A work around for low voltage.

[pianotuna]

Hi All,

Due to the campground refusing to reserve a 50 amp site for me (three months in advance), I've been put where there is only a 30 amp outlet and the campground wiring is, shall we say, "challenged"?

The equipment:

Class C RV
Magnum hybrid inverter/charger
PD 40 amp converter with wizard
"patch panel" wiring for the hot water heater
a stand alone inverter plug in the kitchen.

Initial voltage, on a day when the ambient temperature was 27 C (80 f), was 113 with only the fridge running.

One solution was to turn on load support on the Magnum. However to "trick" the Magnum into doing voltage support to 107 (and above) required throttling back the input amperage to 12 amps. That meant I was drawing 54 amps from the battery bank. Of course, I can't sustain that for very long. Perhaps 4 hours and I'd be pushing the magic 50% number.

Last summer I tried doing "double conversion". Unfortunately the Magnum draws so much power that the reverse polarity fuses in the PD converter blow almost instantly. That is less than convenient. It is something I did regularly with the 2500 watt Cobra MSW inverter--I don't understand why the 3000 watt Magnum works so differently.

So I chose to plug the main power feed for the Rv directly into the shore power. That left me able to run the air conditioner and fridge on electric. But, what to do about the water heater and the induction cooker? Fortunately my system allows me to connect the water heater to a secondary 120 volt 15 amp outlet. But the pedestal only has the 30 amp. So I, instead, plugged it into the inverter.

So, I have almost full functionality for the power system. The solar panels should provide enough energy to keep the water hot. I can run the induction cooker, too.

Fortunately, it cools down at night here (tonight will be 8 c (46 f)), so at night I can redo the "patches" (male and female plugs) to allow me to recharge the battery bank with the OEM pd converter, and operate the water heater.

I'm annoyed at the campground--but in the winter they are "the only game in town", so I need must be careful what I say to them.

How I wish I had the Mexican autoformer right now!

[time2roll]

I would be aiming at 12.1 in the late afternoon. Maybe lower due to voltage sag under load.

[pianotuna]

Hi SMK,

The air does eventually start to cycle--but from 2 p.m. to 6 p.m. it is running flat out to maintain 22 c (72 f). Part of the issue may be the site, which has zero shade in the afternoon.

Recharge was as high as 54 amps but it did not stay there long at all. I have to limit the shore power to 11 amps to "trick" the Magnum into doing not just load support--but voltage support as well.

At 8 p.m. I was able to go back to straight campground power, wiring around the Magnum. My PD converter will get its job back overnight. The batteries are sitting at 12.9 at the moment.

It will be hotter tomorrow so I'll be "doing the dance" for a few more hours. I'll try bumping up the temperature to 23 c (74 f) to try to get the air to cycle.

[time2roll]

Does the air not cycle off? If you are pulling out 35 I would think it would push 70 back in while the air is off for a bit.

[pianotuna]

Hi SMK,

Yes, I'm using the load support feature of the Magnum--but it is drawing 35 amps. I can't keep that up all day long.

I tried double conversion last summer and promptly blew the reverse polarity fuses on the PD 40 amp. That did not happen when I had the Cobra 2500 watt MSW unit--but then I did not run the roof air from it either.

I've been wiring around the Magnum as there was more voltage drop with it that without it. I guess I should have insisted on more than #10 wire to and from the AC side of the Magnum.

[time2roll]

Will the converter not keep up while the air cycles? (double conversion)

[pianotuna]

Voltage hit a new low at the campground. 102 volts running ONLY the roof air.

Fridge on Propane and converter disconnected.

[Big_Katuna]

Watts= V X A (-PF) so when the volts go down the amps go up but with most inductive loads the wattage remains about the same.

Temp makes a compressor work harder = more amps.

When the volts go down, the amps go up so there is more copper losses and heat

Resistive loads like hot water heaters, toaster, the volts go down so does the amps and the wattage drops.

[pianotuna]

As voltage drops amperage increases. There will be more heat losses in the motor and eventually the windings may fail.

What is true is that wattage increases as the ambient temperature rises. The compressor has to work harder and harder, and so does the motor powering it. I have observed between 1200 and 1900 watts for my 13500 btu Dometic unit.

valhalla360 wrote:

Not sure on this but at very low voltage, I suspect the wattage required will increase as the motors turn less efficiently.

So an 1800watt motor may be pulling 1900watts due to low voltage. By giving it full voltage, it needs less watts.

Can any one confirm this?

[road-runner]

In the one time I tested an RV air conditioner down to its lower limit of 103.5 volts, as the voltage went down, the current went up, and the power went down. I have no way of knowing if or how much the motor speed changes with the voltage, so at a lower voltage maybe the motor is running more efficiently, or maybe the air conditioner is moving fewer BTUs, or some combination of both.

[valhalla360]

GordonThree wrote:

2oldman wrote:

GordonThree wrote:
If the wiring at the CG is so bad that voltage is severely sagging, how is an autotransformer going to help? As a step-up transformer, it will be trading amps for volts,...

Not sure I understand your question. The purpose of an AF is to boost sagging voltage, which by definition decreases amps.

Increased voltage reduces amps only on the output side, the transformer's secondary ... but there is no free lunch, that boost in voltage comes from increased current draw on the input side, the primary.

Say an AC needs 1800 watts to operate. At 125 volts that is 14.4 amps. To deliver 1800 watts, the primary winding, operating at 100 volts, needs to draw 18 amps, plus whatever losses there are in the transformer itself.

Not sure on this but at very low voltage, I suspect the wattage required will increase as the motors turn less efficiently.

So an 1800watt motor may be pulling 1900watts due to low voltage. By giving it full voltage, it needs less watts.

Can any one confirm this?

[GordonThree]

smkettner wrote:

GordonThree wrote:
Why would the electrical inspectors have passed such a poorly designed system? 120 down to 102 volts is a huge drop, there must be under-sized conductors in the branch or feeder circuits?

The more connections the more code allows the total to be derated.
The code is just not designed to meet the demands of modern RVs that pull 80+pct of the available power near continuous.

36+ sites on one loop allows for just 41pct of total.

Say 40 sites at 30 amp is 1200 amps. NEC say loop can be 492 amp total. Get a bunch like pianotuna and the voltage just sags away.

NEC 551.73


There are certainly other details and I am not an electrician but I think you get the point. What worked 30+ years ago is not adequate today.

Thank you for the explanation.

[MEXICOWANDERER]

Inductive motor loads are the most fragile. Start your inductive loads, the largest first, then proceed on to resistive loads.

[time2roll]

GordonThree wrote:
Why would the electrical inspectors have passed such a poorly designed system? 120 down to 102 volts is a huge drop, there must be under-sized conductors in the branch or feeder circuits?

The more connections the more code allows the total to be derated.
The code is just not designed to meet the demands of modern RVs that pull 80+% of the available power near continuous.

36+ sites on one loop allows for just 41% of total.

Say 40 sites at 30 amp is 1200 amps. NEC say loop can be 492 amp total. Get a bunch like pianotuna and the voltage just sags away.

NEC 551.73

There are certainly other details and I am not an electrician but I think you get the point. What worked 30+ years ago is not adequate today.

[pianotuna]

Hi,

I find that I can "coast" until bed time--then I turn the water heater on. The next morning after showers and dishes are done I leave the heater on for about one hour. That allows full recovery. The water is hot enough to do the supper dishes.

Snowman9000 wrote:
How often do you need hot water? Considering the ambient temperature, I would think 10 minutes of propane burn prior to taking a shower would be plenty good for the day. That's how we do it, anyway. Does not use much propane.