Casas móviles y casas prefabricadas

Article 550 splits into the factory side, where the home is wired on an assembly line, and the site side, where a service or feeder lands it on a lot. The rule people trip on is that in a manufactured home you keep the grounded neutral and the equipment grounding conductor separate downstream of the service disconnect, unlike an old-style bonded panel, so a broken neutral cannot put voltage on the steel frame someone is leaning against.

These homes move and settle, connections work loose, and the frame is metal underfoot. So I am meticulous about the four-wire feeder, the separate ground, and a solid grounding electrode at the site. It is a small service but the shock-path consequences of getting the neutral wrong are severe.

When it applies

Article 550 covers mobile and manufactured homes, both the wiring built in at the factory and the service and feeder that hook the home up on site. The headline rule is that the grounded and grounding conductors stay separate in the home, the neutral and ground are bonded only back at the service equipment, not inside the unit.

Common mistakes

• Bonding neutral to ground inside the home's panel, in a manufactured home that bond belongs at the service pedestal, not the unit.
• Feeding the home with a three-wire feeder when a four-wire feeder with a separate equipment ground is required.
• Undersizing the feeder or the service pedestal for the home's calculated load.

In the field

The first thing I check on a mobile-home hookup is that the neutral is floating from ground in the home's panel and bonded only out at the pedestal. Get that backwards and the ground wire and metal frame can carry neutral current, which is exactly the shock hazard the four-wire rule exists to prevent.