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Author Topic: wire sizing for a 600 w invertor  (Read 3181 times)
christopher
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« on: March 21, 2010, 02:15:43 PM »

i have a 600 w invertor to power a tv and dvd player. it will be about 10 ft from the battery.
What size wiring do I need? i have an 80 amp fuse for the hot side.
thank you
chris
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robertglines1
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« Reply #1 on: March 21, 2010, 03:28:17 PM »

600 watt ? does that not come with a cigarett lighter plug?electrical guys will chime in but for that size a #10 wire should be over kill but safe...#10 is good for 30 amps at 120 volts...
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« Reply #2 on: March 21, 2010, 04:44:44 PM »

This is an excellent opportunity to try my hand at guessing.  The wire size is determined by the amperage, so 600w @ 120v output would be 5 amps and a 50 amp input at 12v.  According to the below link, you could get by with #12 in and 22 out.  The chart does not say so, but it implies the 120 v figures are for AC and the 12v figures are for DC.  Actually, I would have guessed a larger input wire than a #12.  Proctors, what's my score?

http://www.rowand.net/shop/tech/wirecapacitychart.htm
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« Reply #3 on: March 21, 2010, 05:14:05 PM »

I would play it safe and upsize to #10 or even #8.  Inverters don't like voltage drop.

Disregard this.  I trusted the math someone else did and didn't double check it.  You probably still want to upsize the wire regardless.
« Last Edit: March 21, 2010, 07:36:15 PM by belfert » Logged

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« Reply #4 on: March 21, 2010, 05:14:25 PM »

i say #6  72 cents a foot at h/depot
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« Reply #5 on: March 21, 2010, 07:25:29 PM »

i have a 600 w invertor to power a tv and dvd player. it will be about 10 ft from the battery.
What size wiring do I need? i have an 80 amp fuse for the hot side.


Chris,

I am assuming this is a "portable" inverter with a single NEMA 5-15R receptacle on it.  As such, it needs to be mounted within cord's reach of the devices it powers (TV and DVD).  By contrast, a hard-wire inverter could be mounted closer to the batteries and you could get by with running much smaller wire on the 120VAC side with lower overall loss.  I am further assuming this is a 12-volt model, and not 24.  If either of these is incorrect, please chime in.

600 watts continuous probably implies a 1,000 or even 1,200 watt "surge" rating, but you should check this against the written specs.  1,200 watts at 12 volts is 100 amps, which is pretty significant current.

Since you are using an 80-amp fuse, though, you can size the wires for that amount of current as a maximum.

The size wire you need will depend on the wire and insulation type, but for most stranded copper wire with thermoplastic insulation, 80 amps requires a minimum of #4 for temperatures to 75C (157F).

At 10' one-way distance, 80 amps on #4 will have a voltage drop of nearly half a volt and well over 3%, generally considered unacceptable.  However, the continuous rating of the device is 600 watts, or 50 amps, and likely you will usually be drawing even less.  50 amps on #4 produces a drop of just 1.7%, considered acceptable.

So the short answer is #4 THWN or THHN, available at most home improvement stores.  Note that #4 is extremely thick and unwieldy, so plan accordingly.  Also, #4 will probably not fit the lugs (or whatever connection you have) on the little inverter, so you will probably have to transition to something else to make the connection.

600 watt ? does that not come with a cigarett lighter plug?electrical guys will chime in but for that size a #10 wire should be over kill but safe...#10 is good for 30 amps at 120 volts...

Yes, but it's not 120 volts, it's 12 volts, and 80 amps as stated in  the original question.  So, no, #10 is not sufficient (but it would be for a 600-watt, 120VAC circuit).  Also, a cigarette lighter is not capable of 80 amps; generally, the lighter-plug inverters are 100-150 watts max.

This is an excellent opportunity to try my hand at guessing.  The wire size is determined by the amperage, so 600w @ 120v output would be 5 amps and a 50 amp input at 12v.  According to the below link, you could get by with #12 in and 22 out.  The chart does not say so, but it implies the 120 v figures are for AC and the 12v figures are for DC.  Actually, I would have guessed a larger input wire than a #12.  Proctors, what's my score?

http://www.rowand.net/shop/tech/wirecapacitychart.htm


Your score is low, sorry Grin  In fact, your "gut" was right in this instance (where you said you would have "guessed" larger).  Specifically, the note above the table you linked says "This chart is a simple 'max capacity' chart for a short wire run."  Later he writes "The values here for 12V usage are not yet certified to be correct/valid/safe."  In fact, the capacities listed are for uninsulated conductors in "free air," a condition that only exists theoretically, or for overhead power lines.  Insulated conductors in real applications follow much stricter rules.

This is one reason I urge caution when surfing the 'net to find this information.  A quick survey of five search results for "ampacity" yields five different sets of recommendations.

I would play it safe and upsize to #10 or even #8.  Inverters don't like voltage drop.


Correct on the voltage drop, but way too small on the wire gauge.  #4 is called for, and I would certainly not go lower than #6.

i say #6  72 cents a foot at h/depot


You win the prize for closest.  #6 is good for 65 amps in THWN or 75 amps in THHN.  This wire size could definitely be used, but I would recommend in this case replacing the 80-amp fuse with one rated 65 amps max.

HTH,

-Sean
http://OurOdyssey.BlogSpot.com
« Last Edit: March 21, 2010, 07:32:35 PM by Sean » Logged

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« Reply #6 on: March 21, 2010, 07:40:03 PM »

I would play it safe and upsize to #10 or even #8.  Inverters don't like voltage drop.
Correct on the voltage drop, but way too small on the wire gauge.  #4 is called for, and I would certainly not go lower than #6.

I changed my post.  I make the mistake of trusting the person who said 30 amps.  I should have verified the math.
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« Reply #7 on: March 21, 2010, 08:32:25 PM »

Sean,

I have a portable Xantrex 400 watt inverter I bought about 4 years ago. It came with a cigarette style ligher or hard wire cord. I have used it for hooking up kids x-box and such on trips.

Why would it be sold with that type cord if it is not rated?

Bryan
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« Reply #8 on: March 21, 2010, 09:44:03 PM »

I have a portable Xantrex 400 watt inverter I bought about 4 years ago. It came with a cigarette style ligher or hard wire cord. I have used it for hooking up kids x-box and such on trips.

Why would it be sold with that type cord if it is not rated?


Bryan,

Since it came with both style connections, I would guess the intent was so that it could be used at full power if hard wired, and at some reduced power if used with a lighter socket.

Remember that it is not the rating of the inverter that will dictate how much power can be drawn in this case, but rather the rating of the lighter socket and the fuse behind it.

400 watts at 12 volts would be 33 amps.  If the inverter had a surge capability beyond the 400 watts, it would be even more than that.

I have never seen a lighter socket fused at more than 30 amps.  (Many modern cars, in fact, are equipped with "mini-ATC," aka ATM fuses, where 30a is the highest value.)  So if you plugged this unit in to a lighter socket, then tried to draw the full 400 watts from it, you would blow the fuse in the chassis fuse box.  Not dangerous, but annoying.  In practice, many "lighter sockets" that are not really made for lighters, but rather just accessory power, are fused even lower than that.

Beyond that, most lighter plugs (the one on the end of the device cord) actually have fuses built in.  Usually it is a glass tube type fuse (SAE-J554 or similar) and the end of the plug unscrews to access it.  If you opened the plug on this inverter, you might find a 20-, 25-, or 30-amp fuse built in, which would limit the output to about 90% of 240, 300, or 360 watts, respectively.

So there would, in fact, be no point to selling a full 400-watt inverter that had as its only input option a lighter plug.  And certainly not a 600-watt.  But that could be an option present in addition to hard-wire or clamp-type connections.

-Sean
http://OurOdyssey.BlogSpot.com
« Last Edit: March 22, 2010, 01:15:20 PM by Sean » Logged

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« Reply #9 on: March 22, 2010, 04:55:46 AM »

Sean is right on with his wiring size. I use a 800 watt inverter in my S&S and use 10' of #4 cable and use an extension cord to get the power where it's needed.
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robertglines1
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« Reply #10 on: March 22, 2010, 06:02:24 AM »

I. stand corrected and learned alot in this post...thanks for that;Bob
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« Reply #11 on: March 22, 2010, 06:20:38 AM »

Here is a contrarian point of view.  A typical small LCD TV draws about 100 watts.  A typical small DVD player draws about 25 watts.  Just because you have a 600 watt inverter doesn't mean you have to treat it like one.  200 watts is around 15 amps at 13 volts nominal.  Use 10 or 12 gauge wiring depending on run length and fuse at 20 amps.  Label the outlet "TV and DVD Only" and don't use it for anything else.

As long as the wiring run is fused appropriately, and you arrange things so that the inverter doesn't draw more power than it is fused for, you should be good to go.

Brian
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« Reply #12 on: March 22, 2010, 07:24:24 AM »

Keep in mind that those inexpensive little inverters draw considerable idling current compared to the more expensive units.  You need a simple way to turn off or disconnect from the batteries when not in use.
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« Reply #13 on: March 22, 2010, 12:56:26 PM »

Thank you for all the responses. I will snap a pic of the invertor then we can figure out what to use. I know nothing about electrical.
I just want to do it right.
 Sean, The invertor has 3 plugs built into it as well as the on off switch. So it will be mounted in the cabinet with the tv/dvd.
I dont know which size fuse to use. the 80 amper is the one that someone at the time told me to use on something else.
I think #4 may be to thick but you folks can tell me.
Thanks
Chris
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« Reply #14 on: March 22, 2010, 01:12:56 PM »

Chris,

If #4 is too heavy for your application, you can certainly fuse the whole thing at a lower value, and essentially de-rate the unit to a lower output.  As I wrote earlier, #6 would be good for up to 65 amps (780 watts input or about 660 output, max surge).  Or as Brian suggests, you could go all the way down to, say, #10, good for 30 amps, which would give you 360 watts input or about 300 watts output.  The important thing is to make sure that the fuse is rated no higher than what the wire can safely carry.  If your total load will never exceed 300 watts, this is an acceptable solution.  Although at those numbers, I think I would trade the 600-watt inverter for a smaller model and pocket the difference.

The 80-amp fuse you have may only be used if you run #4.  The good news is that 30-amp fuses and holders are much easier to find and a lot less expensive than values above 60 amps.

-Sean
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« Last Edit: March 22, 2010, 02:18:31 PM by Sean » Logged

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« Reply #15 on: March 22, 2010, 01:31:50 PM »

Just remember-there is no such thing as a too big wire gauge (within reason). But there is certainly a problem with a too small wire.  I would always go to the next size larger suggested.  Good Luck, TomC
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« Reply #16 on: March 22, 2010, 01:56:23 PM »

chris, a little electricity 101 may help you (or it may just confuse you, in which case, sorry in advance).

Things that use electricity use it in watts.  So if you look on the back of your TV and DVD player, you should find their usage rating in watts, or possibly in amps.  Since watts are equal to voltage times amps, if your voltage is 120 VAC and you use 3 amps, you are also using 360 watts.

Wires carry electricity, but they are rated in amps.  Any wire has a rating for the number of amps it can carry, and that is regardless of the voltage that is also being carried.  So once you figure out the power you need, in watts, and you know the voltage you have, you will know the amps you need to carry and can look up the wire size.  Remember - watts equals voltage times amps.  Another way of saying that is amps equals watts divided by voltage.

Wire rating tables are all over the place.  Some things that go into the rating are things like distance - the longer the distance, the bigger the wire has to be, the voltage drop over the distance, and the temperature rating.  You have to know a bit to judge the wire size accurately, or you can just do what most of us do - ask Sean, or go one size higher as a comfort margin.

The size of your inverter is like the speed limit on a highway.  You have an inverter rated at 600 watts, that's it's speed limit.  You can go faster than the speed limit, for a little while, and your inverter can put out more than 600 watts for a few seconds - that's the surge capacity.  If you speed too much, or take out too much power for too long, you get a ticket, or you blow a fuse, or your wiring gets too hot and so on - bad things happen.  But your inverter is like your bus - it can go slower than the speed limit, or run at less than it's rated power output.  So you could decide to run it at no more than 300 watts, put in a fuse and wiring rated for that, and away you go, happy as a clam.  Just like you are in your bus at 50 mph on a 65 mph freeway, smiling at the birds and the cows on the side of the road...

So - what you do is decide how much power your TV and DVD player are going to use.  I already looked that up and thought that 200 watts, including everything, was probably really close, but you can double check.  For 200 watts at 12 volts, you have 200/12 = 16.6 amps.  So you need a wire that can carry 16.6 amps for 10 feet, and I happen to know that 12 gauge wire will do that, I looked it up on a chart, and you could put in a 20 amp fuse.  Or you could say that you have a big roll of 10 gauge wire handy, may as well use that.  You look it up on a  chart, or ask someone who knows, and find that 10 gauge wire will carry 30 amps at 12 volts, so you know that you can put in the 10 gauge wire, a 30 amp fuse, and have your inverter run up to 360 watts.  Or you can say you want the full monte, your inverter is rated to 600 watts and 1000 watts surge, and you want it all, dammit!  So you say, 1000 watts surge, that's 1000/12 = 83 amps, I have a 80 amp fuse handy, and I'll check on a chart, I need 4 gauge wire for that many amps.

So like many things in life, there are a lot of correct answers to your question, you just need to ask it right, and the right answer for what you really need to do pops out.

Hope this helped.

BTW, I do exactly the same thing as what you need to do in my bus, I run a TV and a DVD player off of a 750 watt inverter.  I did the math, I said this sucks, I ain't buying 25 or 30 feet of 4 gauge wire, not this lad, and I put the inverter right beside the batteries (where it belongs anyway) and ran a little extension cord over to where the TV is.  Remember the math - wire carries amps, and 750 watts at 120 volt ac is only 6.25 amps, and just about any little extension cord you can buy will carry that little amount of current.

Brian


« Last Edit: March 22, 2010, 02:11:15 PM by bevans6 » Logged

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