power cords

[bobgay]

I'm really confused. If these electric fireplaces are 5,100 btu and draw 12.5 amps, it would seem that a 13,500 btu air conditioner would draw too much to run on a 30 amp circuit. Is there a direct correlation between BTU's and amps, or I comparing apples and oranges?

[H. John Kohl]

Well I guess I will have to try and confuse the issue more. The one difference between the 12-volt system and the 110-volt system is the wattage. As stated above if the furnace draws 12 AMPS at 12 volts that equals 144 watts. Watts equals Volts times Amps. However, if the Fireplace draws 12 AMPS that is about 1320 watts. 12 x 110 = 1320. You have to keep your voltage sources separate when calculating power (watts). The convert is what feeds the batteries.

Yes if the furnace draws 14 amps and the converter is only supplying 10 amps then the battery is providing the other two, which will eventually draw down the battery. However when the furnace motor stops the converter still continues to provide 10 amps to the batteries and will recharge them some. The cycle time on the furnace motor will impact whether the battery is discharged or does actually take a charge.

I hope this helps and is not more confusing.

[Parrothead]

John, great post and my high school science/technical chemistry learning just topped back into my head. After all it has been a looong time! So how many amps does the convertor use, is it 10? Or were you just using that as an example?
Happy trails.......................

[dsprik]

That's also what I was curious about... what is the Converter's amp pull when charging the batteries?

[fulltimedreamer]

I may resemble the remark that, "Fools rush in where angels fear to trod.", but I'll give this a go.

Ohm's Law says that P=I*E

P - Power in Watts
I - Current in Amps
E - Voltage in Volts

Power describes the energy required to do a certain amount of work, it doesn't matter if you are using AC (Alternating Current) or DC (Direct Current) to accomplish the work.

For simplicity let's say we want to light a 100 Watt Bulb.

In the first example:

A 100 Watt light bulb will use 8.3 Amps of current with a 12 Volt DC source.

100 Watts = 8.33 Amps * 12 Volts

In the second example:

A 100 Watt light bulb will use .833 Amps of current with a 120 Volt source.

100 Watts = .833 Amps * 120 Volts

From these examples you can see why it is important when we talk about amps that we describe whether we are talking DC Amps or AC Amps. Power is always power, but Amps will vary depending on whether you are talking 12 Volts DC or 120 Volts AC.

In the case of the Parallax Power Components converters used in our coaches, the converter is converting 120 Volts AC to 12 Volts DC to run the lights, refrigerator controller circuit, LP Gas Detector and fan on the LP Gas Furnace, etc. and at the same time keep the battery charged for those times when you don't have shore power available.

I do not know if all Montana's use the same model converter so I will use mine as an example. The 7355 Converter has a maximum output of 55 Amps @ 13.2 Volts DC @ fullload.

55 Amps * 13.2 Volts = 726 Watts

726 Watts is the maximum Power that can be generated. Remember that Power is the same whether AC or DC. So how many AC Amps do we need to produce 726 Watts of useable power?

From the formula used above P=I*E we know that I=P/E.

I = 726 Watts / 120 Volts = 6.05 AC Amps. (Converters are not perfect so we must assume a loss of 10-15% so we will be closer to 7 Amps of AC Power)

Sorry if this has become too technical, but this was necessary to explain how a 30 Amp AC shorepower connection can provide 55 Amps of DC current.

Now to the questions:

lstierw wrote:

Just a question from someone who is not an electrician and I am sure that smarter minds will prevail. If your furnace runs on propane and your blower motor is DC power, Where is the amperage draw?

If your furnace blower draws 12 Amps DC. It is using 12 Amps of the available 55 Amps from your converter.

The power used by the fan would be would be: 12 Amps * 12 Volts = 144 Watts.

The amount of AC Amps that is being used via the converter is: 144 Watts/120 Volts = 1.2 Amps.

The 55 Amp DC converter has 43 DC Amps left to power the lights, refrigerator controller, LP Detector, etc. and still has power available to charge your battery(ies).

dsprik wrote:

Alright... you knew this was coming... If you are pulling 12 amps for your furnace fan motor, is this passed straight through, and matched as a demand amp pull by the converter? Or, as I suspect, does the converter/charger have it's own rating/capacity?

In the example above you can see that the converter/charger has its on rating/capacity of 55 Amps of which you are only using 12 Amps in your example.

The power used would be: 12 Amps * 12 Volts = 144 Watts.

The Amps needed on the AC side of the converter would be: 144 Watts/120 Volts = 1.2 Amps AC


desprik continued:

If this last is true, what happens if the motor is pulling 12 amps from the batteries, and the converter is only pulling, say... 10 amps from the CG supply? The batteries will eventually run dead. So I highly doubt any thinking electrical engineer would ever design something like that. Actually, due to the lack of efficiency in an electric system, due to resistance, you will need much more than a 12 amp input (from the CG) to get much less amps to the batteries, right?

If the furnace fan is pulling 12 Amps DC (144 Watts).

The AC Amps would be: 144 Watts / 120 Volts = 1.2 Amps.

Neither the DC nor the AC Amps would exceed the capabilities of the system. In our example the coach has 55 Amps of DC current available and 30 Amps of AC current available. We can run the furnace fan and many other DC Current loads as well as several AC Current loads while connected to shore power.

And finally, you are correct the conversion from AC to DC or AC to DC via converters and inverters does result in energy loss due to resistance and hysteresis (Isn’t that a fun word?).

I hope this has answered your questions. I apologize for the length of the post and for hijacking the original post.

[fulltimedreamer]

quote:Originally posted by bobgay

I'm really confused. If these electric fireplaces are 5,100 btu and draw 12.5 amps, it would seem that a 13,500 btu air conditioner would draw too much to run on a 30 amp circuit. Is there a direct correlation between BTU's and amps, or I comparing apples and oranges?

Bob,

I'll try to answer your question. As you know both Watts and BTUs are measurements of energy.

1 watt = 3.41 BTUs

So your 5,100 BTU Fireplace heater uses 1,495.6 Watts or 1500 Watts.

5100 BTU/3.41 = 1495.6 Watts or 1500 Watts

Thus your 5,100 BTU fireplace heater requires 12.5 Amps AC.

1500 Watts/120 Volts = 12.5 Amps AC

Since your fire place heater is a resistive load the numbers are easy to arrive at.

In the case of your Airconditioner, because it is an inductive load and also due to the added efficiences of the compressor and its ability to remove heat, the formula's are not so straight forward.


We would expect your 13,500 BTU Airconditioner to use 3,960 Watts since:

13,500 BTU/3.41 = 3958.9 Watts or 3960 Watts

Which would result in 33 Amps AC needed to power the Airconditioner which we know isn't possible cause we're only getting 30 Amps from our shorepower connection.

3960 Watts/120 Volts = 33 Amps AC

It looks like there is not enough AC Amps to run the Airconditioner when you do a straight BTU to Watt conversion, but the measured current to run your 13,500 BTU Airconditioner is more like 14 - 16 Amps AC. In this case you are only using 1800 Watts of power to do 3690 Watts of work. An HVAC guy could probably explain the efficiencies of an airconditioner compressor better than I can. I do know that airconditioner/heat-pump compressors are generally very efficient. That is why I use a heat-pump to heat my home in the winter. (Read - CHEAPER than GAS!!) It is adual fuel system that switches to gas when it becomes too cold outside for the heat-pump to be efficient.

Anyway, this leaves 14 - 16 Amps AC on a 30 Amp Shorepower connection available to run your other AC appliances - including the Television, Microwave, 120 Volt Lamps, Converter, etc.

Hope this answers your questions.

[bobgay]

Fulltimedreamer, that explanation helps. I'm in over my head, but I know more now than I did. Thanks for the post.
Bob

[Garin1]

People, this has been one of the best posts of the forum. It's why we come here. A big "KUDOS" to everyone.

[CountryGuy]

fulltimedreamer,

Bit too techy for this ole Gal, but, I know Al is gonna enjoy reading this post! On an attempt to re-read I have to say, GREAT JOB!!!!!!!

[Lstierw]

Fulltimedreamer...thanks for adding clarity to the subject.

[dsprik]

How many amps does the converter draw, then under conditions where the furnace could be cycling in, say... 25 degrees at night... thermostat set at around 68... lights on in the coach (evening here in N. MI watching TV/DVD - I am sure the TV is not 12v)?

[jpbcny]

Has the MARINCO 50F/30M Adapter For: $75.54

And the MARINCO 50F/30M with 25'Cord For: $ 90.80

A few months ago I bought my MARINCO 50F/30M Adapter From: PPL Motorhomes for $64.95, but that was a SALE PRICE, they have since raised it to: $77.29

[Montana_4221]

jpbcny,
Thanks for the link. That's a much better price than what I've been finding.

[Mac]

JPBCNY,

Thanks from me as well, I'd been looking and actually putting it off waiting until spring but I went online and ordered one.

I LOVE this site and all the people!

mac

[Montana Sky]

Dave,
To play the tv and dvd player you will have to be plugged into shore power. Cannot run of of the battery alone.

[don m]

just remember one thing a 50 amp cord has 50 amp wire a 30 amp cord has 30 amp wire if you pull over 30 amps on an 30 amp cord its going to heat up if its alot over 30 amps it could burn!!!

[fulltimedreamer]

quote:Originally posted by dsprik

How many amps does the converter draw, then under conditions where the furnace could be cycling in, say... 25 degrees at night... thermostat set at around 68... lights on in the coach (evening here in N. MI watching TV/DVD - I am sure the TV is not 12v)?

Dave,

I'm not sure I followed the question? If you are connected to shorepower then the converter will draw what it needs to run the furnace fan, lights, etc. as well as keeping your battery(ies) charged. The television and DVD would run off shore power.

If shorepower is not available you could run your furnace fan off the battery(ies) up to the rated Amp/hours of the battery(ies). Amp/hours can be calculated as:

Ah = I * H

Ah - Amp/hour
I - Current in Amps (Current for all 12 Volt Loads)
H - Time in hours (Number of hours to draw load)

You would want to have batteries that can supply approximately twice the number of Ah you expect to use since you don't want to discharge a battery by more that about 50%.

It would also be possible to run your 120 Volt television and DVD player via an inverter which changes 12 Volts DC to 120 Volts AC. An inverter does the exact opposite of what the converter does. Many RVers add these to their coaches for those times when shorepower isn't available.

[dsprik]

Thanks for trying to understand me, Lamar. When you get me figured out, let my wife know, she still struggles in that dept.

Maybe this will help: What's the maximum amps the converter can pull off of shore power, and under what conditions would it do that?

And you just fed my mind another question: What do inverters (given a typical wattage conversion inverter) pull for amps off the batteries? I take it the converter can keep up by keeping the batts charged while the inverter is running?

Hope that's clearer than my last attempt.

[H. John Kohl]

Dave,
Your question is a good one but not so easy to answer. It is like a bank account money or Watts in and money or Watts out.

Your battery is like a storage tank for water. If your useage uses more than can be pumped in at one time then it will go down.

Normally your converter will have a maximum charge rate on the battery. Now the converter is not 100% effecient therefor it uses more WATTS from the "shore power" (120VAC) than it sends in watts to the battery (12VDC).
If it is 75% efficent then 112.5 watts to the battery will require 150 watts of shore power. Some times I have hear a conversion system is only 50% efficent.
That means 9.38 amps at 12 volts on the battery side and 1.25 amps on the 120 volt side. This is a perfect example why it is better to use a common reference of WATTS than thinking amps on both sides. Just remember the AMPERAGE on the battery is 10 times greater for the same wattage on both the battery and shore power side.
When talking about volts and amps you also need to remember that an electrical circuit will normally draw only the amperage it needs to do the job.
I hope this helps. There have been some great clear technical answers on this post.
Good luck and tow safe.

[dsprik]

Thanks, H. John. That was good. Can't remember... I don't think inverters are part of the included pkg when a Montana is ordered. Trying to remember discussions w/o doing a search. What watt rating inverter should we buy, if this is not included.

I'll try to search. I like Parrothead's comments on the new Montanas' 50 amp setup (not being strictly split 30/20). Probably end up wrestling the 50 amp cord as often as I can use it under the new setup.