Repeated Underground Cable Failures at an RV Park, West Texas (2020)

In 2020 the owner of an RV park in West Texas asked us why the underground cables feeding its RV pedestals kept failing, and whether the electrical design of the park was sound. Client details are confidential.

Failures since opening day

The park had opened about a year and a half earlier. According to the owner, groups of sites had needed their underground wiring replaced several times since then, one row of sites three times. Each repair restored power, and none explained the failures. We examined the installation in the field, took custody of a failed cable sample the owner had kept, and confirmed with the local code office which edition of the National Electrical Code^® applied when the park was built. It was the 2017 edition.

Too shallow

At a recently opened pothole the cable lay about 21 inches deep. For circuits under areas used by vehicles, NEC^® Table 300.5 requires 24 inches. A shallow cable runs hotter and is exposed to damage from traffic and digging, and at an RV park heavy rigs drive and park over the runs every day. Some runs had been placed directly under RV lots, which the code does not prohibit but which puts more weight on the cable.

Too small for its breaker

The pedestals were fed in series, seven or eight to a feeder, from 200 A breakers. According to the contractor, the cable left the panel as 4/0 aluminum and later dropped to 2/0. With the demand factors in NEC 551.73, seven sites need about 193 A and eight need 220 A, before any derating for burial depth or the Texas heat. A 4/0 aluminum conductor rated 205 A can serve no more than seven pedestals. A 2/0 conductor rated 150 A cannot be protected by a 200 A breaker under NEC 240.4(B), and the tap rules do not apply, because an RV pedestal does not terminate in a single breaker. The 2/0 sections had to be replaced with 4/0.

No equipment ground

The feeders had three conductors of the same size: two hots and a neutral. At the pedestals, a green jumper tied the case to the neutral to stand in for a ground. NEC 551.76(C) states that the neutral may not be used as the equipment ground in an RV park. Without a separate grounding conductor, fault current from any RV on the circuit returns over the neutral and over every grounded part tied to it. As built, the system was unsafe to operate.

We also found a temporary repair: a bare neutral run between two pedestals to borrow the neutral of another circuit. It was carrying 11 A when we measured it, in a place where anyone could touch it.

What the park needed

Our report called for an equipment grounding conductor to every pedestal, retrenching the cables to the required depth, replacing the 2/0 cable with 4/0, and removing the bare neutral bond. We noted that we had not been given an engineered design for the park, and that a design professional should account for voltage drop and the extra margin a hot climate calls for. Finding causes like these is the point of our electrical failure analysis work, and the fix often goes through our electrical design services.