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Why your RV air conditioner trips the generator

An RV air conditioner doesn't trip a generator because of the power it uses while running. It trips because of the split second when the compressor starts. Dometic rates the compressors in its 13,500 and 15,000 BTU rooftop units at about 10.5–13.3 amps running, but 52–73 amps for a moment at start-up (the "locked rotor" current). A small generator or a 30 amp pedestal that's already feeding other loads sees that spike and shuts off.

A soft start fixes the spike, not the running load. It ramps the compressor up over a fraction of a second instead of slamming it on, so the starting surge is much smaller.

How much a soft start helps

• Coleman-Mach says its Soft Start Kit cuts starting amps by up to 75%, depending on the AC model.
• Dometic says its SmartStart cuts start-up power demand by as much as 75% and can let an AC run on a generator as small as 2,000 watts.
• Without one, Dometic recommends at least a 3.5 kW generator for one AC and 5 kW for two, and notes that generators lose power at high altitude.

A soft start does not lower the running amps. A 15,000 BTU unit still draws roughly 13–14 amps while cooling, and more in extreme heat, so your generator still has to carry that plus everything else that's on.

Which soft start fits your air conditioner?

This is where most people order the wrong part. Dometic's SmartStart kits are model-specific, while the SoftStartRV is universal.

Not sure which you have? The model number is on the label under the shroud or on the ceiling assembly. Coleman-Mach says its 1497-3601 kit fits all of its rooftop air conditioners and heat pumps. You can see all our options on the Dometic soft start kits page, or search "soft start".

SoftStartRV vs. Dometic SmartStart

• SoftStartRV SSRV3T3 is small (about 5-5/8 x 2-15/16 x 2 inches), works on 110/240 volt single-phase AC systems up to 18 amps continuous, and is sealed in resin so it can live inside a rooftop unit. If power to the compressor is interrupted, it shuts the compressor down and tries another soft start after a three-minute delay.
• Dometic SmartStart kits come with the brackets and harnesses for a specific Dometic model, so installation is more plug-and-play on that unit.

Both do the same basic job. Choose the Dometic kit if you want the matched harness for your Dometic, and the SoftStartRV if you have a Coleman-Mach, Furrion, GE or Advent unit, or more than one brand.

Can I run two air conditioners on 30 amp service?

30 amp service is about 3,600 watts. One rooftop AC uses most of that once it's running, so running two at the same time on 30 amps usually isn't realistic, with or without soft starts. What soft starts do help with is the start-up spike, so one AC is far less likely to trip the pedestal or generator when it kicks on. If you regularly need two ACs, 50 amp service (two 120 volt legs, about 12,000 watts) is the normal answer.

Installation tips

• Turn off shore power, the generator and the inverter before opening the shroud. Capacitors inside the AC can hold a charge even when it's off.
• Follow the wiring guide that comes with your soft start. Most connect at the compressor and capacitor inside the rooftop unit.
• After installing, let the AC sit off for a few minutes before restarting. Dometic builds in a compressor delay of about 2 minutes.
• If you aren't comfortable working with 120 volt wiring, have an RV tech install it.

Bottom line

If your generator or campground breaker trips when the AC starts, a soft start is usually the cheapest fix, and often lets you run your AC on a smaller, quieter generator. Match the kit to your AC model, and remember it fixes the start-up surge, not the running load.

Want the background on amps, generators and AC brands? Read the free guide at RV Guidance: RV Air Conditioners and Furnaces.