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Thread: What gauge wire for 800w inverter?

  1. #1
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    Question What gauge wire for 800w inverter?

    Hey guys this is my first post,
    I just ordered a the Cobra CPI 850, 800Watt continuous power inverter.

    I wanted to install it on the passanger side under the glove box of my 1990 Lexus ES 250.

    I went to NAPA auto parts and they handed me 10 gauge wire with a 12 gauge wired fuse box for up to 50 amp fuses.

    My question is, did they give me too small of a gauge?

    What size gauge do I need to run it from the passanger side to the battery and also what size fuse should i use?

    thanks!

  2. #2
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    Check on Google for "Wire Loss Table". You will find tables that will help you find the wire size required to run a certain current over a certain distance.

    If you really plan to use 800W continuous, then you will be drawing somewhere between (800 / 13.8 = ) 57 Amps and (800 / 12 = ) 67 Amps. A quick lookup shows that you're not to use #10 wire for that, it will overheat.

    By the way, I'm fairly sure you won't be using 800W continuous, unless you plan to run your kitchen coffee machine from it. Do the math and use a lookup table.

  3. #3
    FLAC
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    I'd just be sure to put a fuse on the source-side of the power wire, and choose it based on the current limit of the length and AWG of the wire I selected.

    The manufacturer lists 1600W surge capabilities. I imagine that the inverter has fuses on the input of the unit, but I doubt that they are rated for that.

    Be careful with your current calculations as well. It lists a voltage range of 10-14 volts (Low voltage alarm at 10.5V and Low voltage cut-off at 9.5V). And one more consideration is the efficiency of the device, which is 88%.

    So here's what I'd do:

    Take the worst case scenario- 9.5V which could be easily obtained in most vehicles during starting.

    surge rating:

    1600W/9.5V/.88 = 191 Amps

    normal rating is half of that or:

    800W/9.5v/.88 = 95 Amps



    That’s the last I’ll speak of the 1600W surge rating, you would need welding wire 00/0000 SAE to accommodate that.



    And normally you are going to see (if you intend to use all 800W)

    800W/13.8v/.88 = 65 Amps



    Ampacity tables like the ones you find in NFPA and NEC documents are sorted by type and do not include the automotive grades such as the PVC insulated TWP, GPT, HDT, and SGT and the XPE Cross-Linked Polyethylene insulated SXL, TXL, and GXL, which meet the SAE J-1128 standards. A Google search will take you to the only one I could find online, which is at http://www.kayjayco.com/techAmpsNSAE2Metric.htm.



    Using that chart, I see that for 95 Amps you would need 2 AWG SAE wire.

    And for normal (13.8V => 65 Amps) use, you can probably use 6 AWG SAE.



    What is what I would do, is use 6 or 4 AWG SAE as possible and fuse for 70 or 80 (respectively) Amps at the battery. You will probably blow that fuse if you try to start the car with full load applied. But at least you’re safe. Actually I wouldnt need 800W so I wouldn't...but you can.

    and remembering that this wire goes through the engine compartment and taking into account the highest temperatures the wire will see (im going to use the NEC’s requirement for engine compartment wiring in an engine compartment of an RV which is 125C or 194F (it requires ensuring the conductor does not go within 450mm (18in) of the engine’s exhaust system as well) {551.10(B)(4)}(it derives that from SAE J-1127/1128 standards). You may want to bump it up to 4 just based on this. Your call. This all agrees with another document I found; http://www.mobileaudio.com/rac-faq/

    P.S. Be sure to ground to bare metal and get a really good crimping tool, high resistance connections are important to avoid here.

  4. #4
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    Thanks for the reply!

    that was the most informative post i have ever seen.

    I think I'll go with the 4 awg wire and a 70 amp fuse.

    Now the new question is where can I get the cheapest 4 awg wire?

    Thanks.

  5. #5
    FLAC
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    With 4 AWG you can bump up your fuse rating to 80 Amps conservatively.

  6. #6
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    Quote Originally Posted by h3rk View Post
    With 4 AWG you can bump up your fuse rating to 80 Amps conservatively.
    Don't I want it to blow if the draw is over 800 Watts? Which is about 67amps, if I go with the 80 amp fuse, if the draw is over 800 how am I certain the inverter will shut off, couldnt it fry since 1600 peak is only for a certain ammount of time.

    Thanks.

  7. #7
    FLAC
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    the inverter has fuses on it right?
    the voltage protection and its own fuses are there to protect the unit, the fuse we figured out protects the car, you and perhaps the inverter.

  8. #8
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    Quote Originally Posted by h3rk View Post
    the inverter has fuses on it right?
    the voltage protection and its own fuses are there to protect the unit, the fuse we figured out protects the car, you and perhaps the inverter.
    O gotcha. thanks for your help. Do you think the cheapest place for 4 awg is ebay? They run for about 1.38 a foot, thats some expensive wire lol.

  9. #9
    FLAC
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    I'd be sure to check what kind of wire you are ordering
    your 2 main choices are SGT and SGX.
    SGT is the PVC option but it does not have the higher heat rating and is not as cool as the Cross-Linked Polyethylene SGX, but it's about 20% more expensive.

    That should be enough info to your way to whatever bargain you can find. PVC I've seen for about $2+/ft



    http://www.wiringproducts.com/
    www.anaheimwire.com/SXLCROSSLINK.htm

  10. #10
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    If you want some serious stuff, consider this stuff from Aircraft Spruce:
    http://aircraftspruce.com/catalog/el...hieldlwire.php

    It's resistent to high temperatures and oil/gas spills.

    Stay away from PVC insulated wires, as PVC causes toxic fumes when it overheats. You don't want to die from saving $10 ...

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