Alternator wire size?

bottomacher · October 03, 2011, 03:44:51 PM

I'm fixing to add a 12 volt Leece-Neville alternator to my MC9 to provide a 12 volt house system in addition to the 24 volt system now installed. I may also add a second inverter to this system, if I ever work up the nerve or need. The two 12 volt batteries will be about 30 feet from the alternator. I want to use a 100 amp or greater alternator. I hoped to use a #2 copper welding wire to connect everything. Is that going to be suitable? Thanks

DW SD · October 03, 2011, 05:35:50 PM

Here's a chart that seems to suggest your sizing would be acceptable. I'd probably have run larger to be safe, but maybe not a lot bigger. it also suggests this is open to the air, which isn't ideal for chassis wiring - I'd want to cover it to protect from cuts to the insulation and shorts to ground, which could lead to a fire. Sean will probably weigh in with a better response.

http://www.powerstream.com/Wire_Size.htm

Sean · October 03, 2011, 05:36:51 PM

Not even close. You will need a minimum of 2/0 to get the voltage drop under 5% at that distance. Your proposed #2 would have a drop of over 8%.

FWIW, the math is much better for 24 volts. You already have a 24v alternator capable of producing over 7 kW -- why add this kind of complexity?

-Sean
http://OurOdyssey.BlogSpot.com

Jerry32 · October 04, 2011, 03:58:30 AM

I'm with sean there as i just put a converter pluged into the inverter output to run the 12v battery for the minimal amount of 12 volt use.

bottomacher · October 04, 2011, 06:01:45 AM

Thanks to all for the replies. It appears that I can do what I want to do by using a Vanner equalizer, so I will.

Sean · October 04, 2011, 07:16:43 AM

I think that is a better choice. It will cost you less overall and is a simpler system. I would also recommend that you do as much as you can on 24v rather than 12v. Not only will you avoid the conversion loss, but you'll have fewer issues with voltage drop, and you'll be able to use smaller gauge wire for many items.

-Sean
http://OurOdyssey.BlogSpot.com

rv_safetyman · October 04, 2011, 09:11:23 AM

For those following this thread (and planning to add a second alternator) there are really two issues to address. First is the wire size - you need welding cable type/size wire.

The second is circuit protection. You will be running the wire from the alternator to the house battery bank. Lots of places on the route to cause chafing if not done correctly. If somehow the positive cable shorts, your will almost certainly have a fire. Being anal, I put a large fuse at each end of the positive cable.

BTW, you need to run both a positive and negative cable to the house battery bank.

Jim

bottomacher · October 04, 2011, 01:35:57 PM

I already use 24 v for everything possible, including my inverter. I find that I need about 50-60 amps (potentially) of 12 volt for headlights (I can't live with this MCI diode setup), cabin lights, radios, fans, etc. I couldn't figure out how to move my start batteries to the rear and incorporate the equalizer into a 12 volt system, but I've seen the light.

Sean · October 04, 2011, 02:16:09 PM

Quote from: bottomacher on October 04, 2011, 01:35:57 PM
... I find that I need about 50-60 amps (potentially) of 12 volt for headlights ...

Whoa... the brightest (legal) incandescent lamps use around 75w of power, so a pair of them draws less than 12 amps. Even if you have dual high-beams with older, less efficient tungsten lamps, the most they could draw would be 19 amps for the set.

That said, if I were going to change an MCI headlamp setup from the weird center-tap arrangement with relays, I would just get H4 housings and use 24v H4 bulbs. Cheap, easy to get, and draw just 3 amps or so apiece on high beam.

FWIW and YMMV.

-Sean
http://OurOdyssey.BlogSpot.com

bottomacher · October 04, 2011, 02:43:00 PM

My 4656 bulbs use 45 watts each at high beam, with an additional 45 watts for the secondary 4651 high beam, for a total of 180 watts. I have never seen them in an h4 configuration. In addition, interior lights, radio, exhaust and circulating fans, backup spot lights, water pump, etc, increase the possible total to close to about 30 amps. I know everything won't be on at the same time. In addition, I hope to be charging the 12 volt battery bank while underway, even in the middle of nowhere with the high beams, radio, CB, and reading lights on. Further, I have a backup 12 volt inverter that I would like to be able to use if needed, and I want full batteries if I do. Like most amateurs, I'm prone to overdoing the system, but I don't have the knowledge to refine the setup. What I do have is two inverters and two equalizers that I have collected over the years, and I don't fancy selling them at the prices I've been seeing lately.

Bill B /bus · October 06, 2011, 04:54:15 PM

I'm partially with Sean on 24V. However, I'm using 12V interior lighting, 12 V LED for clearance/turn and stop lights, all 12V for house. I have two batteries 1- 2 Group 31's for bus stuff starting and Vanner equalizer for all the 12V bus stuff, 2- 6 8D AGM's for the house with a Vanner equalizer. Can crosstie both the 12 and 24V systems as necessary. When traveling normally the 24V is tied together to charge the house batteries. The inverter/charger is 24V from the house batteries and alternator. Fairly simple system with redundancy. Besides I have the 50DN alternator ready to supply lots of amps.

Bill