Calculating alternator requirements

[mad accountant]

I'm OK with mechanical stuff, but electrical is just to the left of witchcraft if you ask me. So, how do I go about figuring out how large an alternator I need, based on the accessories I have now and what I may do in the future. I have a relatively new 60 amp alternator and have added no electrical accessories as of yet. I may look at things like electric fan, electric water pump, stereo, etc., and want to make sure my current alternator is up to snuff, and/or upgrade when necessary. I just have no idea how to know how much is enough, and don't really want to go buy a bigger alternator if I don't need to.

 

[67Dart273]

FIRST question is "what about wiring?" If you have a factory harness, running more than about a 45A is foolish:

On this website:

MadElectrical.com - Electrical Tech

read this article:

Catalog

On a side note, before you "jump into" electric fans consider this:

They are not a cure all, and there's been "several" members on here who have had nothing but troubles. Not enough fan, restricted airflow, etc. Bear in mind YOU are "re-engineering" the entire cooling setup. So base your fans on what others have done "that works." GOOD fans draw LOTS of current. There is no way around it. You might just consider 'not' using them at all!!

Once you know what you are going to do in that regard, really, it's a simple matter of adding up accessories and deciding "what all" you will have online at the same time, plus some headroom to charge the battery afer starting.

 

[greg r]

There are reference charts, materials, etc., that have amp draws for various electrical loads such as head lamps, tail lights, blower motors, and other stock equipment. You can also reference loads for cooling fans, sound systems, power seats, a/c clutches and on. Tally up the loads on one side of some paper. Then look up wire gauges and what current they can handle and see what you have in the vehicle. It comes down to protection and distribution. Think of it as sizing water pipe for a home or sprinkler system; current becomes gpm. The more water you need, the bigger the pipe to get it there. However unlike pipes which if too small you run out of water; in conductor layout, too small means hot terminals and leads to melted wiring; especially if the alternator far exceeds the system's capacity.

 

[crackedback]

Mad has some good info, I don't agree or like their approach on the wire around.

Look for a site that has an amp draw chart. This one will give you a general idea of charging requirements.
Calculate the Amperage You Need

Sizing the wire is important, the difference in performance between a properly sized and undersized system can make for big changes. Look at my headlight relay kits as an example.

 

[72bluNblu]

I try not to disagree with Del on electrical matters, because I'm not anywhere near his level of knowledge. But, my experience with alternator output and the stock systems has been a little different...

I run 100 amp alternators on both my Challenger and my Duster. My challenger has ~60k miles on it with that alternator, and it's harness is completely stock. Now, it doesn't have a ton of electrical draw either, it still runs a mechanical fan, but the wiring configuration is stock otherwise. My Duster has been modified a bit to relocate the battery, with help from Del to make sure I didn't burn my car down (and I probably would have left to my own). But it still retains the amp gauge and it still sends its power through the bulkhead connector. It has a ford contour dual electric fan set up as well, which draw a bunch at start up, ~40 amps depending on who you ask, with around 20 amps running. The fans pull through a continuous duty relay, but that's still hooked into the stock system. The electric fans on that car outperform any mechanical fan I've run, they're a great addition if you take the time to set them up properly. But it does require some some work to set them up properly.

The later A-bodies came with 60amp alternators right from the factory. I still plan on doing the MAD bypass on all of my cars, I'm not saying its a bad idea at all or that the bulkhead connectors aren't problematic, they absolutely are. The Challenger is apart now and with the dash out I'll do the conversion before it goes back together. I've already done the gauge conversion on a spare amp gauge I had for the Duster, I just have to pull the dash again. There's a few things I'm going to do when I do that, so it's on the list.

But if you check the bulkhead connection, clean up the terminals, and make sure that everything has a good, clean, tight connection I don't think they're a time bomb. I'm pushing my luck, no doubt, with the 100 amp alternators. But I've been monitoring the connections since I installed those alternators, and I haven't noticed anything burnt, melted, or damaged. If I'd ever come across anything I'd have done the bypass immediately. I'm not saying every car can run a 100 amp alternator without problems, just that I have.