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Topic: When is voltage too low? (Read 589 times) previous topic - next topic

When is voltage too low?

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?
1998 U295 36' No slides
Towing 2018 Jeep Wrangler
Previous rigs:
  1995 26' Fleetwood Flair
  1998 34' Fleetwood Bounder
  1997 40' Bluebird BMC
  1999 24' Winnebago Minnie Class C

Re: When is voltage too low?

Reply #1 –
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.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: When is voltage too low?

Reply #2 –
Quote from: wolfe10 –
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.
1998 U295 36' No slides
Towing 2018 Jeep Wrangler
Previous rigs:
  1995 26' Fleetwood Flair
  1998 34' Fleetwood Bounder
  1997 40' Bluebird BMC
  1999 24' Winnebago Minnie Class C

Re: When is voltage too low?

Reply #3 –
Quote from: Dan & Annie –
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.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: When is voltage too low?

Reply #4 –
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.

Re: When is voltage too low?

Reply #5 –
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

1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: When is voltage too low?

Reply #6 –
Quote from: Dan & Annie –
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. My EMS will shut things down at 104, but what are the recommendations/advice about running for any length of time at 107-108?
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.
1992 Foretravel Grand Villa
U225 SBID Build No. 4134
1986 Rockwood Driftwood
1968 S.I.A.T.A. Spring
1962 Studebaker Lark
1986 Honda VF700C
1983 Honda VF750C
Charlie, the Dog was broken out of jail 24 Oct 2023
N1RPN
AA1OH (H)e who must be obeyed.

Re: When is voltage too low?

Reply #7 –
Quote from: Old Toolmaker –
The accepted rule of thumb is plus or minus 20% of the rated voltage.

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?
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: When is voltage too low?

Reply #8 –
Quote from: wolfe10 –
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.

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

Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: When is voltage too low?

Reply #9 –
BTW, relying on an EMS that turns off at 104volts is set too low.

Re: When is voltage too low?

Reply #10 –
Also keep in mind that all voltage readings should be under a load.
Bruce, Linda, and Macy
Zoey RIP 1/20/19
1999 U295 40' build #5400
2017 silver Jeep Wrangler, 1260 watts of solar on top
Moving around the country

Re: When is voltage too low?

Reply #11 –
Quote from: Barry & Cindy –
BTW, relying on an EMS that turns off at 104volts is set too low.

Agreed.  Should be 108 low, 132 high.
Learn every day, but especially from the experiences of others. It's cheaper!  - John C. Bogle

2000 U320 36' non-slide / WildEBeest Rescue
2003 U320

Re: When is voltage too low?

Reply #12 –
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.
1994 U280, Build 4490
Deming, NM.
 

Re: When is voltage too low?

Reply #13 –
Quote from: Twig –
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.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020