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Old 07-06-2016, 09:51 PM   #1
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Inverter breaker/ wire size

I recently added a 2000 watt inverter to my rig. The inverter manufacturer recommend AWG 1/0 cable for runs up to 9 feet. I used AWG 1/0 cable and my run is about 5 feet from inverter to battery bank. I installed a 250 amp breaker on this run. My battery bank is 420Ahr.

The inverter runs fine except when I try to run my 1500 watt microwave. The microwave works fine but the 250 amp breaker pops after about 30-40 seconds despite the inverter showing its only at about 75% capacity. It's my understanding a 250 amp breaker should be able to handle up to 3000 watts.

Is my issue the 1/0 cable creating too much resistance? If so I guess I could double up the runs with two 1/0 cables for each leg.
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Old 07-07-2016, 12:11 AM   #2
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Hi,
We only have a 1000W inverter (Magnum) but in reviewing the manual they are requesting the # 1/0 AWG up to 5ft.. Then 5-10ft., #2/0 AWG with a 300A fuse (*rated for DC) protection device.


Best of luck.
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Old 07-07-2016, 11:08 AM   #3
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I suspect that the breaker isn't suited for the application. A fuse may work better for you or a different or larger breaker.
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Old 07-07-2016, 10:32 PM   #4
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Originally Posted by John Hilley View Post
I suspect that the breaker isn't suited for the application. A fuse may work better for you or a different or larger breaker.
Thanks for the input. I think the wire size was too small and creating excessive heat causing the breaker to pop. I doubled up the cables and all seems ok now.
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Old 07-08-2016, 04:18 PM   #5
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Good deal, thanks for the update!


Happy motoring.
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Old 07-08-2016, 04:58 PM   #6
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FWIW I'm trying to figure out how a warm wire would cause the breaker to pop. 1/0 copper has 0.09827 ohms/1000 ft. so 5 ft is ~0.0005 ohms.

1500 W @ 12 VDC = 1500/12 = 125 A

line drop = 125 x 0.0005=0.0625 V

line drop @ 250 A = 0.125 V

pwr loss @ 250 A = 31.25 W

The line drop does not look like enough to be an issue. OTOH you are dissipating enough power in the wire at a very low resistance to get me looking at the actual connection to the breaker. I would be cleaning and/or tightening up any screw type connectors as the contact resistance could be an issue. That is a good place for a non contact thermometer or an educated finger.
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Old 07-08-2016, 05:08 PM   #7
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"I would be cleaning and/or tightening up any screw type connectors as the contact resistance could be an issue."


Good observation.
This can often be an issue with other RV electrical related problems. Shore power, pedestal connections etc. another example.
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Old 07-08-2016, 05:14 PM   #8
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Agree that it is not a wire problem.
By adding cable, you actually reconnected the breaker, probably a poor connection the first time.
Be sure you use copper wire.

Dan
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Old 07-08-2016, 08:55 PM   #9
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All new wires and connections so corrosion wasn't an issue. I can confirm all connections were tight and solid. I also ran the math and would have thought a single 1/0 wire was sufficient but all I know it wouldn't work. In hindsight I should have just used 2/0 or 3/0 cable as my starting position.
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Old 07-09-2016, 08:13 AM   #10
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My 2000W inverter requires 2/0 cable.
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Old 07-09-2016, 10:08 AM   #11
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typically a 2000 w inverter requires 2/0 wires with 300a class t fuse. mine is that. i'd rewire if i were you.
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Old 07-09-2016, 08:33 PM   #12
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Darn...I don't want to start an argument...

But the reason you would use a larger wire is to have less voltage drop across the wire. Having too small of a wire will also cause the wire to heat up because of the power dissipated (P=E x I)

A circuit breaker actually has a bi-metal strip inside which heats up as current increases through it. When the current gets high enough, the Bi-Metal strip bends enough to trip the switch (breaker).

The resistance is given as ohms/foot, so this is why you would pick a larger wire if it was a longer run.

There may be other factors at play here on your installation, and a larger wire can't cause any problems.

It is possible the breaker is a little weak, and the larger wire is acting more like a heat sink and keeping the breaker cool. However, the coffee maker was still well below the trip point of the breaker.

But if you ask any electrician, they would never say you can fix a tripping breaker by using a larger wire....it is just ohms law stuff.

Important thing is it is working

Regards,

Dan
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Old 07-09-2016, 10:34 PM   #13
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A few other numbers.

The average efficiency of an inverter is 10% or better.
Assuming 10%, supplying 2000 watts, would be using 2200 watts from your batteries.

2200 watts from 12V requires 183.3 amps

Dan
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Old 07-09-2016, 11:45 PM   #14
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The Magnum MS 2012 recommends 2/0 up to 5 feet and 4/0 from 5'-10". I would say that 1/0 is too small for any 2000W inverter.
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