12 Volt by Wire or Converter?

Iver · January 13, 2011, 03:39:10 PM

I need to get 12v to the rear engine compartment from my battery bay on my MCI-9. I have a bank of 12v house batteries and also 2- 8D starting batteries 24v.
I want to have the 12v to operate several different things so I was planning on running a 1/0 or 2/0 wire. Distance is @ 35 feet.
There is already a heavy 24v wire in the engine compartment for the starter. Can I get 12v from that with a converter of some sort
or would it be best to run a separate wire.
Thanks, Iver.

gus · January 13, 2011, 03:54:13 PM

Since the starter tap is used so seldom it is an ideal power source for the engine compartment, surely better than 35' of heavy cable.

You probably could use a resistor of some sort to step down the starter 24v to 12v but I'm sure there is a better way, some of our more electrically astute posters will let you know soon.

FloridaCliff · January 13, 2011, 03:59:40 PM

Quote from: Iver on January 13, 2011, 03:39:10 PM
I need to get 12v to the rear engine compartment from my battery bay on my MCI-9. I have a bank of 12v house batteries and also 2- 8D starting batteries 24v.
I want to have the 12v to operate several different things so I was planning on running a 1/0 or 2/0 wire. Distance is @ 35 feet.
There is already a heavy 24v wire in the engine compartment for the starter. Can I get 12v from that with a converter of some sort
or would it be best to run a separate wire.
Thanks, Iver.

What are you planning on running with 12v?

Lin · January 13, 2011, 04:02:34 PM

Check your manual. There is probably a 12v terminal in the rear junction panel.

bevans6 · January 13, 2011, 04:10:31 PM

The answer is it depends. I do both (MC-5C, remarkably similar in many respects to your MC-9). I use a voltage converter to obtain 12 volts from 24 volts in the engine compartment for my tail light adaptor for the trailer. It is a high quality, expensive way to get a highly reliable 20 amps of 12 volts directly where needed. Powered from the 24v buss in the rear electrical panel. http://www.solar-electric.com/12to24or24to.html

I also pulled multiple 10 gauge to 14 gauge wires from the old AC condenser compartment to the engine compartment using the AC freon lines as conduit. Using them for trailer brake controller wiring, etc. there is 12v available in the AC electrical panel, stud 6 I think, if your bus has an operating 12v tap off the start batteries. You could use that, it is fed by 10 gauge wire up to 30 amps. Some buses may have that mirrored in the engine bay electrical panel, mine does not.

1/0 wire can support around 150 amps. What do you have in the engine compartment that needs 150 amps of 12 volt DC? If I needed 150 amps of 12 volt DC the only way I would do it is a separate alternator and battery system dedicated to it.

Brian

gumpy · January 13, 2011, 04:14:31 PM

There's a 16 ga 12v feed in the rear junction box on stud #41. Good for about 10 amps. You may need to connect it in the A/C J box in the front driver's side bay as it was an
option. If you need more, you can run a 10 ga wire from the A/C J box for 30 amp. Put a circuit breaker in at the source to protect it. It would be best if you have an equalizer
on the house batteries, as all 12v is off the center tap.

Alternatively, you could hook up a 20a 24v-12v converter in the rear bay area. I don't advocate putting those in the engine compartment unless you can build a weather proof
enclosure that allows it to breath and dissipate heat. Maybe you could put it in the rear cap area.

I vote for the 10 ga wire from the A/C J box. You can bring it into the rear J box and connect it to stud 41, the same terminal that the 12v option is on. With that approach, you can
then run your toad lights or anything else in the rear of the bus.

craig

luvrbus · January 13, 2011, 04:49:31 PM

Bet these guys can tell you how to tap the 24v at the starter and convert it to 12v without spending a bundle www.texasindustrialelectric.com

good luck

Iver · January 13, 2011, 07:35:55 PM

Actually, I didn't realize what I was typing. I meant to say #2 or #1 wire not 1/0 or 2/0.
Now that I look at my conductor size chart, even a number 2 wire will give 40amps at 40 feet.

Really all I need is to supply 12volts to Craig's tail light converter and to the Stone Bennett shifter.
I'll check for the 12v supply in the AC junction box. However, the OEM a/c has been removed and I'm not sure what wiring is still original.

The main problem with this coach is that much of the original wiring was changed/modified by the previous owner who did all the conversion himself.
When we originally purchased the coach, the PO was at the age where he couldn't remember much detail about anything and he has since passed away.

So everything has to be traced manually. I have the book which helps for anything that hasn't been modified.

Anyway, thanks for all the suggestions, Iver.

Tony LEE · January 13, 2011, 10:23:29 PM

Not really a good idea to tap 12V from the centre of a 24V battery because it WILL cause an imbalance in the condition of the two batteries that may be cumulative and may be difficult to correct.

Easy and cheap to buy a 24V to 12V converter and do it properly.

PS DON'T use a resistor to drop 24V down to 12V unless you are feeding a single load that never varies.

JackConrad · January 14, 2011, 04:45:18 AM

Quote from: Iver on January 13, 2011, 07:35:55 PM
So everything has to be traced manually. I have the book which helps for anything that hasn't been modified.
Iver.

Lowes (and electrical supply stores) sell a gizmo to trace wires. You clip the main part to one end of the wire and to ground. Then you take the remote probe and start touching wires at the other end. You get an audible signal when you touch the correct wire. Jack

Lin · January 14, 2011, 08:32:36 AM

It's called a tone generator. You're crazy to try to trace wires without one.

fe2_o3 · January 14, 2011, 11:28:57 AM

Would this help?
http://www.weisd.com/store2/VELPSDC20.php
...Cable

gumpy · January 14, 2011, 11:59:15 AM

I tried one of those tone generators from Home Depot on my bus. Ended up taking it back. It was useless. I could not trace any wires with it.
Never figured out why. Maybe it was me.

I believe I indicated that the 12v center tap is best accompanied by an equalizer.

Iver, if you have the original 12v in the RJB, it will only support 10 amps max. Good enough for LEDs, but if you're pulling a newer vehicle, you might
see problems with the blinkers. We pulled a 91 Explorer on that circuit with no problems, but when we upgraded to the 03 Explorer we found that
when the blinkers were used with the tail ligts or brakes on, the opposite side would dim in sympathy due to voltage drop, making the blinkers almost
useless on the toad. I eventually ran a 10 ga wire from the A/C J box and that cured the problem.

If you need help with figuring this out, feel free to give me a call, or email.

craig

bevans6 · January 14, 2011, 12:56:36 PM

what I find is that you get a lot of cross-talk on the tone to adjacent wires in a bundle. I have one, but I haven't played with it enough to figure it out right.

Iver · January 14, 2011, 08:28:33 PM

I have one of those tracer units. It is a 120v. I use it around the house to check outlets relation to breakers. It works ok for that.

I do have a Vanner equalizer for my coach batteries. The thing is it's still on my workbench. It's on my long "to do" list though.

I also looked up a Victron 20amp 24-12v converter I can pick up for @ $90. And thanks for the other link Cable.
But for the cost I think running a wire as Craig says would probably be more practical.

My house batteries are about the same distance away as the A/C junction box so I guess I will have my choice.
Again, thanks, Iver.