What is the minimum wire size for a 100 amp mobile home service?

There are two different runs in a mobile home installation and they follow different rules. Getting the conductor count wrong on the second one is the most common failure.

The service to the pedestal

The service equipment for a mobile home is installed adjacent to the home, not in it. Section 550.32(A) requires the service equipment to be located in sight from and not more than 30 feet from the exterior wall of the mobile home it serves, and 550.32(B) permits it to be mounted on the home only where specific conditions are met.

For a 100 ampere single phase 3 wire service, meaning two ungrounded conductors and a grounded conductor, Section 310.12(A) allows service conductors to have an ampacity not less than 83 percent of the service rating. That is 83 amperes. Table 310.16 in the 75 degrees C column gives 4 AWG copper at 85 amperes and 2 AWG aluminum or copper clad aluminum at 90 amperes. So the minimum is 4 AWG copper or 2 AWG aluminum.

The grounded conductor does not automatically match. Section 250.24(C)(1) requires the grounded service conductor to be not smaller than the value in Table 250.102(C)(1) based on the size of the ungrounded conductors, and it must also carry the maximum unbalanced load calculated per 220.61. For 4 AWG copper ungrounded conductors, Table 250.102(C)(1) gives 8 AWG copper as the floor, but the unbalanced load calculation usually drives it larger.

The feeder from the pedestal to the home

This is where the 3 wire assumption fails. Section 550.33(A) requires the mobile home to be supplied by a feeder assembly consisting of either a listed 50 ampere mobile home power supply cord or a permanently installed feeder of not more than four continuous insulated color coded conductors, one of which is an equipment grounding conductor.

Section 550.16(A)(2) requires the grounded conductor to be insulated and prohibits connecting it to the equipment grounding conductor, the chassis, or any enclosure inside the mobile home. The neutral and the ground are separated at the home. A 3 wire feeder where the neutral doubles as the ground has not been permitted for mobile homes for decades, and there is no grandfather clause when you replace the feeder.

Size that 4 wire feeder the same way. If it is the main power feeder carrying the entire load of the dwelling, 310.12(B) permits the 83 percent allowance, so 4 AWG copper or 2 AWG aluminum for a 100 ampere feeder. The equipment grounding conductor comes from Table 250.122, which is 8 AWG copper or 6 AWG aluminum for a 100 ampere overcurrent device. If you upsize the ungrounded conductors for voltage drop, 250.122(B) requires the equipment grounding conductor to be increased proportionally.

Grounding at the pedestal

The service equipment is where the main bonding jumper lives, per 250.24(B). Drive the grounding electrodes there, not at the home. Section 250.32(B)(1) requires the equipment grounding conductor to run with the feeder to the mobile home and prohibits re bonding neutral to ground at the home. Section 550.16(C) requires the mobile home distribution panelboard to have an equipment grounding terminal bar separate from the neutral bar.

Field takeaway

Service conductors at 4 AWG copper or 2 AWG aluminum for 100 amperes under 310.12. Feeder to the home is four conductors, insulated neutral, 8 AWG copper equipment grounding conductor, and neutral isolated from ground inside the home. Service equipment within sight and within 30 feet of the exterior wall.