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Car Forum / Driving, Maintenance, Tuning / RVs / October 2007Voltage conversion for RVs
I'm planning some rv-like modifications to a Ford Bronco and have a question. I presume that rv's have converters for turning 120v 30amp ac shore power to 12v dc power, for use in the vehicle's accessory circuit (am I already wrong?). I see ac to dc converters online that only describe themselves as '55 amp', and I don't know what they mean by that. In stepping down from 30a 120v ac to 12v dc power, what kind of amperage can I hope to get? Is amperage lost in the conversion? >I'm planning some rv-like modifications to a Ford Bronco and have a >question. I presume that rv's have converters for turning 120v 30amp >ac shore power to 12v dc power, for use in the vehicle's accessory >circuit (am I already wrong?). I have never seen such a thing... I have seen more systems that use inverters to crank the 12V accessory power up to 120VAC and use 120V accessories both on the road and on shore power. But there's no reason not to build such a thing. >I see ac to dc converters online that >only describe themselves as '55 amp', and I don't know what they mean >by that. In stepping down from 30a 120v ac to 12v dc power, what kind >of amperage can I hope to get? Is amperage lost in the conversion? Power is always conserved, and power is current times voltage. So if you take a 30A 120V circuit, step it down and rectify it, you should be able to get 300A at 12V, if there are no losses. This being the real world, 250A is all you can reasonably expect. But that is a hell of a lot of power. --scott  Signature"C'est un Nagra. C'est suisse, et tres, tres precis." >>I'm planning some rv-like modifications to a Ford Bronco and have a >>question. I presume that rv's have converters for turning 120v 30amp [quoted text clipped - 5 lines] >accessories both on the road and on shore power. But there's no reason >not to build such a thing. Most RVs have one. The converter takes the 120 VAC and converts it to 12 VDC for the 12 volt house circuits (lights, fans, furnace, water pump, etc.) and usually includes a charger for the on board 12 volt batteries. >>I see ac to dc converters online that >>only describe themselves as '55 amp', and I don't know what they mean [quoted text clipped - 7 lines] >of a lot of power. >--scott Alex >> I'm planning some rv-like modifications to a Ford Bronco and have a >> question. I presume that rv's have converters for turning 120v 30amp [quoted text clipped - 5 lines] > accessories both on the road and on shore power. But there's no > reason not to build such a thing. Never been around RV's, have ya? http://www.progressivedyn.com/ Signature"You can say any foolish thing to a dog, and the dog will give you a look that says, 'My God, you're right! I never would've thought of that!'" --Dave Barry http://www.bobhatch.com http://www.tdsrvresort.com > Power is always conserved, and power is current times voltage. AC power is voltage times current times the cosine of the phase angle between the voltage and current. http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powerac.html By adding a capacitor to the load in your house it is possible to make the power meter read zero and still power your house. >> Power is always conserved, and power is current times voltage. > [quoted text clipped - 5 lines] > By adding a capacitor to the load in your house it is possible to make the > power meter read zero and still power your house. Yep, when you have a phase difference between current and voltage ( a reactive rather than pure resistive circuit), power is not what would be expected by the resistive example (Ohms law). I am told that, at least in the early days, the old style power meter could also register incorrectly excessive consumption of power for the same reason. I dont know whether the meters are now better able to reflect a true value, or not. >>> Power is always conserved, and power is current times voltage. >> [quoted text clipped - 18 lines] > I dont know whether the meters are now better able to reflect a true > value, or not. Apparently the new meters are quite sophisticated. Here's a device for sale that reduces power factor and your utility bill: http://www.on-timeelectric.com/powersave.html?mode=preview The old meters could only read real power not reactive and could be tricked. There are still ways to get AC power into your RV for free. One could wind a large coil on the perimeter of the roof and park close (but asymetrically) to power lines and couple in power that way. A friend of mine had a set of copper ground rods driven in under high tension lines and another set several hundred feet away from the lines. He could pull quite a bit of power from it, enough to light a shed and some lawn lights. >> Power is always conserved, and power is current times voltage. > >AC power is voltage times current times the cosine of the phase angle >between the voltage and current. > >http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powerac.html This is correct but basically irrelevant to the subject at hand where we can basically assume everything is real power. >By adding a capacitor to the load in your house it is possible to make the >power meter read zero and still power your house. This seems like a good way to get a massive surcharge in your bill for having a reactive load (which the power company WILL do). --scott Signature"C'est un Nagra. C'est suisse, et tres, tres precis." >I'm planning some rv-like modifications to a Ford Bronco and have a >question. I presume that rv's have converters for turning 120v 30amp >ac shore power to 12v dc power, for use in the vehicle's accessory >circuit (am I already wrong?). You are correct, if you limit your phrase "accessory circuit" to the RV furnishing -- house lights, water pump, furnace blower, etc. -- and not to the 12v system normally used with the engine and chassis. GB in NC > I'm planning some rv-like modifications to a Ford Bronco and have a > question. I presume that rv's have converters for turning 120v 30amp [quoted text clipped - 3 lines] > by that. In stepping down from 30a 120v ac to 12v dc power, what kind > of amperage can I hope to get? Is amperage lost in the conversion? What they mean is that they will convert 120 vac to 12 vdc up to 55 amps.It draws about 6 amps of ac power to do this. I don't think you'll ever have to convert all 30 amps of ac power to dc that would require something the size of stick welder, it would make close to 300 amps. Tinker The 55 amp would relate to the 12v output. Thusly, you'd have about 5.5amp on the 120v input side. s > I'm planning some rv-like modifications to a Ford Bronco and have a > question. I presume that rv's have converters for turning 120v 30amp [quoted text clipped - 3 lines] > by that. In stepping down from 30a 120v ac to 12v dc power, what kind > of amperage can I hope to get? Is amperage lost in the conversion? You seem to have gotten a fine set of half answers. Most RVs a have a converter or converter/charger. That device can be what ever maximum current capability the owner chooses to purchase. > I'm planning some rv-like modifications to a Ford Bronco and have a > question. I presume that rv's have converters for turning 120v 30amp > ac shore power to 12v dc power, for use in the vehicle's accessory > circuit (am I already wrong?). So - This is a correct assumption. >I see ac to dc converters online that only describe themselves as >'55 amp', and I don't know what they mean by that. This world is all 12V (actually 13.5v nim.), so a 55 amp converter will supply 12v equipment up a maximum of 55 A (after that all bets are off). > In stepping down from 30a 120v ac to 12v dc power, what kind > of amperage can I hope to get? As another posted, you will not convert all of a 120V 30A feed to 12VDC with anything smaller than a welder. You can count on getting what ever the converter is rated to do. >Is amperage lost in the conversion? It is not lost (expect a small part as heat), but you will only get what the system can use. >I'm planning some rv-like modifications to a Ford Bronco and have a >question. I presume that rv's have converters for turning 120v 30amp [quoted text clipped - 3 lines] >by that. In stepping down from 30a 120v ac to 12v dc power, what kind >of amperage can I hope to get? Is amperage lost in the conversion? volts times amps equals watts. Output watts equals input watts times efficiency. Apply some seventh grade algebra: Aout = Ain (Vout/Vin) * Efficiency. 120V * 30A = 3600W. w/ 75% efficiency, expect 2700 watts out. That's 12V @ 225 A. In a typical house, there is no such thing as 120 volts 30 amps. the biggest breaker in my power box for 120 volts is 15 amps. There might be a a 30 or 40 amp breaker at 240 volts for the stove dryer or air conditioner. If you need 12 volts when you are out camping then a battery charger should keep a 12 volt auxilary battery charged up. I have even seen some people use a solar panel to keep the extra battery charged when camping. > In a typical house, there is no such thing as 120 volts 30 amps. > the biggest breaker in my power box for 120 volts is 15 amps. There [quoted text clipped - 4 lines] > seen some people use a solar panel to keep the extra battery charged > when camping. Go to the breaker aisle at home depot or lowes and LOOK at the breakers. The only reason there is no such thing as 120V 30A in your house is nobody needed a circuit of that rating. I have several in my house, not the least of which is the one my RV plugs into! I thought RORT got bad sometimes but, Alt.RV makes it look positively sane. Biggest collection on morons I have ever seen! SignatureMatt Delete nospam for email "Where did you say we are going again, Dear?" snipped > Go to the breaker aisle at home depot or lowes and LOOK at the > breakers. The only reason there is no such thing as 120V 30A in your > house is nobody needed a circuit of that rating. I have several in my > house, not the least of which is the one my RV plugs into! snipped Gotta go along with this. You can put any 120v breaker in a box, regardless of what the wires are rated for. Kinda like the old fuse days when a penny could be used to keep an overloaded circuit live <g>. Anyone could add or substitute a 30 amp breaker for one of lesser rating as long as the wiring to the outlet(s) were changed to 10 ga. Our first house had only a 120v service. No. 6 wires from transformer to house broken down to two 30 amp fuse boxes. All circuits from those two boxes were 15 amp. Former owner had added a third apartment to the house which strained the service a lot. Replaced a few 30 amp fuses before folks got used to the idea they couldn't run every darn appliance at once. Hugh > Go to the breaker aisle at home depot or lowes and LOOK at the > breakers. The only reason there is no such thing as 120V 30A in your > house is nobody needed a circuit of that rating. I have several in my > house, not the least of which is the one my RV plugs into! Hmmm - your unit must not be wired for 50A @ 240V like mine is... For that matter, you must not do much heavy welding or machine [lathe, mill, etc.] work either... Since a 30A @ 240V breaker is actually 2 30A @ 120V breakers that are "twinned", take a closer look at those used for Central Air Conditioning systems. (Some of these are wired for as much as 100A - especially those in "all-electric" houses that use Electric Central Heating. [BTDT]) > I thought RORT got bad sometimes but, Alt.RV makes it look positively > sane. Biggest collection on morons I have ever seen! Only lately - since the TROLLs moved in - but, for the most part, this NG has, usually, been more polite. Killfile the TROLLs and things get a lot quieter. <G>
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