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Old 26-04-2016, 15:38   #1
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Generator starter allowable voltage drop

I'm installing a 4 KW generator. The manual lied to me about the starter ampere, so even though I used a size larger than what based on the manual it is too small.

Anyone have a idea of what starter voltage drop is allowable? I want to calculate for the right cable (not too small, but not too large).

The supplier is now saying the starter is 1.1 KW so that would be 90 amps and I would need a 1 AWG cable to keep voltage drop to under 3%. That seems like a big cable for a little generator.
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Old 26-04-2016, 15:51   #2
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Re: Generator starter allowable voltage drop

About 10 volts at the starter during cranking. Any less will affect starter life, more is better.
1 AWG isn't large by battery cable standards. It may be 1.1kw rated but how much is it loaded? 60% ? 80%?
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Old 26-04-2016, 15:58   #3
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Re: Generator starter allowable voltage drop

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1 AWG isn't large by battery cable standards. It may be 1.1kw rated but how much is it loaded? 60% ? 80%?
Not by battery cable standards, but pretty big for a little generator starter. Apparently it is a secret as to what the load is.
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Old 26-04-2016, 17:44   #4
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Generator starter allowable voltage drop

Well if it's that important, you can use jumper cables and a clamp on multimeter to measure load.
I would have guessed 100 amps myself, but that's for just a few seconds and you have to assume if it's a single cylinder like mine, it stopped on compression, meaning of course the starter may not get a running start, but immediately have to deal with compression and accelerate the mass of the motor and generator, big flywheel momentum at the same time.

My windlass is I believe 1200 W or 1.2 KW for example.


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Old 26-04-2016, 18:54   #5
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Re: Generator starter allowable voltage drop

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Well if it's that important, you can use jumper cables and a clamp on multimeter to measure load.
You have a pair of 15' jumper cables?


Back to the question, is there an accepted allowable voltage drop for a starter circuit?
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Old 26-04-2016, 20:44   #6
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Re: Generator starter allowable voltage drop

Don't know an actual number, but in my experience, for a 12V system, I'd say that at somewhere in the range of a .3 -.4 v less than 12.5, you're gonna start getting slow cranking speeds or worse, that dreaded clicking sound...

I'm pretty sure you're supposed to use round trip lengths when calculating wire size, so 1/0 for 91.66 amps at 30 feet doesn't seem so big. This is a very intermittent circuit, so maybe you could get away with something a little smaller, but it starts to look like a false economy pretty quickly.

Can you move the generator starting battery closer?
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Old 26-04-2016, 21:16   #7
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Re: Generator starter allowable voltage drop

You may see instantaneous starting voltage drop below 10V.

Loaded starter speed is the limiting factor. Too slow and you wont fire.

If it starts and temp in the starter cables are not greater than ambient will be fine.


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Old 27-04-2016, 04:47   #8
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Re: Generator starter allowable voltage drop

If the available voltage at the starter is not within 0.5 to 1.0 volt of battery voltage, there is excessive voltage drop in the circuit.
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Old 27-04-2016, 05:02   #9
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Re: Generator starter allowable voltage drop

Well I did a few Google searches, rewording the search to get different answers, and read the best sounding hits for the first 4 pages of results. I couldn't really find a actual answer, just "noise". So since this is a critical electrical load I'm going to say the allowable voltage drop is 3% at the max power of the starter (I have the curve).
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Old 27-04-2016, 05:17   #10
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Generator starter allowable voltage drop

Quote:
Originally Posted by sailorboy1 View Post
You have a pair of 15' jumper cables?





Back to the question, is there an accepted allowable voltage drop for a starter circuit?

Yes, actually 25' but they were used for farm equipment where you couldn't always get that close. Last truck, I had a large deans connector in the grill so I didn't even have it open the hood, but neither here or there, but if you have cables, they need to be long enough I believe to reach the furthest thing that may need them.
Your generator is 15' from your engine? Cause I'd tap 12v off of the starter solenoid of the big engine as opposed to going all the way back to the bank, plus by tapping off of the solenoid, then your connected to whichever bank the battery switch has selected.
Many will tell you that you need a separate bank for the generator, but those same would tell me I need a separate bank for my Windlass too, and four banks on a 38' boat is I believe excessive.

My run to my 1200W windlass is probably 40' total and #2 wire seems to hold that load fine if that helps.


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Old 27-04-2016, 05:28   #11
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Re: Generator starter allowable voltage drop

[QUOTE=a64pilot;2107451]
Your generator is 15' from your engine? Cause I'd tap 12v off of the starter solenoid of the big engine as opposed to going all the way back to the bank, /QUOTE]

closer to 12' by the time it gets neatly routed, and that is tapping off the engine starter
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Old 27-04-2016, 06:29   #12
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Re: Generator starter allowable voltage drop

Sounds like your getting close
Although I feel guilty about running mine silly as that sounds, it does make things significantly different in the boat, my belief is it makes the boat a whole lot more self sufficient.
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Old 27-04-2016, 10:55   #13
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Re: Generator starter allowable voltage drop

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Originally Posted by sailorboy1 View Post
Not by battery cable standards, but pretty big for a little generator starter. Apparently it is a secret as to what the load is.
I can not imagine that type of draw on a starter for a gen. of that size. As fare as load, you don't start a gen. under load. JMHO
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Old 27-04-2016, 15:06   #14
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Re: Generator starter allowable voltage drop

Well it was really 33' round trip. Used 1 AWG cabling and it turns over just fine and started up, then died, started then died, started then died, started then died, started then died. So I started thinking fuel problem and went on a tighten hoses mission. Then it started and got to speed and died, started and died, started then died. So I drained the fuel separator/filter since it is early spring and it always has some water after sitting all winter. Then it started and ran almost 10 seconds at speed, then died, started then died, didn't start, didn't start, didn't start.

So I got pissed off and decided to come home.

I don't know if the line to generator got water in it or what so I'm going to hook a fresh pail and see what will happen. Looking through the manual again it may be that I'm at the extreme limit of the fuel line length. I didn't really notice that as I figured as long as the suction lift wasn't a problem it would be OK, but who knows.

But I've drifted way off topic and am venting
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Old 27-04-2016, 16:12   #15
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Re: Generator starter allowable voltage drop

Some have a low oil switch, both pressure and quantity if it's not fuel delivery issues.
I first tapped fuel supply into the extra bungs in my fuel tank, it would always start, then run slowly for awhile and maybe die and have to be restarted and on the second attempt continue to run.
I have since tied into the main fuel Racor filter which cut fuel line length in half and since then it starts and runs like it should.
I think installing an electric fuel pump as a boost pump will confirm if it's just too long a fuel line run.


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