Best practices for installing service mast

Service masts take abuse. Wind, ice, tree limbs, and a utility drop pulling hard against the weatherhead all work against you. Install it wrong and the homeowner calls you back in two years with water in the panel or a mast bent like a fishing rod. Here is what holds up in the field and what the NEC requires.

Size the mast for the drop, not just the load

The service drop applies real mechanical force, and the mast has to take it without flexing. For most residential 200A services with a standard utility span, 2 inch rigid metal conduit (RMC) is the minimum that will hold up. Anything longer than a 10 foot span or in heavy ice country, step up to 2.5 inch or 3 inch RMC. EMT is not acceptable as a service mast above the roof, full stop.

NEC 230.28 requires the mast to have adequate mechanical strength to withstand the service drop without damage. That is deliberately vague, which means the utility's service requirements usually dictate the actual size. Check the POCO handbook before you pick up pipe.

• 2 inch RMC: standard 200A residential, span under 100 feet
• 2.5 inch RMC: longer spans, heavier conductors, or utility requirement
• 3 inch RMC: 320/400A services or extreme exposure
• Schedule 80 PVC: only where utility explicitly permits, never as sole support

Flashing and roof penetration

Water follows the pipe down. If your flashing fails, the service entrance cable soaks, the meter base rusts from the inside, and you will be back with a sawzall. Use a proper lead or aluminum roof jack sized to the conduit OD, set under the upslope course of shingles and over the downslope course. Seal with roofing mastic, not silicone.

Keep the mast plumb. A leaning mast puts uneven load on the flashing and the through-roof seal cracks within a season. Use a 4 foot level on two sides before you tighten the roof brace.

Pro tip: run a bead of butyl sealant between the flashing flange and the roof deck before you reset the shingles. It buys you another decade when the mastic eventually fails.

Clearances above the roof and around the weatherhead

NEC 230.24 drives mast height. Conductors passing over a roof need 8 feet of vertical clearance in most cases, but there are carve-outs worth knowing. If the roof has a slope of 4/12 or greater, you can drop to 3 feet. If no more than 6 feet of conductor passes over the roof and it terminates through-the-roof at the weatherhead, you can go as low as 18 inches, provided the voltage is 300V or less to ground.

Also watch NEC 230.9 for clearance from windows, doors, porches, and fire escapes: 3 feet horizontally from windows that open, and conductors must be out of reach from balconies and similar surfaces. The weatherhead itself goes above the point of attachment whenever possible so water drains away from the service conductors, per 230.54(C).

1. Verify roof slope and pick the right clearance rule
2. Locate point of attachment (POA) at least 10 feet above grade, 12 feet over driveways
3. Set weatherhead above POA, drip loop below
4. Confirm 3 foot horizontal clearance from openable windows

Bonding, grounding, and the meter base

The mast is a raceway for service entrance conductors, which means it sits on the line side of the service disconnect. Bonding requirements in NEC 250.92 are strict: standard locknuts are not enough. Use bonding locknuts, bonding bushings with a jumper to the neutral, or a bonding wedge where the mast lands on the meter base.

Torque the meter base hub and the couplings. A loose hub is the most common cause of moisture intrusion at the meter, and it is the first thing a sharp inspector checks. If you are running a mast kit with a pre-drilled meter base top, seal the threads with an approved thread sealant rated for wet locations, not teflon tape.

Pro tip: after you dress the service entrance conductors, pull a drip loop inside the meter base so any water that tracks down the conductor jackets falls away from the lugs instead of pooling on top of them.

Guy wires, braces, and structural support

Any mast projecting more than 36 inches above the roof needs support. Most POCOs require a roof brace (angled strut back to a rafter) or guy wires rated for the pull. The drop applies horizontal load, so the brace or guy must resist in the same plane as the drop, not perpendicular to it.

Lag into rafters, not just the decking. A 3/8 inch lag, 3 inches into solid framing, with a sealed washer. If you cannot hit a rafter, add blocking from inside the attic. Never rely on the flashing or the through-roof seal as structural support, they are there to keep water out, not to hold the mast up.

• Brace or guy required when mast exceeds 36 inches above roof (verify POCO spec)
• Brace must oppose the direction of the service drop pull
• Lag into rafters with sealed fasteners, minimum 3/8 inch
• Re-check tension after the first winter

Final checks before the POCO connects

Before you call for the utility to make the connection, walk the install. Mast plumb, weatherhead above POA, drip loops formed, conductors have at least 36 inches of free lead outside the weatherhead for the lineman to splice, and the meter base is sealed and grounded per NEC 250.24. Verify the grounding electrode conductor is sized per Table 250.66 and run without splices to the rod or ufer.

Photograph everything. Meter base interior, mast base at the roof, brace attachment, ground rod connection. When the inspector wants a clarification three weeks later, or the homeowner has a question about the work, the pictures end the conversation fast.