Installing an inverter...
Cut & Paste Expert
Joined: May 2007
Posts: 13,322
From: NW Arkansas
Here is a good table to use for this from over at the12volt.com(this is intended for car amplifiers however, so keep that in mind, for many listed there 1 gauge lower would be fine for this). Figure, usual voltage (optimal) for automotive use is 13.8v, so, 400w divided by 13.8, gives you the rough outline of it´s supposed draw(in amps, so around 30 as best case, more than that is far more likely).
What planet are you on?
Most run from the cigar lighter jack and thats
limited to 20Amps DC on most vehicles....
This math wont work for 12V input and 120VAC 400W output.
AC and DC are different pigs.
400W divided by 120VAC gives you the AC amps available to you.
Not the DC amps needed for 400 Watts of 120VAC.
Read the AWG on the input wires.
10G =s 30A
12G =s 20A
14G =s 15A
16G =s 12A
18G =s 10A
The fuse rating will tell you the12VDC input amps
to get the 400Watts of 120VAC output.
My 400Watt inverter if fine with 4Amps of DC input.
18G wire is fine for the 12V side.
Unless the run is more than 6 feet or so.
My TrippLite 400w is as clean as a bean on the scope.
But it did run $40 new on eBay....
Last edited by 157dB; Jun 23, 2011 at 12:00 PM.
Thread Starter
2nd Gear Member
Joined: Jul 2009
Posts: 341
From: CA
Normally Newegg is a fantastic site (computer nerd hangout), with great reviews and the best prices. But I didn't pay that price for the inverter, I got it off amazon for $27.
Thread Starter
2nd Gear Member
Joined: Jul 2009
Posts: 341
From: CA
30A DC for 400 Watts of 120VAC?
What planet are you on?
Most run from the cigar lighter jack and thats
limited to 20Amps DC on most vehicles....
This math wont work for 12V input and 120VAC 400W output.
AC and DC are different pigs.
400W divided by 120VAC gives you the AC amps available to you.
Not the DC amps needed for 400 Watts of 120VAC.
Read the AWG on the input wires.
10G =s 30A
12G =s 20A
14G =s 15A
16G =s 12A
18G =s 10A
The fuse rating will tell you the12VDC input amps
to get the 400Watts of 120VAC output.
My 400Watt inverter if fine with 4Amps of DC input.
18G wire is fine for the 12V side.
Unless the run is more than 6 feet or so.
My TrippLite 400w is as clean as a bean on the scope.
But it did run $40 new on eBay....
What planet are you on?
Most run from the cigar lighter jack and thats
limited to 20Amps DC on most vehicles....
This math wont work for 12V input and 120VAC 400W output.
AC and DC are different pigs.
400W divided by 120VAC gives you the AC amps available to you.
Not the DC amps needed for 400 Watts of 120VAC.
Read the AWG on the input wires.
10G =s 30A
12G =s 20A
14G =s 15A
16G =s 12A
18G =s 10A
The fuse rating will tell you the12VDC input amps
to get the 400Watts of 120VAC output.
My 400Watt inverter if fine with 4Amps of DC input.
18G wire is fine for the 12V side.
Unless the run is more than 6 feet or so.
My TrippLite 400w is as clean as a bean on the scope.
But it did run $40 new on eBay....
3rd Gear Member
Joined: Apr 2011
Posts: 917
From: Wisconsin
30 Amps is the correct current draw on the DC side. If you have an inverter that can supply 400W @ 120V, it will also draw 400W, or higher. Since Wattage = Voltage x current, the DC into the inverter will max out at 33A at 12V. You can easily confirm this by looking at the inverter, it will tell you the max current input, and the instructions will tell you the size of the fuse to use. You stand a real good chance of lighting your car on fire if you use 18 gauge wire. I use a cigarette lighter inverter that can supply 1 amp at 120V, not wiring is needed. You can pick one up at Best Buy or Walmart for under $30.00.
Joined: Oct 2010
Posts: 18
From: AL
This math wont work for 12V input and 120VAC 400W output.
AC and DC are different pigs.
400W divided by 120VAC gives you the AC amps available to you.
Not the DC amps needed for 400 Watts of 120VAC.
Read the AWG on the input wires.
10G =s 30A
12G =s 20A
14G =s 15A
16G =s 12A
18G =s 10A
The fuse rating will tell you the12VDC input amps
to get the 400Watts of 120VAC output.
My 400Watt inverter if fine with 4Amps of DC input.
18G wire is fine for the 12V side.
Unless the run is more than 6 feet or so.
AC and DC are different pigs.
400W divided by 120VAC gives you the AC amps available to you.
Not the DC amps needed for 400 Watts of 120VAC.
Read the AWG on the input wires.
10G =s 30A
12G =s 20A
14G =s 15A
16G =s 12A
18G =s 10A
The fuse rating will tell you the12VDC input amps
to get the 400Watts of 120VAC output.
My 400Watt inverter if fine with 4Amps of DC input.
18G wire is fine for the 12V side.
Unless the run is more than 6 feet or so.
You're doing your math wrong. The amp draw isn't going to be the same on both sides of the inverter. The inverter is quite literally converting amps into voltage. So you're looking at 30+ amps on the DC side and then only 4 amps on the AC side.
Cut & Paste Expert
Joined: May 2007
Posts: 13,322
From: NW Arkansas
Joined: Oct 2010
Posts: 18
From: AL
Most vehicular accessory sockets that are controlled through a vehicle’s ignition switch are fused at 15 amps. This limits inverter output to approximately 140 watts. A user has some options to power appliances to 400 watts.
1. For temporary use, connect the battery clip adapter cord to the inverter and
follow the procedure located under the “Connecting the Inverter” section.
2. Install a separate fused DC accessory socket wired to the vehicle’s battery. Fuse this circuit at 50 amps.
There is no danger in leaving the inverter directly wired to the battery if the inverter is turned OFF after use. Further, all inverters will shut off when the battery discharges to 9.5 volts. This lower battery voltage still allows the vehicle to start.
1. For temporary use, connect the battery clip adapter cord to the inverter and
follow the procedure located under the “Connecting the Inverter” section.
2. Install a separate fused DC accessory socket wired to the vehicle’s battery. Fuse this circuit at 50 amps.
There is no danger in leaving the inverter directly wired to the battery if the inverter is turned OFF after use. Further, all inverters will shut off when the battery discharges to 9.5 volts. This lower battery voltage still allows the vehicle to start.
4th Gear Member
Joined: Dec 2010
Posts: 1,597
From: Gallifrey
Originally Posted by From the manual again
This inverter is powered from 12 volts DC and it will continuously deliver 400
watts (800 watts momentary peak) AC power at 120 volts, 60 Hz.
AND
If the tool or device is rated in amps, multiply the amps by 120 (120V AC) to determine the watts. For example, an appliance rated at 0.5 amps will draw 60 watts.
AND
AC Outlet — This outlet can supply up to 3.3 amps at 120V AC 60 Hz
watts (800 watts momentary peak) AC power at 120 volts, 60 Hz.
AND
If the tool or device is rated in amps, multiply the amps by 120 (120V AC) to determine the watts. For example, an appliance rated at 0.5 amps will draw 60 watts.
AND
AC Outlet — This outlet can supply up to 3.3 amps at 120V AC 60 Hz
Originally Posted by OP's cobra manual
12 volt DC power source (such as a vehicle battery).
The power source must provide
between 11 and 15 volts DC and
be able to supply enough current to
run the test load. As a rough guide,
divide the wattage of the test load by
10 to get the current (in amperes)
the power source must deliver.
The power source must provide
between 11 and 15 volts DC and
be able to supply enough current to
run the test load. As a rough guide,
divide the wattage of the test load by
10 to get the current (in amperes)
the power source must deliver.
I certainly wouldn't use any lower gauge wiring for the same run that comes with the OP's inverter. And likely 157db's is far more efficient, as it lists 90%. That cobra probably isn't.
However, 400w at 12v does not equate to the inverted wattage of 400w at 120v for amps drawn. This rating is derived from the output AC voltage, so your usable max (except surge) is around 4amps for the AC current. They won't be an exact correlation. Sorry OP, I was a bit inebriated when I posted, though I was helping at the time since nobody else responded the original time. And I never actually installed the one I bought. =)
The gauge wiring listed should still hold for distance that I originally linked from the12volt.com. I re-read your manual (also in my original post), but it doesn't give as many specifics as 157db's.
OP's doesn't have a fuse upon looking at the manual, looks like it actually uses a breaker for the circuit. Normally you could just look at the blade fuse most come mounted directly on them, the same as a decent sound amp. Which would tell you it's max capable DC (before inversion) amperage that the circuit was set to tolerate. I would use an in-line fuse at the battery in addition that is slightly less than that, and slightly higher (better, as in lower #) wiring gauge for the length of the run you plan to do. I assume the original point of this was you wished to (semi?)permanently wire it up directly to the battery, yes?
10guage might be overkill, but if it's "iffy" I'd err on the side of larger for the length of the run, as less resistance means less heat anyhow. Not only avoiding fire, but lessening the heat factor for the unit itself. But yeah, 18 gauge..no.
I'd set it up with it's max wattage/load in mind regardless of the draw you plan to put on it. So bottom line, 40amps according to the manual, and I'd fuse it at the battery less than that.
Last edited by wayne613; Jun 25, 2011 at 10:59 PM.