Motors, Motor Circuits, and Controllers)

Requirements for AC motor circuits, including disconnect sizing.

Article 440 (Air-Conditioning and Refrigeration Equipment)

Additional requirements specific to AC equipment.

Article 250 (Grounding and Bonding)

Essential requirements for grounding.

Working Space (110.26)

Ensure adequate working space around the disconnect for safe maintenance.

Wet Locations (Article 100)

The roof is considered a wet location. Use appropriate wiring methods and equipment listed for wet locations.

Support (314.23, 344.30, 352.30)

Follow code requirements for support spacing.

4. Disconnect Mounting Options



Direct Mounting to Strut

The most common and generally preferred. Attach strut to the roof with appropriate supports and sealants, then mount the disconnect enclosure directly to the strut.

Elevated Platform

Construct a small elevated platform using treated lumber or composite materials. Mount the disconnect to the platform. This provides extra clearance and helps prevent water from pooling around the disconnect.

5. Important Considerations



Roof Warranty

Modifying the roof can void the warranty. Consult with the roofing manufacturer or a qualified roofing contractor before starting work.

Professional Help

If you are not comfortable working with electricity or on a roof, hire a qualified electrician. Improper electrical work can be dangerous and can cause fires.

Local Codes and Permits

Check with your local building department for any required permits or inspections.

Summary and Recommendation

Running conduit on a flat roof is a practical solution for your situation, but it requires careful planning and execution. Use

RMC for the conduit run whenever practical for maximum durability. Prioritize safety throughout the process. If you're not completely confident in your ability to handle the electrical and roofing aspects, it's best to hire a qualified professional. Ensure all work is done according to the National Electrical Code (NEC) and local codes. A properly installed system will provide years of reliable service. Good luck! Flag for review