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Old 12-19-2010, 02:21 PM   #1
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Need help with wire size and fuses

I have a '78 Tradewind 25 ft that I am going to move the battery, converter, and add an inverter. Need some help with wire size and what fuses to use and where.

See attached sketch. I am going to an 8D Lifeline AGM 255 amp-hr, located under the curb side twin bed, directly over the twin axle. The Iota 75 amp converter and the WFCO 600 watt inverter will be in the closet between the bed and the bath closet. I will provide for air movement.

We boondock four or five times a year avoiding the rv parks mostly. I will use a Trimetric2025 battery monitor to help with power usage and a small generator to recharge every third day or so. Someday I'll add solar.

Here's some questions that I need help with:

1. I was thinking of the 75 amp Iota for quicker battery charging of the 8D battery. Does this make sense or should I get a 55 amp Iota?

2. The Airstream 12 volt panel has 50 amp fuses between panel and battery and I am not adding appliances to the trailer, so I am thinking I only need to supply 50 amps to the existing 12 volt wiring system. Does this mean I can use a 55 amp Iota instead of the 75 amp unit?

3. Question 2 is important. The wire distance from the new battery location to the existing 12 volt panel is approx, 23 ft. For 75 amp supply I'll need 3/0 wire. For 55 amp supply I'll need 2/0 wire. Or - here's the question: can I put a 50 amp breaker at circles 3 and 4 (see attached sketch) to limit the current to the existing 12 volt panel and then I will only need a #1 guage wire?

4. The WFCO inverter will have a 110 amp class T fuse block on the (+) leg between it and the battery (I don't show it on my sketch). Is this correct?

5. The Iota converter will have a 100 amp fuse on the (+) leg between it and the battery (at circle 1). Is this correct?

6. Should there be any fusing on the battery (+) leg (at circle 2)?

Sure seems like a lot of fuses....

7. Does the sketch seem correct?

Thanks, John H
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Old 12-19-2010, 03:03 PM   #2
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1. I was thinking of the 75 amp Iota for quicker battery charging of the 8D battery. Does this make sense or should I get a 55 amp Iota? Seems like 75 is over kill to me. I'm not sure about the faster charge with the larger converter. I would lean more toward the condition of the battery and how low the charge is for a recovery time.

2. The Airstream 12 volt panel has 50 amp fuses between panel and battery and I am not adding appliances to the trailer, so I am thinking I only need to supply 50 amps to the existing 12 volt wiring system. Does this mean I can use a 55 amp Iota instead of the 75 amp unit? I doubt you will ever draw that kind of current on the old AS system. The 50 amp fuse is for protection of the wire between the battery and the panel.

3. Question 2 is important. The wire distance from the new battery location to the existing 12 volt panel is approx, 23 ft. For 75 amp supply I'll need 3/0 wire. For 55 amp supply I'll need 2/0 wire. Or - here's the question: can I put a 50 amp breaker at circles 3 and 4 (see attached sketch) to limit the current to the existing 12 volt panel and then I will only need a #1 guage wire? Yes But with a question: How many branch circuits leave the existing panel and what are the sizes of the fuses in those existing circuits?

4. The WFCO inverter will have a 110 amp class T fuse block on the (+) leg between it and the battery (I don't show it on my sketch). Is this correct? 600 watts divided by 12 volts is 50 amps. I don't think you need a 110 amp fuse. 600 watts divided by 120 volts AC is 5 amps on the AC side: when you factor in the efficiency of the Inverter @ 80% at best, you have a 4 amp capacity on the output of the Inverter.

5. The Iota converter will have a 100 amp fuse on the (+) leg between it and the battery (at circle 1). Is this correct?

6. Should there be any fusing on the battery (+) leg (at circle 2)? NO, It looks like it is redundent.

I do have a question: Where does the TV charge line come into your circuitry?

Look at my next post.
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Old 12-19-2010, 03:16 PM   #3
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Quote:
Originally Posted by johnhh View Post
I have a '78 Tradewind 25 ft that I am going to move the battery, converter, and add an inverter. Need some help with wire size and what fuses to use and where.

See attached sketch. I am going to an 8D Lifeline AGM 255 amp-hr, located under the curb side twin bed, directly over the twin axle. The Iota 75 amp converter and the WFCO 600 watt inverter will be in the closet between the bed and the bath closet. I will provide for air movement.
Consider also the combined inverter-charger units which may give you more performance for your dollar. I would look at Magnum Energy and Xantrex.

Quote:

We boondock four or five times a year avoiding the rv parks mostly. I will use a Trimetric2025 battery monitor to help with power usage and a small generator to recharge every third day or so. Someday I'll add solar.

Here's some questions that I need help with:

1. I was thinking of the 75 amp Iota for quicker battery charging of the 8D battery. Does this make sense or should I get a 55 amp Iota?
A 75 amp converter may charge slightly faster. Since wire size will be driven by the inverter's requirements the only problem with the 75 amp unit is its cost.

Quote:

2. The Airstream 12 volt panel has 50 amp fuses between panel and battery and I am not adding appliances to the trailer, so I am thinking I only need to supply 50 amps to the existing 12 volt wiring system. Does this mean I can use a 55 amp Iota instead of the 75 amp unit?
Either size converter will work. The advantage of the 75 amp unit is potentially shorter charging times, especially if you are using the lighting and other 12 volt stuff in the trailer while charging.

Quote:
3. Question 2 is important. The wire distance from the new battery location to the existing 12 volt panel is approx, 23 ft. For 75 amp supply I'll need 3/0 wire. For 55 amp supply I'll need 2/0 wire. Or - here's the question: can I put a 50 amp breaker at circles 3 and 4 (see attached sketch) to limit the current to the existing 12 volt panel and then I will only need a #1 guage wire?
Using a 3% table will give you unnecessarily large wire.

Wire size will be driven by the inverter. Most 600 watt inverters will draw around 100 amps during short surges of up to a second or so, and you want to limit the round trip voltage drop to around 1 volt. There's a calculator here. 4 gauge wire should work fine if there's only 7 feet or so between the inverter and battery. The 23 foot run to the existing panel can be 6 or 8 gauge. It won't be carrying the full converter output. I would put in a 50 amp circuit breaker in the (+) line to the existing panel.

Quote:

4. The WFCO inverter will have a 110 amp class T fuse block on the (+) leg between it and the battery (I don't show it on my sketch). Is this correct?

5. The Iota converter will have a 100 amp fuse on the (+) leg between it and the battery (at circle 1). Is this correct?

6. Should there be any fusing on the battery (+) leg (at circle 2)?

Sure seems like a lot of fuses....
I would not fuse the system that extensively or that tightly. I would use a single 150 amp t-class fuse in the battery + line before any junctions. It isn't necessary to have a separate fuse for the converter, inverter, and trailer distribution block. You're protecting from shorts, not overloads, and this isn't an easily replaced fuse so pick a size that won't blow except in an emergency. The converter and inverter both have some kind of internal overcurrent protection.

As noted above, if you're using smaller wire to the trailer distribution block, a 50 amp breaker in the (+) line might be prudent (circle 3). I would not fuse the (-) line at this point (circle 4).

Quote:

7. Does the sketch seem correct?

Thanks, John H
Looks OK to me except as noted above
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Old 12-19-2010, 03:31 PM   #4
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Here is a drawing to look at. It shows wiring to the 12volt panel , but does not show a Converter or Inverter.
The 3% table is calling out wires sizes much to large for your application.
There is an Inteli-Power model PD9245C Converter on EBay for $129.56 including shipping. I think it would work for you.
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Old 12-19-2010, 04:29 PM   #5
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Here is a drawing I made up for my '74 Argosy. It does not include an Inverter. It will replace the original Univolt unit.
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Old 12-19-2010, 07:19 PM   #6
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I concur with most of the above notes...

I wouldn't use any wiring smaller than 4 AWG between the battery and both forward and aft runs in your diagram.

I would think a 55 amp converter/charger would have plenty of capacity to charge your AGM 8D battery in a timely manner -

50 amp fuse in the POS circuit should be plenty...

One IMPORTANT note - if you are going to plug the AC output of the inverter into one of your AC receptacles to power up the others - don't forget to protect the exposed MALE prongs of you shore power cord, as they will then be HOT! Tripping the main breaker will turn off only one leg of the AC...

Don't forget to tie down that heavy 8D battery in a secure manner - that's a lot of weight to be bouncing around when on the road...

Ray
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Old 12-19-2010, 08:57 PM   #7
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Thanks for all the comments.

Using the wire size calculator, I'll use #1 wire for the 23 ft. from the battery forward to the 12 volt panel. #4 wire for the 7 ft. connections near the battery.

I'll go with the 55 Amp Iota converter. 50 amp fuses.

And I will tie down the battery.....

Merry Christmas.

John H
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Old 12-20-2010, 02:18 AM   #8
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johnhh, I don't really think you'll need 1 AGW cable going up front - you just won't have that much of a load up there to need that much 'copper' cable...

The largest wire you probably have in your TV is probably 10 ga running from the front to the rear trailer harness, and it works OK, for the most part...

I'll bet AS doesn't use any wire heavier that 10 or 8 gauge, except for the cable from the batteries to the fuse panel...4 AGW should be plenty, IMHO...

Ray
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Old 12-20-2010, 09:00 AM   #9
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I agree with Mexray. It's a waste of copper to run anything larger than #8. If you count up everything you have in the original trailer that uses 12 volts I think you will find that it doesn't add up to more than 30 amps if it were all on at the same time. For example:
1141 light bulbs = 18.4 watts = 1.5 amps
Suburban gas furnace blower = 4.5 to 6.0 amps

My '74 Argosy 26 has the following 12 volt fixtures:
Bath:1 fixture 2 bulbs = 3 amps
Berthing Area: 3 fixtures 8 bulbs = 12 amps
Galley and Dinette: 4 fixtures = 6 amps
Furnace Blower = 4.5 amps
Roof vent fans: 2 = 4 amps

3+12+6+4.5+4 = 29.5 amps with everything on. Who has everything on at once?

Have you totalled up your requirements to see what you need?
How many circuits do you have running out of the fuse panel?
Would it be better to install a new fuse panel aft and then run 3 or 4 #12 wires up front and connect them into the old fuse panel location with a terminal strip?
Just food for thought.
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Old 12-20-2010, 09:58 PM   #10
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The West Marine catalog has a very good chart in it regarding 12V wire sizing, you might take a look at that . The loads in a trailer do not require the sizes you are talking about using. Your major electrical loads are on the 120 V side for things such as the A/C . I do like to upsize wire for things such as that because of the confined spaces and heat that the wire will be in. I also like to fuse for less than the capacity of the wire as an extra margin of safety. Wire prices are out of sight.
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Old 12-21-2010, 11:18 AM   #11
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Well, I just checked out the size and price of #1 guage wire. Holey crap that stuff is big, and expensive..... Time to start agreeing with you guys and down-size the wire....

So, for the circuit going forward from the battery to the trailer fuse panel, 23 ft of wire length one-way, and using Mr. Twinkie's thought to design for an actual maxi current of 30 amp, I can use a #4 AWG and have a 2.93% voltage drop which is the target maxi. drop. If I actually run 40 amp thru the #4 I'll get a voltage drop of 3.9% or from 12vdc to 11.7 vdc. I can live with that as well or turn off some appliances to lower the draw back to 30 amp.

And I will use a 50 amp fuse on this circuit to protect the wire. This matches the 50 amp fuses in the existing fuse panel.

And, I will actually use #3 wire, not #4 because the panel shop here at work just happens to have some that can be used for home projects...

Guys, thanks for the education.

John H
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Old 12-21-2010, 08:43 PM   #12
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Johnhh! I think you will do fine with that. When you calculated your loses you centered it around 12 volts. If you are plugged into the grid the actual potential will be over 13 volts if the converter is functioning properly. So that makes it even better. If you have more questions or need help with anything. There are plenty of qualified people on the forum willing to lend a hand.
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