Why you should installing service mast

When a Service Mast Is the Right Call

Service masts earn their keep on homes with low eaves, detached garages fed overhead, or any structure where the service drop can't meet the 10 ft clearance above finished grade required by NEC 230.24(B). When the roof pitch, soffit depth, or driveway below forces the drip loop too low, a mast through the roof gets the attachment point up where it belongs.

Before cutting anything, verify the POCO's point of attachment requirements. Most utilities publish a service standards booklet with minimum mast height above roof, approved mast materials, and guy wire rules. What the NEC allows and what the utility accepts are often two different things.

Check clearances in all directions: 3 ft from windows that open per 230.9(A), 10 ft above grade, 12 ft over residential driveways, and 18 ft over public roads per 230.24(B). Triplex over a pool deck or hot tub has its own rules in 680.9.

Sizing the Mast and the Conductors

The mast has to carry two loads: the service drop tension and the weight of ice and wind. Rigid metal conduit is the standard. For a standard 200A residential service with a short span, 2 in. RMC is the typical minimum, but utilities often require 2-1/2 in. or 3 in. if the drop is long or the mast extends more than 24 in. above the roof. If you need more than 36 in. of exposure, you're almost always into guy wires or a back-brace.

Conductor sizing follows 310.12 for dwelling services. A 200A service runs 2/0 copper or 4/0 aluminum SER equivalent in the mast, though verify with the AHJ since some inspectors still want full 310.16 ampacity. Use XHHW-2 or THWN-2 for wet-location compliance inside the mast per 230.43.

Tip: Pull your conductors with at least 36 in. of free conductor at the weatherhead per 230.54(B). Utilities will reject a short tail, and coming back with a bucket truck to splice isn't a callback you want.

Roof Penetration and Flashing

The roof jack is where most mast jobs fail, not electrically but through water intrusion. Use a mast flashing rated for the roof pitch and the pipe size. Neoprene boots for asphalt shingle, lead for tile, standing-seam adapters for metal. The shingle courses above the flashing must lap over the upper half.

Caulk is not a flashing. Sealant fills gaps in the flashing's final detail, it doesn't substitute for proper shingle weaving. If the homeowner calls about a leak six months later, you own it.

Inside the attic, the mast has to be supported within 3 ft of the weatherhead and at intervals per 344.30. A single strap at the roof deck is not support, it's penetration sealing. The real load transfer is at the meter base and any intermediate attachment to structure.

Bonding, Grounding, and the Meter Base

The neutral is the grounded conductor at the service, and the meter base is where the grounded and grounding systems tie together per 250.24. Use a listed bonding bushing on the mast threads into the meter base, or a bonding locknut if the hub is listed for bonding. Standard locknuts are not acceptable for service bonding per 250.92(B).

Grounding electrode conductor sizing comes from Table 250.66. For a 200A copper service, that's #4 copper to rod and #4 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, which almost nobody measures.

• Bonding bushing or listed bonding hub at the mast-to-meter connection
• Main bonding jumper inside the service disconnect per 250.28
• GEC from neutral bar to rods, water pipe, and any Ufer or structural steel present
• Intersystem bonding termination per 250.94 for CATV, phone, and satellite

Weatherhead, Drip Loop, and POCO Connection

The weatherhead must be above the service drop attachment so water drips away from the fitting per 230.54(C). The drip loop forms below the weatherhead, with enough slack that water runs off before reaching the insulation terminations. If the weatherhead sits below the triplex tap, you're pulling water straight into the mast.

Conductor identification at the weatherhead matters. The grounded conductor needs to be identified per 200.6, typically with white tape wrapped at the terminations if the utility conductors are all black. Some inspectors want it identified along the full length inside the mast, others only at terminations.

Tip: Tape your drip loops before the POCO shows up. A clean, taped loop with proper sag tells the lineman this job is ready and cuts your wait time on a reconnect.

Common Field Mistakes

The mast gets installed plumb and the drop is tensioned, and then the mast bends. That's an undersized mast, a missing back-brace, or a drop span that exceeds the utility's tension table. Fix it before the inspector shows up or before the first ice storm.

Other repeat offenders worth walking before you call for inspection:

1. Standard locknuts instead of bonding bushing at the meter hub
2. Weatherhead facing the wrong direction, forcing an uphill drip loop
3. Mast height above roof exceeding 24 in. with no guy or brace
4. GEC run through ferrous conduit without bonding both ends per 250.64(E)
5. No working clearance at the meter per 110.26, especially with A/C units or fences nearby
6. Shingles cut but not re-lapped over the flashing upper edge

Service mast work is one of the last jobs where the utility, the AHJ, and the homeowner all sign off independently. Get the clearances, the bonding, and the flashing right the first time and the rest is paperwork.