There are times like today, when I need to park the RV out front of the house where the only available electricity is a 120v outlet on a 20 amp circuit. I can reach it with my 50 amp shore power cord plus dogbone adapters to fit the outlet, and get I 120v inside the coach, but my question is whether I should be running even one of the overhead A/C units. I know I CAN, but with the compressor running it's pulling 15 amps and dropping voltage down to 107 or 108. These are brand new A/C units on the roof, and I don't want to ruin them. My EMS will shut things down at 104, but what are the recommendations/advice about running for any length of time at 107-108?
108 is still within specs (120 +/- 10%).
Be sure EVERYTHING else is turned off. If an inverter charger that doesn't allow you to turn off charge function, be sure to go into programming and "power share/power save" feature and select 5 amps. That is 5 amps of 120 VAC which will still charge the batteries at over 40 amps at 13.5 VDC.
I believe my Heart Interface panel controls the inverter/charger, and it only performs either of those functions when I press Invert or Charge.
Yup, depends on exactly what Xantrex inverter/charger it is. Many automatically charge, but inverter function has on/off switch.
The owners manual for your particular model (your hard copy or on-line) will tell you for sure.
Motors will usually rotate at low voltages, BUT they will heat their motor windings by increasing amps to compensate for the lower voltages and WILL cause some wear damage. The lower the voltage, the increased wear.
If you asked about a heating element like an incandescent, toaster, etc these will not heat as much or be dimmer but will not cause any damage.
Do not recommend starting or running roof air with home or campground low voltage much below low end of 110-120 volt range. I would not run them below 110 volts.
Know and turn off all the other loads on the 20-amp house breaker that controls the outside outlet. You should not have voltage drop from your shore cable and dogbone if all connections are tight and shinny.
A/C compressor startup drawing down for a moment is okay as long as running voltage is above 109 volts. Keep roof air running at max cool to keep it from stopping & starting.
You might look into adding a "soft start" module to one of your roof air units. Doing so will not reduce the running voltage, but will greatly reduce the spike in amps required to start the compressor (LRA).
There are several popular brands. Do a Google search to see if this mod would be of any interest or value to you. See below for one informative blog:
The Best Soft Start for RV AC - TheRVgeeks.com (https://www.thervgeeks.com/soft-start-for-rv-ac/)
The accepted rule of thumb is plus or minus 20% of the rated voltage.
But you know the answer yourself: Have a licensed electrician install a 240V outdoor outlet on the front of your house. If you're running through your shore power cord with no extension cords intervening, then the problem is the wiring in your house not being sized to carry the rated current flow. IF you're using smaller gauge extension cords to bridge the gap and that's where the loss is then buy the correct size extension cords "I" have used a Buck/Boost transformer to give me 120VAC at the load on a construction site at the end of a too small extension cord, but only because I had one in my truck. That was done only while I was assembling my 30' by 70' steel arch building and I needed my 2HP air compressor because battery operated tools weren't a thing yet and I had air powered tools.
I have never seen the safe range for AC current at +/- 20%, though some appliances are less affected by out of spec voltage than others.
Do you have a link from reputable site suggesting an electric motor/electric compressor can be safely run on 96 VAC?
I completely agree with Brett.
Running +/- 20% is significantly out of spec for NEC/NEMA. Suggesting this is okay is not okay.
+/- 10% at the branch/point of use is acceptable.
Don't Let Voltage Drop Get Your System Down | EC&M (https://www.ecmweb.com/national-electrical-code/code-basics/article/20892807/dont-let-voltage-drop-get-your-system-down)
BTW, relying on an EMS that turns off at 104volts is set too low.
Also keep in mind that all voltage readings should be under a load.
Agreed. Should be 108 low, 132 high.
I believe the weak point here would be the dog bone that delivers the 120V and whether it gets hot or not. If not, I would run it.
Ya, not all dog bones/adapters are of the same quality. Better ones has a short length of wire between male and female end so the two "areas of contact" are separated.
Said another way, be leery of inexpensive/off-brand adapters-- like many things today, quality is all over the place.