alternator wire size MC 8

[rcbeam]

For the background:  The P/O replaced the original alternator with a 75 amp model as he also removed the bus heat/air.  He replaced it because he kept burning out alternators.  Now, I am having to replace his 75 amp alternator.  I bought a 150 amp Leece Nevile.  In this process I have found some bad wiring that needs to be replaced in the engine compartment.  The wire that goes from the alt pos to the rear panel box stud is too small, and the wire that goes from the rear panel box stud across the bus to the engine compartment stud is really in bad shape.  Someone covered it with the plastic wire wrap stuff and once I looked inside I saw why.  All of the insulation is complete gone for about half the length of the wire.  So next week I will have a new cable made up to replace this. 

My question is:  according to my book the cable from the alt pos to the rear panel box stud is 1/0 and the cable from the rear panel box stud to the engine compartment stud is also 1/0.  This wire size is too small according to what I find on the net for a 225 amp alternator, but I can't imagine MCI putting in wire that is too small.  What  do I not understand about this?

It seems the further I look, the more I find wrong.  I'd rather find it now than later, but sometimes I feel like it will never end.  Can anyone explain the wire size issue to me?

Tx,
Russell

[pvcces]

I know that chassis sizing is smaller than house wiring, but I don't think that using the smaller wire is smart. You have the reason why in front of you.

I think that the chassis sizing does not figure on a constant load. However, in our buses, we might have a constant or near constant load pumping our house bank back up.

Our coach is 12 volt, uses a 220 amp 50DN, and uses 4/0 wire between the generator and the starter circuit. That makes me think that I would probably use 4/0, if I was replacing that wiring.

I would be conservative in choosing wire for any constant load to avoid overheating the wire.

For what it's worth.

Tom Caffrey

[buswarrior]

Also, a bigger wire can help to delay the effects of deterioration of corrosion that those of us in damp or salty environments have to deal with.

Coach re-furbishers regularly pull new heavy cables as part of the job.

Busnuts are well served to do likewise.

happy coaching!
buswarrior

[rcbeam]

All good points, but maybe I didnt' ask my question correctly.  Why does the manual call for 1/0 cable from the alt pos to the rear panel stud, and from the rear panel stud to the engine compartment stud when the bus originally had a 50DN alt?  The books also shows a 4/0 from the batts back to the eng comp stud, and a 2/0 from that stud to the starter.

So since I am having a new cable made from the alt pos to the rear panel stud, and a cable made from the rear panel stud to the eng comp stud, which size should I use?  The orig 1/0?

Also, I no longer have the 50DN, but I have a 24v 175amp alt.

MCI using 1/0 cable makes no sense to me.  Can anyone explain this?

Thx again,

[stevet903]

How long are the cables?

[JohnEd]

Russel,

A sage  old black man I used to work with explained that to me.  He said "cause you weren't here when they did that".

You have the info on what size you should have in there.......get-r-done!

Its a kinda "God grant me the will to change the things I can and the patience to put up with those that I can't." or something like that.

Be well and happy,

John

[zubzub]

The onl 2 cents I have are that it's 24V so the wire sizes are going to be about half of what you are used to from 12V vehicles.  The other cent is buses manufactured are penny pinchers like everyone else, they may have specked it pretty close to the  bone.  i would probably just follow the manual, or I might go a little bigger.

[rcbeam]

ok guys, thx for the opinions.  The cable that is bad measures out to be about 10 ft long.  I guess I'll just go back with what the manual says should be there, which is 1/0.  I hope to do this only once. 

I just can't believe people would cover up such a wiring mess.  It's prob a days work and maybe $75-100 for the cable.

Russell

[stevet903]

The original 50DN 24V alternator could put out a max of 240A.  If you use this calculator ( http://www.powerstream.com/Wire_Size.htm ) and a 10 foot 1/0 wire, you will get a 2% voltage drop at max amps.  I would guess that most engineers would size for a 2-3% loss at max amps, figuring that the alternator would rarely run for long at 240A. I'd guess that with A/C, blowers, headlights, etc, that the steady state draw on the alternator would end up around 70 or 80A.  Rounding up to 100A with the same 10 ft 1/0 wire you get about a .8% drop.  I'd bet that the engineers decided that a bigger wire, costing more, wouldn't be cost effective.  I'd say you'll be fine with a 1/0 wire and a 175A alternator.

I would check the other connections, grounds too, for corrosion, etc before putting this all back together.  Continually burning out alternators, and what sounds like a wire that got hot enough to melt the insulation, sounds like something is not right.  Bad connections can waste a lot of amps, creating heat at the bad connection.

Steve

[rcbeam]

Today I went to a good local heavy duty electrical shop and ordered the 10 cable, a 4ft cable to go from the alt pos, and a 4ft ground cable from the alt gnd.  He looked up the welding cable that he uses and said that 1/0 would be good for up to 300 amps at my distance, and since I'm running a 150 amp alternator, I'd be more than fine.  They do really good work and he made them up on the spot.  All three cables with proper ends to fit what I'm hooking to was about $110.

On the ground for the alternator, I was going to bolt it from the alt gnd to a bolt on the engine gnd. He suggested I go to the starter ground, but that would be all the way back across the engine to the other side and I only had them make up 4ft. 

Now my next question.  I can't find a ground strap off the engine to the chassis anywhere like I had on my GMC.  Any MC8 owners care to tell me where the ground strap should be located?

[bevans6]

On my bus (MC-5C) the ground strap was directly to the ground lug on the starter motor, to a chassis lug welded on the frame near where the air conditioner frame is.  If you think about it, just about the only thing that needs to be grounded on the engine is the starter motor so that is going directly to the need.  On the wire size, the wire leading to the bus electrical system that runs from the main battery switch to the AC junction box, with all the feeds to all of the electrics at the front half of the bus, is a 1/0.  The wire between the main battery switch and the engine bay stud that feeds the starter motor is 4/0.  I guess they felt that the main load is the starter, and the actual loads that are continuous can be fed fine by the 1/0 cable.  You size the cable for the design load, not the max output of the alternator sometimes.

Brian

[rcbeam]

THX Brian.  I'll check to see what kind of gnd wire I have on the starter. 

[HB of CJ]

Ten feet is pretty long.  Anyway the length can be reduced?  FWIW, we tried to keep the DC voltage drop less than one percent when using off the grid house PV panels and storage panels.  HB of CJ (old coot)

[rcbeam]

Since it was originally installed that way by MCI I figure 10 ft isn't too long.  Not nearly as long as the run from the batts to the rear.  Besides, I replaced it with welding cable that was capable of higher amps that the original, and the alt that I put is is 150amp instead of the 225 or so original one.

I'm really glad I found this mess now instead of later on down the road with a fire or something.

Next to check on the grn wire from the starter to the frame.  If it'll EVER quit raining.