Cobra CPI 1575 1500 Watt 12 Volt DC to 120 Volt AC Power Inverter

12VDC Wire size and fuse considerations, October 4, 2009

By 

D. Warwick (NC USA) –

  

This review is in response to some comments regarding wire and fuse size selections for this inverter and others like it.

In response to other review quoting a table from the National Electric Code (NEC). While the table that was posted is correct for 115VAC (household voltage) is does not translate directly to 12VDC (automotive voltage). When dealing with a 12 volt system there is less concern with wires overheating, at least in the way that the NEC considers it, and far more concern with voltage drop. When running wiring a reasonable distance (a couple hundred feet or so) there is little concern with voltage drop with AC power. But when dealing with DC, simply increasing your wiring distance 5-10′ can have a drastic effect on voltage drop. That is why this inverter should be wired with, at the very least, #4 wire and really should be installed with #2 or thicker, especially if you need to run wire 15 or 20 feet or more.

The purpose of a fuse in a DC system is to protect the rest of your electrical system in the event of a short. That is why you always want to install your fuse as close to the battery as possible (no more then 3′ if you can help it). As far as the amperage goes, there is a really good quick reference chart for automotive wire size that can be found at Tessco’s web site here:[Turns out I can’t link to an outside page so I’ll paste the contents of the table at the bottom]. But to boil it down, you can safely fuse a 12VDC circuit at 150, or 200 amps, or more on #4, #2 or #0 wire.

Now this unit at full load is something like 85% efficient. So if you do the math — 1500 Watts / 12 Volts / .85 efficiency = ~147 amps, A 150 AMP FUSE IS REQUIRED. If you take peak power into account you would need to double that to a 300 amp fuse, but because the inverter cannot run at peak power for more then a moment, a slow-blow 150amp fuse may work just fine, or perhaps you could step up a 200amp. I wouldn’t go all the way to a 300 amp fuse unless you really found that you needed it and had the inverted is wired in such a way that it could handle that level current (read: installed using very heavy gauge wire). Now if your vehicle is running while your inverter is working the amperage will be a little lower because your dealing with 14.4 VDC rather then 12 volts (watts = volts x amps -or- amps = watts / volts). But at the very least, a 150 amp ANL fuse is very good idea and not at all over sized.

Just a FYI on fuses, most car stereos are installed with AGU style fuses, but they only go up to 100 amps.

You will need an ANL style fuse to get one rated at 150 amps or more. See these links for examples:
Kicker 09ANL150 150 Amp ROHS Compliant ANL Fuse – Package of 2
Scosche EWFH Single ANL Fuse Holder

Wire Gauge Selection Table 12 Volt Circuit

Wire Distance
AMPS 3′ 5′ 7′ 10′ 15′ 20′ 25′
—————————
<5 18 18 18 18 18 18 18
6 18 18 18 18 18 18 18
7 18 18 18 18 18 18 18
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 14 14 12
18 18 18 16 16 14 14 12
20 18 18 16 16 14 12 10
22 18 18 16 16 12 12 10
24 18 18 16 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

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