Alternators

[Ex Sailor]

While upgrading various things on our ( new to us ) boat, something jumped out at me & specifically: We have twin 210 HP Cummins with what I was told were twin 120 Amp alternators, but the charging wire from each alternator looks like # 14 AWG maaaaybe # 12. Is that typical ? Out of common sense, I was going to change them to at least # 6 or # 4 but one is going to be tough to access with my waistline ..

So again, should I get into this thing ?
How would I find out what size the alternators really are ?

Thanks in advance .... Frank B.

 

[DavidM]

#14 or even #12 is woefully small for almost any size alternator. Are you sure that is the positive output wire for the alternator? If so, then you need to fix it because as is, it is a safety hazard. #12 can safely carry only 38 amps inside an engine room. See Conductor Sizes | Ancor.


You will need at least #4 wire if it really is a 120 amp alternator.


And you should question how anyone would wire such an alternator with small wire. It couldn't possibly be Cummins OEM wiring. Someone had to have changed it, maybe when they installed a different alternator.


Can you read the part number and manufacturer on the side of the alternator? Cummins uses Delco alternators and Sbmar's site indicates 19SI at 105 amps and 22SI at 135 amps were used on Cummins engines. See Alternators for Cummins Marine Diesel Engines.


David

 

[Ex Sailor]

#14 or even #12 is woefully small for almost any size alternator. Are you sure that is the positive output wire for the alternator? If so, then you need to fix it because as is, it is a safety hazard. #12 can safely carry only 38 amps inside an engine room. See Conductor Sizes | Ancor.


You will need at least #4 wire if it really is a 120 amp alternator.


And you should question how anyone would wire such an alternator with small wire. It couldn't possibly be Cummins OEM wiring. Someone had to have changed it, maybe when they installed a different alternator.


Can you read the part number and manufacturer on the side of the alternator? Cummins uses Delco alternators and Sbmar's site indicates 19SI at 105 amps and 22SI at 135 amps were used on Cummins engines. See Alternators for Cummins Marine Diesel Engines.


David

Thanks David, Yes I'm sure it's the +, and I think you're right re: them being changed, BUT Both !? Even considering the " free air " rating of a conductor, you're right about the # 12 +/- a few amps. I looked around and I have some ultra flex marine # 4 in my shop so I will change that this w/e ( I think that may be close to what I need ) and I will look at the nameplates as well ........... but the Stbd. engine will be tough to get to. Thanks. Frank B.

 

[DavidM]

Ex-Sailor:


Given the sizing screw up, I would take a hard look at where that alternator output wire goes as well. In most cases the alternator output terminates at the big positive lug at the starter, which then goes to the starting battery with 2/0 usually.


But who knows. Sometimes the alternator output goes to a 1,2,all switch, and sometimes to an isolator, ACR or battery combiner. And maybe those are wired improperly. And even if wired ok, the length may be such that you need more than #4 for a decent voltage drop.


But if this is a simple 3 wire alternator, then just worry about ampacity which #4 covers. The output will rarely if ever really be 120 amps and even if it tries to put out 120 amps (into a big almost dead battery bank) then all that will happen is that the voltage drop will limit the current to something less than 120 amps.


David

 

[Bacchus]

Ex-Sailor
Agree w/ David - he's right on as always!
Here's a link to the ABYC table for 3% V drop which is what you should be shooting for w/ an alternator. #4 is minimal but possibly OK if the distance is only to the starter lug as David mentioned - that's exactly the way mine is wired.

ABYC 3% V Drop Wire Size

Also consider that the distance is total loop - pos to battery & neg return path for the sizing.

 

[Ex Sailor]

Ex-Sailor
Agree w/ David - he's right on as always!
Here's a link to the ABYC table for 3% V drop which is what you should be shooting for w/ an alternator. #4 is minimal but possibly OK if the distance is only to the starter lug as David mentioned - that's exactly the way mine is wired.

ABYC 3% V Drop Wire Size

Also consider that the distance is total loop - pos to battery & neg return path for the sizing.

You're both on point, thank you for your input. The distance is to the starter terminal and only about 2 - 2.5' so I'm thinking now that # 4 will do. My main reason being that the terminal post on the Alternator is 1/4" and that's the largest size of cable lugs that I have for that post. Rest of the boat c/w new 8D's etc. I have just rewired to all @ # 4-0. cheers .....

 

[Bacchus]

My main reason being that the terminal post on the Alternator is 1/4" and that's the largest size of cable lugs that I have for that post.

Sounds like you are on the right track...

Relatively easy to measure the actual V drop after completion just to confirm

For future reference remember that if needed (and you already have the #4 & lugs available) you can use 2 - #4's to increase ampacity / reduce V drop...provided you have the post length to accommodate them.

 

[rwidman]

I would expect to see the current output rating (and other specifications) on a plate riveted to the alternator case. If you can't read it, try taking photos from various angles.