Step-by-step: installing service mast

When a mast is the right call

Service mast installs show up when the drip loop height can't be met any other way. Minimum clearances live in NEC 230.24: 10 ft over walkways at 150V to ground or less, 12 ft over residential driveways, 18 ft over public streets. If the eave is too low to hit those numbers, you're going through the roof.

Confirm with the POCO first. Some utilities require their own mast spec, riser height above the roof, weatherhead model, and meter position. Get the service drop height commitment in writing before you cut a hole in someone's roof.

Size the service per NEC 230.42 and the load calc in Article 220. For a typical 200A residential, that's 2/0 copper or 4/0 aluminum SER/SE or service conductors in the mast.

Materials and sizing

Mast pipe is almost always rigid metal conduit (RMC) for strength, not EMT. NEC 230.28 requires the mast to have adequate strength or be supported to withstand the strain of the service drop. Most AHJs read that as 2 inch RMC minimum for 200A, 2-1/2 inch if the span is long or exposed. Check your local amendment.

Standard parts list for a straight-shot roof mast:

• 2 in or 2-1/2 in RMC, threaded both ends
• Weatherhead (service entrance cap) sized to conduit
• Roof flashing (lead or rubber boot rated for the pitch)
• Meter base with hub, or reducing washers plus locknut and bushing
• Guy wire kit or roof brace if span over 36 in above roof line
• Ground bushing at the meter end
• Service conductors sized per 310.12 for dwelling, or 310.16 otherwise

Bring extra weatherhead gaskets and a roll of Teflon tape for the threads. You will need them.

Step-by-step install

1. Kill the service. Coordinate with POCO for a cut and reconnect, or pull the meter if allowed in your jurisdiction. Lock and tag.
2. Mark the mast location plumb below the intended drop point, directly above the meter base.
3. Mount the meter base per 230.70 and local height rules, typically 4 to 6 ft to center.
4. Dry-fit the RMC from meter hub up through the soffit and roof. Cut the roof opening oversize by 1/4 in for the flashing collar.
5. Slide flashing over the pipe before threading the weatherhead. Ask any electrician who forgot this once.
6. Thread the weatherhead on, orient the knockouts so drip loops point down and away from the building.
7. Seat the mast into the meter hub, tighten, and confirm plumb in two planes.
8. Pull service conductors. Leave 36 in of tail at the weatherhead for the POCO to make up the drop.
9. Flash and seal the roof penetration with compatible roofing sealant. No silicone on asphalt shingles.
10. Install guy wires or roof brace if the mast rises more than 36 in above the roof, per NEC 230.28 strain requirement.

If you're working alone, thread a piece of mule tape through the mast before you stand it up. Trying to fish 2/0 through a 20 ft vertical pipe from a ladder is how people end up calling for help at 4pm on a Friday.

Bonding, grounding, and the meter

The mast itself is not a grounding electrode, but it must be bonded. At the meter base, the neutral is bonded to the enclosure per 250.24(B). A ground bushing at the top of the hub with a bonding jumper back to the neutral bar handles the conduit bond if you're using concentric knockouts, per 250.92(B).

Grounding electrode conductor (GEC) sizing comes from NEC 250.66. For 2/0 copper service, that's a 4 AWG copper GEC to the ground rods and a 4 AWG to the water pipe within 5 ft of entry per 250.52(A)(1). Two rods 6 ft apart unless you can prove 25 ohms or less on one.

Label the disconnect per 230.70(B) and mark the service with maximum available fault current and date per 110.24 if it's commercial or required by your AHJ.

Clearances you can't skip

After the drop is made up, walk the clearances with a tape. NEC 230.9 governs clearance from windows, doors, porches, and similar: 3 ft horizontal from any window that opens, above or to the side. Conductors above a window that doesn't open are allowed to pass.

Vertical clearance to the point of attachment from final grade is 10 ft minimum for 120/240V drops per 230.24(B)(1), and the point of attachment itself must be at least 10 ft above grade per 230.26. The drip loop's lowest point has to meet the same numbers.

• Above roofs with 4/12 pitch or greater: 3 ft above the roof surface, 230.24(A) exception
• Above roofs under 4/12: 8 ft above roof
• 18 in allowed over 4 ft of roof if drop terminates at through-the-roof mast, 230.24(A) exception 3

POCO inspector failing you on clearance after the cover is on is the worst outcome. Shoot the heights with a laser before you call for inspection, not after.

Inspection checklist

Before you call it done, walk it. Weatherhead oriented correctly, flashing sealed, mast plumb, guy wires tensioned if required, meter base level, conductors labeled at both ends, GEC continuous and unspliced per 250.64(C), bonding jumpers torqued.

Take photos of the GEC connections at the rod and the water pipe before backfill or cover. Inspectors love buried grounds. Keep the torque spec sticker on the meter base visible for the final.

File the permit close-out the same day. Open permits on services have a way of biting the next electrician who touches the panel.