3 Air Conditioners

[azbuzzard]

Can you run all 3 AC units at the same time?

On 50 amps?

On generator?

[wolfe10]

Welcome to the FMCA Forum.

The answer is MAYBE.

First question is what is the amp draw of each A/C, particularly the two on one "hot"? Big difference between new-generation efficient 13,5 K BTU units and inefficient 15K BTU units.

What other 120 VAC items will be running at the same time, particularly on the hot supplying the two A/C's?

On 50 amp shore power, you have TWO 50 amp hots. As long as your draw from each is under 50 amps total, yes you can. BUT, the wiring in many CG's is old/small enough that as you approach that 50 amp draw the voltage drops quite a bit. As voltage drops, your A/C will consume more amps.

As far as generator, you will need to determine its watt output and also how it is wired to your house system. Probably the best answer is to ask your coach maker who designed and installed your electrical system.

[Guest BillAdams]

As eluded to, the answer is....you should be able to. I can in my coach (shore power or generator) if I am very careful about what else is running. I would never leave the coach unattended in this condition as the chance of blowing a circuit when something kicks in unexpectedly or the campground power simply gives up the ghost is very real. If you can monitor the voltage on each leg (a 50 amp circuit gives you 50 amps on each leg or a total of 100amps. A 30 amp connection will provide a total of 30 amps combined) you can adjust what is and is not allowed to come on while attempting to use this much power simultaneously.

Of course, this also depends upon the way your coach was setup by the manufacturer and a built in energy management system my help or hinder you in this goal.

[hermanmullins]

Two thoughts. 1) On conversion coaches and larger coaches made for passengers service like MCI. Prevost, and others, they are able to run their A/Cs all at the same time, check out their wiring diagrams.to see how they are configured. 2) In the past I have seen coaches with 30 amp service that had two A/Cs that could be on at the same time but it would cycle between the units so they both were not on at the same time.

[chucknewman]

Most of the "entertainer" bus conversions I've seen had a 50 plug available, but generally they don't park at a power post. And most of those had a single "basement" AC of several tons powered by a 12KW generator or greater in a sound box so it could be run while parked (as well as on the road) and not bother anyone.

A 12KW generator will give you 100 amps on L1 and 100 amps on L2. 24,000 watts total. More than enough to run a large basement AC or 4 or 5 roof top units plus electric HWH, electric cook top, etc. that you generally find in this class of coach.

A 50 amp post supplies 6000 watts per leg. 12,000 watts total. If you put two med efficiency roof top AC's on the same leg, on a hot day, with high humidity, you would find each AC would pull about 17 amps. Mine do! With 50 amps per leg you will be OK with very little voltage drop in from the post. At those currents, little voltage drop can be engineered, but in the real world is not generally what you will find in any park.

As stated, with a large enough generator, the sky is the limit. But no park I've been in will allow you to run it in the park. As opposed to casino dry parking, truck stop, and rest stop areas.

Chuck

[hermanmullins]

Chuck, I understand your post, that your A/Cs pull 17 amps while running. My question is how many amps do they pull when they cycle on? That might effect how many amps is required.

[chucknewman]

Herman,

In my case, each AC (only 2) is on a separate leg and I haven't measured the starting current. But you are correct in that it does affect the amps required for compressor start. Keep in mind the starting current duration for a typical RV roof top AC is 500 to 1000 milliseconds. Not enough heating time for a typical thermal circuit breaker.

It has been a very long time since I studied this particular part of NEC, but the standard was to design a circuit to accommodate 1.25 times the running current of the motor. So my 17 amp draw AC's individually would draw 21.25 amps on compressor start. Theoretically!

In the real world we found some inductive devices required less than 1.25 times the running current, and some required up to 8 times the running current. It depended on several factors: Age, mechanical drag, load, stator and rotor efficiencies, type of start circuit, amount and duration of inductive kick (if any), and regulation of supply voltage during start. The supply voltage regulation being a major factor as Brett and others have pointed out.

The point I was trying (maybe poorly) to make was I've seen many bus conversions with three AC units that run fine on (most) 50 amp plugs. Typically you would have 2 AC's on one leg with no other loads, or minimal loads. The third AC would be on the other leg. And then the wiring may be totally different relying on a pricey load management system. And, in general, the coaches with 4 or 5 AC's don't connect to a post -- at least without some type of load management.

On a power post, you have finite volts and amps, and they don't stay the same around the clock and with different loads in the coach. One of the best statements here was Bill's point that you must be aware of your working loads vs the supply -- whether 30A or 50A service. I can run the fans in my two AC's simultaneously (with the compressors off), heat my coach, run the microwave, wash the clothes, and charge the batteries while on a 15/20 amp service. I just cannot do it all at the same time. The same when connected with 30 amps. But most users don't want to be bothered with load management.

I have found measuring the voltage drop vs amp draw using my HW50C that the greater the amperage available at a post, the less voltage drop in general. This is because NEC requires larger conductors for applications using larger amperage. But other factors in any particular park can affect the voltage drop even more than your individual loads.

Chuck