inverter question

[BurbanAZ]

I have just a basic inverter that I got from an auto parts store. It came with short cables with clamps to connect to the battery. I want to hard wire it permanently, would it damage the inverter?, also would it be a fire hazard or something constantly connected, or drain the battery? This is what i have.

http://www.oreillyauto.com/site/c/d...p?ck=Search_inverter_-1_3750&keyword=inverter

 

[dwh]

Sure, inverters are often hard-wired (the bigger ones pretty much always are).

BUT...

800w / 12v = 67 amps

So you'll want to use some fairly hefty wire/fuse to feed that thing. I can pretty much guarantee that whatever size wire they gave you with the alligator clips is way too small to really handle 800w@12v. Cut a short section from a set of 4 gauge jumper cables and use a 70a breaker or fuse and you'll be good to go.

You can get a set of sacrificial jumper cables pretty cheap:

http://www.google.com/products/cata...=X&ei=GUWQT9SxJO_ZiQLL9OTvAg&ved=0CJABEPMCMAA


As for draining the battery - it depends. If it's switched off, it shouldn't be drawing from the battery at all. Just remember to switch it off, otherwise it will parasite drain the battery just sitting there.

EDIT: And don't even think about running an 800w load unless the engine is running.

 

[BurbanAZ]

Sure, inverters are often hard-wired (the bigger ones pretty much always are).

BUT...

800w / 12v = 67 amps

So you'll want to use some fairly hefty wire/fuse to feed that thing. I can pretty much guarantee that whatever size wire they gave you with the alligator clips is way too small to really handle 800w@12v. Cut a short section from a set of 4 gauge jumper cables and use a 70a breaker or fuse and you'll be good to go.

You can get a set of sacrificial jumper cables pretty cheap:

http://www.google.com/products/cata...=X&ei=GUWQT9SxJO_ZiQLL9OTvAg&ved=0CJABEPMCMAA


As for draining the battery - it depends. If it's switched off, it shouldn't be drawing from the battery at all. Just remember to switch it off, otherwise it will parasite drain the battery just sitting there.

EDIT: And don't even think about running an 800w load unless the engine is running.

thanks for the input, yea the wires they provided are 8ga so i was just going to use longer 8ga to supply power to it, also would it have a integrated fuse or breaker since its meant to be a stand alone thing? either way doesnt hurt to add a fuse. I didnt even think about it being that high a load since it was a smaller inverter.

 

[dwh]

It will likely have a fuse to protect its internal circuitry. That's a totally different thing than a fuse to protect the wire that feeds power to it.

 

[4RunAmok]

I think you're overestimating the need for larger wires... especially 4g.

100 amps over distance:
3ft-5ft 12 gauge wire
7ft-10ft 10 gauge wire
15ft-20ft 6 gauge wire
25ft 4 gauge wire

Anything you run should have a 70 amp breaker at each end, or a breaker at the battery end and a 70amp maxi fuse at the device end.

 

[BurbanAZ]

Do you think 8ga wire then for a run of about 6ft would be heavy enough? Ill definitely add fuses, is there a reason you recommend a breaker at the battery end instead of 2 fuses? sorry if some of these questions seem dumb but the more i can learn about these small projects the easier and safer future larger electrical projects will be, electronics has always been my trouble area when building stuff.

 

[lqdchkn]

Sure, inverters are often hard-wired (the bigger ones pretty much always are).

BUT...

800w / 12v = 67 amps

In straight math yes but batteries are do not always put out 12V. Also inverters are not 100% efficient so input could be higher than what appears to be needed to make the rated output. Always make your plans on the safe side. Use 10 volts whenever sizing wire for your inverter. Thus your 800W inverter could draw as much as 80 amps. So I'd suggest sizing you wire according to that.

According to the table I use (near the bottom) http://www.rbeelectronics.com/wtable.htm

I'd be safe and run 6 gauge, which should also be easy to source by getting an amp install kit from your local car audio dealer

 

[dwh]

A fuse near the battery is all you need.
There is NO need for fuses at both ends.

The purpose of the fuse or breaker is to keep the wire from melting if there is a short. How? Disconnect the power.

If the fuse is at the battery end of the wire, then when there is a short the fuse blows and the *whole wire* is now without power.

If the fuse is at the far end away from the battery and the short is say, somewhere in the middle of the wire (which, it MUST be), then the power will still flow from the battery to the short and the fuse at the far end won't blow because the overload is not flowing through it.

If you have fuses at both ends and there is a short, then the overload will flow through the fuse at the battery end causing it to blow. Now what is the fuse at the far end supposed to protect? Nothing because the *whole wire* is now without power because the fuse at the battery end shut the whole thing down.


As for wire size...

Inverters are a somewhat special case because they pretty much all have a built-in "low-voltage disconnect". So a dip in voltage which most things like lights and radios would ignore, is often enough to cause the inverter to shut down. So - IF you intend to run that inverter at full load - then you have to provide wire large enough to prevent a voltage dip that will put the inverter into shutdown.




Now, admittedly, I only put in 24,000 hours as a journeyman electrician and field supervisor before I switched over to network engineering...but that's my considered (and educated) opinion.

 

[BurbanAZ]

Ok ill just see if i can get some 6 guage, i dont want to be worrying about my truck burning down especially not to safe a quick trip to the store to get more wire lol.

 

[dwh]

According to the table I use (near the bottom) http://www.rbeelectronics.com/wtable.htm

The table at the bottom of that page is a fire waiting to happen. That table says that it's okay to run 200 amps though a 10 gauge wire if the wire is 3 feet long.


According to the U.S. National Electrical Code, #10 copper is allowed to carry up to 55 amps, IF the wire insulation is rated to 90 degrees centigrade. 50a with 70 degree rated insulation and 40a with 60 degree rated. And even then - those ratings are only for conductors in "free air" (not enclosed).

It's covered in Section 310:

http://www.houwire.com/products/technical/article310.html

The specific section we're looking for is 310.17:

http://www.houwire.com/products/technical/article310_17.html

But even that only covers up to an ambient temperature of 30C (86F). For hotter ambient temps you have to use a different table.



That's for "power transmission". There is another table which allows #10 to carry a few more amps (but NOT 160 more!) if it's used as "chassis wiring". Chassis wiring is "short lengths inside of a machine". I.e., inside of a Mr. Coffee, welder, radio, inverter, etc.

Automotive wiring is NOT "chassis wiring" it's "power transmission".



But of course...the NEC doesn't really care about anything below 50v. Down there, you're on your own and can get away with just about anything. And why not? Auto, truck and RV manufacturers do all sorts of nutty stuff; why not us?


200 amps through a #10? You can. I won't.

 

[BurbanAZ]

A fuse near the battery is all you need.
There is NO need for fuses at both ends.

The purpose of the fuse or breaker is to keep the wire from melting if there is a short. How? Disconnect the power.

If the fuse is at the battery end of the wire, then when there is a short the fuse blows and the *whole wire* is now without power.

If the fuse is at the far end away from the battery and the short is say, somewhere in the middle of the wire (which, it MUST be), then the power will still flow from the battery to the short and the fuse at the far end won't blow because the overload is not flowing through it.

If you have fuses at both ends and there is a short, then the overload will flow through the fuse at the battery end causing it to blow. Now what is the fuse at the far end supposed to protect? Nothing because the *whole wire* is now without power because the fuse at the battery end shut the whole thing down.


As for wire size...

Inverters are a somewhat special case because they pretty much all have a built-in "low-voltage disconnect". So a dip in voltage which most things like lights and radios would ignore, is often enough to cause the inverter to shut down. So - IF you intend to run that inverter at full load - then you have to provide wire large enough to prevent a voltage dip that will put the inverter into shutdown.




Now, admittedly, I only put in 24,000 hours as a journeyman electrician and field supervisor before I switched over to network engineering...but that's my considered (and educated) opinion.

thanks for point on the 2 fuse thing, i never understood that. The way i always understood it that any break anywhere in the wire will obviously stop all current and 1 fuse would do that so i never saw the need for 2. I dont plan on really running it at full load since i dont really have it for anything specific just there incase we need to charge a phone or laptop maybe and just small things. Are you thinking 6 gauge also for a good wire size then?

 

[dwh]

Well it's a different story if you're connecting two batteries. In that case you'd have two sources of power so you'd want a fuse at either end. Other than that, not needed.

6 will be fine.

I only suggested the #4 because, A) It's big enough to handle the max load (though, so is 6), B) voltage drop will be less under max load (less likely to trigger a shutdown on the inverter) and C) it's hard to beat the price on those jumper cables - 40' of #4 for under 20 bucks.

 

[evldave]

Don't forget that inverter has a 1,600W peak capacity... you hit that (or even much over your 800W) for any length of time and you will pop your 70A fuse. You indicated you aren't planning on running full load, but it can't hurt to be ready for the future.

4GA with a 100A fuse would treat you right and be fairly cheap. Yes, it will pop if you hit your peak capacity on the inverter, but you have to decide how much power you'll use and then decide how much you want to pay for cabling for the inverter, then fuse based on your cabling.

One common mistake I see in wiring cars, people fuse based on the rating of their inverter, not the rating of their wire. If you use 6GA wire and a 60A fuse, you'll pop the fuse before the wire runs a risk of shorting out (the inverter can 'pull' way more than 60A). If you use that same 6GA wire and fuse it at 100A, there's a good change the inverter will 'pull' that current, but the wire isn't big enough to handle it - it will get hot and either right away or over time the insulation will fail and it will short, causing a fire, burning up your car and killing your family (this happens way more often than anyone wants to admit).

Also, make sure you use the same gauge wire for your ground as your power wire.

As dwh indicated, while you can technically get away with running smaller wire, the larger the wire, the less voltage drop you will get (the voltage drop changes with wire size, voltage, temperature and a whole bunch of other factors). I generally go 1-2 wire sizes up from my rated - that reduces the potential voltage drop (not a huge deal for your inverter, but just something to think about if you are wiring anything else, especially lights). You can also run a longer wire with less voltage drop if you use a larger wire.

Yeah, way too complicated. Buy some jumper cables, cut the ends off and wire it with a 100A fuse

 

[4RunAmok]

I was merging info.. The breaker/fuse at each end is from a dual battery installation, where if something shorts in between the run, you want to kill power from both ends.

 

[Jay H]

I have found that in my experience the cheap inverters all draw a good bit of power even when they are switched off. Mine was
900ma when off that's not acceptable to me. I wired my cheap inverter with a relay connected to an ignition line so that my inverter can only be switched on when the key is in the ignition and switched on. Of course there is also a fuse at the battery.