Saturday, May 14, 2005

It's another beautiful sunny day here in southern California.

Which is great, except the beautiful sunny days have also been quite warm, which has presented something of a problem. I know a lot of bus conversion folks are reading the blog, and this situation may be of interest to some of you.

As you know, our main engine is down for maintenance, and we are parked out in the yard at PEDCO. Daytime temperatures the last few days have been running in the high 80's to low 90's, and we are parked in full sun, so we've been trying to keep cool with our air conditioners.

Now, normally when we are boondocking, we simply start the generator if we need to run air conditioners for more than a few minutes (we can run one AC on batteries for a few hours without too much trouble), and certainly if we need to run more than one at a time. As I mentioned here yesterday, that's not an option right now, because guys have been working under and behind our coach, and the genny would either smoke them out or cook them.

Virgil offered us power from an outlet in the shop, and he showed me an outlet that he had installed for the purpose. Unfortunately, the electrician that put it in for him misunderstood what was asked for, and the outlet was not really suitable. It turned out to be a 208-volt outlet daisy-chained off an unused 30-amp, 208-volt three-phase receptacle that was in the building before PEDCO took it over. To make matters worse, the new outlet was a 50-amp style with no neutral.

In an effort to make use of what was there, I ran down to Home Depot and bought a plug to mate with this receptacle, a receptacle to mate with our shore cord, and 15' of 10/3 SOOW cord to make myself an adapter. Of course, the outlet had two hots and a ground, with no neutral, and the hot-to-hot voltage was 208, not 240. Suffice it to say, once I got this all wired up, and tried to run one AC per leg, I kept tripping the 30-amp three-phase breaker on the panel. The breaker was warm to the touch -- it was clearly tripping thermally.

I gave up on trying to make use of the hokey jury-rigged three-phase receptacle, and plugged into a standard 15-amp job elsewhere in the shop. At which point, Virgil and I had a brief discussion about how the outlet he thought was going to be for visiting coaches would neither fit any standard shore cord, nor provide enough juice to even a modest coach.

After a few minutes of discussion, I agreed to re-wire the circuit in the shop to something more standard in the RV industry. We agreed that, since the wire in the conduit was only rated for 30 amps, and the phase-to-phase voltage was not in the 220-240 range, that I would just put in a single-phase, three-wire 30-amp "RV" receptacle.

Putting in the correct circuit was straightforward -- I taped off one of the three phase wires at both ends, re-tasked one of the others as a new neutral wire (wrapping both ends in white tape) and connected it to the neutral bus in the panel, and pulled out the three-phase breaker and substituted a single-pole 30-amp breaker. It was a Square-D "QO" panel, which are easy to work hot.

After getting the new circuit and receptacle in and tested, I dragged out my trusty 30-amp adapter and plugged Odyssey in. Within minutes of cranking the AC's up, the incoming voltage dropped to a bit over 100, and the inverter dropped sync. Harumph.

Well, now I was certain I had the right kind of circuit, and it was good all the way from the panel to the receptacle, and, furthermore, everyone had already left for the day so no equipment was running in the shop to drag down the voltage. At which point it occurred to me to pace the circuit out...

Turns out the circuit in question runs about 110' from the shop panel to the receptacle. The original circuit that I re-tasked had been run with #10 wire. Add another 35' for my shore cord and adapter, a few feet here and there inside the coach, and then throw 30 amps of load at it, and, voila -- 11 volts of voltage drop across the circuit between the shop panel and my inverter. (Try this handy voltage drop calculator.) Shop voltage was around 117-118, which meant 106-107 volts aboard Odyssey. Of course, AC's draw a fixed amount of power, so if the voltage drops, current increases.

There's really no way to fix this -- I'm stuck with the 110' of #10 from the panel. I'm using a #6 shore cord and 15' of #10 adpator/extension to reach the outlet from the coach. I could beef up my 15' extension to #8 or #6, but the improvement would be minimal.

Fortunately, the SW4024 inverter allows us to set the amount of current we draw from the mains. As I type, I've got the input set at 25 amps, even though the two running air conditioners are drawing 28amps combined. That seems to be keeping the 30-amp breaker in the shop panel happy, and has the input voltage at the inverter at around 108-109 volts. The ACs are struggling at that voltage, but at least we're somewhat cool. When the ambient temperature drops, we'll drop one of the ACs and then the inverter can begin replenishing what it is now using from the batteries.

So there are some lessons learned here that I will pass along to other bus and RV folk:
  1. Carry the heaviest gauge shore cords and adaptors you can.
  2. Two short lengths can be better than one long one.
  3. Having a variety of plugs on hand, along with a receptacle to mate with your shore cord and some wire to connect them, can get you out of a jam.
  4. (Drum roll) Don't underestimate the usefulness of a load-supporting inverter. I don't think we'd be getting any cooling at all without it.
On the plus side, Virgil is happy with his new outlet, and the next coach that stays here will at least have a 30-amp outlet they can use.

We're looking forward to sleeping in past 0700 tomorrow -- there won't be anyone running air tools under our bedroom at that hour on Sunday.

1 comment:

  1. Most of the discussion about amps, voltage, etc. for the shore power is fascinating Greek. Those of us without EE degrees need a lot of help. You're nominated to write "Shore Power For Dummies."
    --Reader Jon (BA in humanities!)


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