What size wire do I need for a 125 amp feeder?

1 AWG copper or 2/0 aluminum at 75 degree terminations per NEC Table 310.16, in SER, MC, or as THWN-2 conductors in conduit. The equipment ground is 6 AWG copper or 4 AWG aluminum per Table 250.122.

Can SER cable run through a crawl space?

Yes if the crawl space is a dry location. SE cable is not permitted in damp or wet locations, so a moist vented crawl space pushes the job to jacketed MC cable or THWN-2 conductors in PVC conduit.

What Are My Wire Options for Running a 125 Amp Feeder Through a Crawl Space?

A user asked the app about wire options for a 125 amp run through a crawl space. Three methods dominate: SER cable, MC cable, or individual conductors in a raceway. The deciding factor is whether your crawl space is a dry or damp location, because that single judgment call knocks options off the list.

Option one: SER cable, the budget pick

For 125 amps, 1 AWG copper or 2/0 aluminum SER covers it using the 75 degree column of Table 310.16, where 1 AWG copper is 130 amps and 2/0 aluminum is 135. Note that the old 83 percent dwelling service rule in 310.12 applies only to services and whole dwelling feeders, so a 125 amp subpanel feeder is sized straight from the table. SE cable installed in interior locations follows Article 338, is supported every 30 inches by 338.10(B)(4) through the NM rules of 334.30, and runs through bored holes or on running boards in the crawl. The catch: SER is for dry locations. A vented crawl with standing moisture that an inspector calls damp takes SER off the table.

Option two: MC cable, happier in rough spaces

2/0 aluminum or 1 AWG copper MC handles 125 amps on the same table, and interlocked MC is permitted in dry locations generally, with listed jacketed MC acceptable in damp and wet locations. Support follows 330.30 at 6 foot intervals. For crawl spaces with moisture history, a PVC jacketed MC or the conduit option below is the defensible choice.

Option three: conduit, the bulletproof route

Schedule 40 PVC or EMT with individual THWN-2 conductors works in any crawl space condition. 1 AWG copper or 2/0 aluminum THWN-2 meets the ampacity, and a 1 1/2 inch PVC handles the four wire pull comfortably. If the space is damp, 300.9 and the W rating already have you covered, and PVC shrugs off the contact with soil moisture that rusts EMT. Strap PVC every 3 feet for 1 1/2 inch per Table 352.30 and within 3 feet of boxes.

The ground and the physical protection

Whatever method you choose, the equipment grounding conductor for a 125 amp overcurrent device is 6 AWG copper or 4 AWG aluminum per Table 250.122. Keep cables out of the bottom of joist bays near access openings where they invite damage, and sleeve anything passing through the foundation wall.

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