aka The frog wrote:Hi Les
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Tear Les wrote:I don't believe for the majority of folks that it has much merit since the wiring size to run most of what's in small trailers for lighting is in the 16 to 14 gauge size range; hardly an argument for smaller wire size when the AC wiring is likely to as large and there are 3 conductors rather than two (and if using the same wire; then more expensive and heavier)..
For a given quantity of power transmitted, the wire size would be inversely proportional to the voltage used; or to put it another way, the allowable length of a circuit, given a wire size and allowable voltage drop, would increase approximately as the square of the distribution voltage.
To put that simply for any given ampage AC wiring can be a lot smaller and lighter than DC wiring
Tear Les wrote:They also don't point out inverter losses; the cheaper models have more losses than the more expensive inverters in general and if it gets to the 10% or 15% range it isn't trivial..
All invertors have losses the best if matched to load exactly can Achieve only 7% loss, but virtually every invertor is losing 10% Plus it goes with the territory.
There is a physical limit to the size of wire than can or should be used. I'm well aware of the limitations imposed since I design electrical systems for mobile applications for a living utilizing AC and DC circuits.
In therory you could use much smaller wire for AC lighting than is necessary for DC lighting. In the real world that won't happen because for physcial strength and for convenient connections you wouldn't drop smaller than 16-gauge on either the AC or the DC circuit for general use (small gauge or sensor wiring excepted). If you used 14-gauge for DC you could certainly use 16-gauge for AC but you'd also add a conductor so the complexity, weight, and cost has gone up. How much smaller the wire "could" be if AC were used is therefore a moot point relative to the lighting and small appliance (fans, etc) systems typically installed in small trailers.
And that's the gist of my comments. It isn't that some can't be done but rather that what is done is not necessarily a practical application of availalbe technology; unless it's just something you "want" to do (then it ought to be labeled "experimental").
Inverter losses are "part of the territory" which is exactly why they ought to be limited for use in those applications where they're necessary rather than because they're available; a loss is a loss and there's no reason to build losses into a system that didn't require it in the first place.
The safety issue is the single biggest reason I'd hate to see a bunch of recreational trailers set up with large storage banks of batteries and inverters used to power the unit for general purposes (rather than a few necesary applicances). The more often a user interfaces with a system like that the more likelihood there is for a shock hazard.
A mobile unit isn't like a house where a good ground system is an easy thing to accomplish and without a proper ground system it's a rolling death trap. Most folks don't know or understand the regulations involving establishing grounds (that is when to establish ground to the unit and when to establish ground to a grid) in a mobile unit and what they don't know in this case could kill them. For instance; if you have a shore power cable and a built-in inverter what do you have installed that allows the ground to switch from shore power ground to chassis ground since each of those systems has a different requirement?
This doesn't apply to 100% AC units since the only way they can get power is though a shore power cord which (hopefully) has been wired properly. But, one also has to hope that the last person to handle the wiring at the power pole also got the receptacle wiring right. A crossed ground and neutral wire could have dire results. Polarity should alaways be checked (with a plug-in polarity checker) before energizing any mobile unit on AC power.
My professional opinion is that AC power should stay in houses and DC should be used in mobile application whenever possbile (and always staying in an RV park with shore power is not really mobile in the electrical sense; they're moving houses from that persepctive). AC power should be reserved for those appliances that really need them and then connected to power poles or inverters where the polarity has been tested for a proper ground path.
AC power in a mobile unit is the single most dangerous thing that most folks ever install. It's also the system that's least understood and most taken for granted since we (typically) live with it everyday in our homes and offices. It's not to be trifled with; it can and will kill if not properly implemented.
by RAYVILLIAN » Sun Jun 24, 2007 11:27 am 
