This came to me via PM, thought I would make a thread out of it.

 

What is the amperage draw of a 400 watt inverter vs an 800 watt inverter?

 Also if I use 2 strands of 14 gauge wire twisted together, will the resistance decrease and by how much. The dia. of 14 ga. is .0641, double would be .1282, will this have the same characteristics of 8 ga wire?

 Thanks

 

Most consumer inverters run on 12 volts DC.  To calculate for watts, you use the formula P=IE, where P is power (watts), I is current (amps), and E is volts.  So here we have 400 = I x 12.  400/12 = 33.3 amps.  And 800 = I x 12, which equals 66.7 amps.

 

There are tables that show the maximum current on wires at each gauge of wire (here is one: http://www.powerstream.com/Wire_Size.htm - http://www.powerstream.com/Wire_Size.htm ).  Wire gauge is opposite what you might think, the smaller the number the larger the wire diameter.  The rule-of-thumb when you use two strands of the same gauge wire is to move 2 sizes larger on the chart (2 numbers smaller).  Two 14 gauge wires roughly equals one 12 gauge wire (compute for area of the cross-section).

 

When you use a portable 12volt dvd player how much current does it draw; say a 9' dvd player and also a 15' tv/dvd player?

 

If it has a standard 12 volt plug for a cigarette lighter it doesn't use more than 12 amps.  Those outlets have 15 amp fuses on them, and they shouldn't be more than 80% loaded.  15 x .8 = 12.  However they most likely draw far less than that, I would guess around 5 amps.  If a DC appliance comes with its own "wall-wart" you can look to see what its maximum amperate output is to get a feel for what the appliance uses.  If the transformer puts out 5 amps max, the appliance probably uses an amp or two below that.  You might find the exact number in the user manual on the specifications page.  DC appliances aren't as easy as AC appliances, which usually have nameplates with this info on the back.

 

By twisting 2 wires together you are doubling the total surface area (low voltages tend to travel on the outer surface of a wire), so you are cutting total resistance.  Melting the wires together would do the same, by doubling the copper involved just in a different way.

 

8 gauge, 55 amps @ 12 volts, 15 foot run one-way gives a 10.7% voltage drop.  Dropping to 400 watts (33 amps), gives 6.4%.  I would use something larger.  Especially if the amp is driving a subwoofer which pretty much needs peak power or little at all, so you could get clipping or an amp shutdown if you crank it up.