View Full Version : Alternator Loads and Sizing

July 25th, 2017, 20:08
After some discussion on another thread about larger than stock alternators and wiring, and some disagreement about output, I wanted to post up some numbers for how much current the alternator was producing with different loads.

Measurements were taken with an Extech MA120 AC/DC Clamp meter. It's no Fluke, but should be good enough for these purposes.

I tested on both my trail rig and DD XJ, with the clamp meter attached between the fuse/breaker and the PDC.

Test Setup 1: '97 XJ, stock alternator (unknown if it's the 90 amp or 117 amp version, I've never bothered to check), upgraded 4 gauge wiring harness, 120 Amp circuit breaker between alternator and PDC post. Aftermarket radio and speakers but no external amps. Headlights are Autopal H4s with the bulbs they came with, upgraded Putco relay harness.Measurements:

While cranking: 95-100 amps

These are incremental so each measurement includes the previous loads as well

Hot Idle, nothing else on: 13 amps
Headlights on High: 33 amps
A/C on high (cooling fan on and blower motor on high): 70 amps
Radio on w/volume turned up: 71 amps

Test Setup 2: '99 XJ, 160 amp Durango alternator, upgraded 4 gauge wiring harness, 150 amp ANL fuse between alternator and PDC post. Aftermarket radio and speakers, but no external amp. Headlights are Autopal H4s with Hella 85/100W bulbs with DIY upgraded harness.Measurements:

While cranking: 95-125 amps

These are incremental so each measurement includes the previous loads as well-

Hot Idle, nothing else on: 14 amps
High Beams on: 33 amps
A/C on (cooling fan on, blower motor on high): 71 amps
Radio on w/volume turned up: 74 amps


Higher current while cranking on the '99 XJ. The meter doesn't have the response time to see what's really happening so I just made a note of the peak measurement I saw. Out of curiousity, I swapped the 120 amp breaker from the '97 XJ in place of the 150 amp fuse on this XJ and after starting it 5 or 6 times in a row, it never popped the breaker. So that current didn't last very long, and the breaker doesn't trip instantly.
Radio made a bigger impact on the '99 than the '97. I suspect this is mostly due to differences in standby power usage, and what the radio is doing when it's actually "off", as the headunits are different brands but both rated for similar output on each channel.
I revved the motor while taking the top readings to see if I noticed any changes in current, and there weren't any on the '97. The '99 made a few extra amps when the radio was on but it was less than 5 amps extra.
Apparently higher wattage bulbs aren't really higher wattage. Expected a difference between the headlight measurements but there wasn't a measurable one.


The max power draw on a mostly stock '97-'01 XJ with no fog lights or driving lights appears to be about 75 amps.
Stock alternators are capable of producing ~70 amps at idle
A 150 amp fuse on a 160 amp alternator is fine if you don't need the last 10 amps (apparently I don't, as it hasn't blown once in 4+ years, but I've never run a winch either).

I didn't expect the current draw to be as high as it was with everything turned on, and I didn't expect the alternators to produce that much power at idle.

I used to have two extra sets of driving/flood lights on the front of this jeep with upgraded bulbs in both, and a set of reverse lights, I would have liked to see the measurements with all of those running but I'm in the process of re-wiring some switches and upgrading to an LED lightbar so it'll be a while before I can get a measurement on extra lighting. Based strictly on the bulb ratings, I had about 400 watts in lighting, which would have been around another 30 amps.

My trailer isn't here or I'd have measured with the trailer lights running as well, not sure when I'll be able to get those measurements but I'll update when I can.

Anyone want to see something more specific tested? Problem with my measurements? Let me know. I looked around and didn't find anyone who had actually posted all the stock numbers so figured some might find it useful.

July 25th, 2017, 20:22
So here's couple thoughts. You just demonstrated that 10 gauge could be shortchanging the system. Also there is an additional benefit of thicker wire - you have less of a loss on the run.

Green XJ Jeep
July 25th, 2017, 20:53
I ordered a 2ga wire set from Jeepcables so I will try and do a before and after amp and voltage drop test.

July 26th, 2017, 15:29
The overall results are what I expect i.e. the original 90 AMP alternator is capable of powering everything that are standard XJ on a XJ although the head light draw is a bit more than I expected but,....after having calculated the total wattage, it seem accurate.

The results also proves another area of the discussion correct or incorrect depending on you individual prospective. Form an OEM alternator to an aftermarket 200+ amp, the output power at any given time will be dictated by the amount of load/demand on the electrical system and thus a low amperage fuse or breaker is fine as long as the demand remains below the rating of such fuse or breaker.

July 28th, 2017, 08:10
Just did a 4g battery cable and 136amp alternator upgrade. Havent got to turn key yet but good info. I also upgraded the head light harness to hopefully reduce draw. A winch and OBA are in my near future (within a year hopefully) so the extra amps are more of a failsafe for me than anything else. Appreciate you taking the time to test all this.