240V AC Power Distribution and Field Day

ARRL Field Day 2021 is upon us, and we are listening carefully to the various planners. The tower trailers are popular in two roles- towers and generators. Beams are a nice add on for station planners and a 30+ foot place to hang a dipole- why not. Light towers have accessory outlets, and 15-20A 120V AC outlets are common. With 30+ gallons of diesel and .45 GPH consumption, you do not need to refuel for the 24 hour event.

One use case for field day- powering campers and motorhomes. We have at least one small camping trailer and a large motorhome with a defective main generator.

The average small camping trailer has a TT-30 power cable. This is two 115V, 15A circuits. The camper usually has one “side” for the 11 -13.5 amp air conditioner, and the other “side” for the rest of the loads, such as the 12V battery charger and outlets. Newer 3600 RPM consumer generators (such as the Sportsman 4000DFs we use) have a TT-30R jack. The older (2012) diesel tower trailers do not.

The larger trailers and motorhomes use the 50A plug. Again, the average ten year old light tower does not have a 50A 14-50R motorhome outlet. These are two large 115V legs.

The light towers in the 6KW class do often have a 240V optional outlet at 30A. On the older trailers this is L6-30R. Hot (R), hot(B) and ground. There is no neutral, or proper adapter for TT30R. Newer light towers have a L14-30R, which is hot, hot, neutral and ground. These are easier to adapt for RV use.

The issues for us are two. If most of our tower fleet has 115V outlets, you can adapt any motorhome or trailer to 115V home style outlets, but a single RV AC unit will use most of the circuit and two AC units will probably blow the fuse. The other concern is unbalanced loads. There are reports if you fully load one leg of a 30A generator (AC unit) and have a light load on the other leg that can damage the generator or burn out a winding. The light tower generators have two 115V AC windings and are usually capacitor regulated.

In a portable generator, the rule of thumb is you tie neutral to ground. This is common in light towers. It will say that on a sticker.

We took several approaches as recommended by Doug N0NAS who is an expert here.

  1. We have a supply of tower trailer “stock” 115V outlets, usually 15 or 20A. These may just use one leg of a 30A generator but are breaker protected. (It is reported some transient loads will burn out a winding before the breaker trips). #16 100′ extension cords are tacky but are probably fine for 500 watt loads. #12 cords are better.
  2. We bought a power yellow distribution box for L14-30 to two 115V outlet banks. You have 25-50′ #10 four wire 240 cables cables and can split the power. Lots of breakers to protect the generator
  3. We made a load balancer out of a 3.5KW transformer. We just used the secondary to derive a neutral. 240V AC (red/black) in, two 120V with neutral out. So this is a proper L6-30 to TT30 adapter. But you can still overload it.
  4. For the 50A motorhome with a bad generator- we did find an 11,000 watt gasoline Honda at a garage sale – it has an L14-30R and a “California” 50A jack- more to follow on that.
  5. We are going to not connect our 6KW diesels to the 50A motorhome for the moment- except maybe on a 115V outlet.
  6. Larger, 15 and 20KW light towers are occasionally available.
  7. The next frontier is three phase 208V and or 480V

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