Electrical calculations - R-pod Owners Forum

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techntrek

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Topic: Electrical calculations
Posted: 15 Jan 2010 at 8:41am

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). 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).

Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual

TerryM

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Posted: 15 Jan 2010 at 9:03am

Interesting! I think that if you twist 2 wires together you would increase the resistance of the wire. If you could melt them together into 1 wire it should decrease the resistance compared to the above wire.

I'm in the process of hooking up an amplifier in my Jeep. I'll be running about 15' of 8Ga. wire. At full output the amp. is pulling about 50 t0 55amps. The chart I use goes from 5amp. to 200amps. But it jumps from 50amps. to 100amps. So to be safe I'm using the 100amp. figure to come up with the 8Ga. wire. Bigger is better some times! Big smile

Terry

PS: the amp at full output would be 650 watts. I'll run it at 400 most times.

RP-175 W/Lift Kit 2011 Ford F-150 4X4
Saint Augustine, FL: The first permanent European settlement in the USA: 1565

rpodcamper.com View Drop Down

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Posted: 15 Jan 2010 at 9:27am

Terry it sounds like you are living a second childhood. I should hook you up with my son that blew his hearing out from his amp! Star

(Says the man who owns and rides a motorized bar stool around)

TerryM

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Posted: 15 Jan 2010 at 9:46am

Originally posted by rpodcamper.com

Terry it sounds like you are living a second childhood. I should hook you up with my son that blew his hearing out from his amp! Star

(Says the man who owns and rides a motorized bar stool around)

No, it's for a CB radio.

Terry

RP-175 W/Lift Kit 2011 Ford F-150 4X4
Saint Augustine, FL: The first permanent European settlement in the USA: 1565

TerryM

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Posted: 15 Jan 2010 at 9:55am

Circuit Amps      (Wire   Guage   For   Desired   Length)

       3’   5’      7’   10’   15’   20’   25’
0 to 5          18   18   18   18   18 18   18
6       18   18     18   18   18      18   16
7       18   18   18   18    18      18   16
8                       18   18   18    18   18      16    16
10         18   18   18   18    16      16    16
11       18   18   18    18   16      16   14
12                     18   18   18   18    16      16    14
15       18   18   18   18   16      16    14
18                     18   18    18    16   14      14    12
20       18   18   18   16   14     12    10
22         18   18    18   16   12      12    10
24         18   18    18   16   12     12    10
30                     18   16    16   14    10      10    10
40         18   16   14   12    10      10 8
50       16   14    12    12    10      10      8
100       12   12   10   10      6      6   4
150         10   10      8   8       4      4   2
200       10   8   8   6       4      4   2

The chart above offers a good rule of thumb for selecting the proper gauge wire for a specific use length and amp. Load on a 12v DC circuit.

Terry

RP-175 W/Lift Kit 2011 Ford F-150 4X4
Saint Augustine, FL: The first permanent European settlement in the USA: 1565

techntrek

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Posted: 15 Jan 2010 at 1:04pm

Another question from the same person via PM:

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.

Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual

TerryM

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Posted: 15 Jan 2010 at 1:12pm

Lets see....a 15 FOOT TV & a 9 FOOT DVD...hmmm, I should know that. I know! It's 15 million amps!! Tongue

Terry

RP-175 W/Lift Kit 2011 Ford F-150 4X4
Saint Augustine, FL: The first permanent European settlement in the USA: 1565

techntrek

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Posted: 15 Jan 2010 at 1:21pm

Originally posted by TerryM

Interesting! I think that if you twist 2 wires together you would increase the resistance of the wire. If you could melt them together into 1 wire it should decrease the resistance compared to the above wire.

I'm in the process of hooking up an amplifier in my Jeep. I'll be running about 15' of 8Ga. wire. At full output the amp. is pulling about 50 t0 55amps. The chart I use goes from 5amp. to 200amps. But it jumps from 50amps. to 100amps. So to be safe I'm using the 100amp. figure to come up with the 8Ga. wire. Bigger is better some times! Big smile

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.

Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ Pod instruction manual

TerryM

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Posted: 15 Jan 2010 at 3:28pm

I knew there was a voltage drop. That does concern me. The amplifier's output goes into antenna, not a sound system. I guess I'll just have to deal with the problem somehow. I might be able to shorten the power run some.

Terry

RP-175 W/Lift Kit 2011 Ford F-150 4X4
Saint Augustine, FL: The first permanent European settlement in the USA: 1565