90 amp altenator

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domnic

Senior Member
Occupation
Electrical Contractor
I just put a new alternator in my truck it is rated at 90 amps the wires coming from the alternator look like #14 wire how can this put out 90 amps?
 

cadpoint

Senior Member
Location
Durham, NC
Think DC first, or remember it is

Ohm's Law

V=IR

12= 90(R)

R= 7.5

Opps, no home work problems! :)

Even on the high side or average 14 V (what one might see on the dash)

14=90(R)

R= 6.42 even better
 

cadpoint

Senior Member
Location
Durham, NC
So, put 120V in the equation, that would make things exciting!

Your forgetting that your only at 12V!

Your forgetting that in the end it's still only one part of the ratio side of an equation problem, because
you have to do something to get across the equal sign!
Because of this, if one side goes up the other side goes down.

We use this at work, we use it every day!

IE we use and solve for numbers and the number don't lie.

That's why it's a "LAW". It's been proofed.

It's a simple A=BC equation.

V=IR

Power = Watts
I = Amps
Energy = Voltage
Resistance = Ohms

I've been corrected before here for stating PIER/WAVO respectfully, But if one can remember the plain
and simple reference starting terms, it might really help in the end.

Remembering when, where, why, to use all that might make up the underlined
initials; that's out of my control .
 
Last edited:

domnic

Senior Member
Occupation
Electrical Contractor
90 AMPS

90 AMPS

I HAVE NO CLUE WHAT YOU ARE TALKING ABOUT ?????????????????????????????
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
141119-0646 EST

donmic:

My ITT Reference Book provides the following information for copper:

.... # 14 ......... # 12 ......... # 10

2.525 ........... 1.588 ........ 0.9989 .............. ohms / 1000 ft at 20 C
166 .............. 235 ........... 333 .................. amperes fusing current

I doubt # 14 for the main current wire.

Five feet of # 10 at 100 A is a voltage drop of 0.5 V . Of # 14 is 1.25 V .

.
 

kwired

Electron manager
Location
NE Nebraska
I am not up to speed with current designs of these vehicle systems - the answer may lie in some of the design details.

You said the "wires" and not the "wire" - very possible no single conductor there carries 100% of the 90 amp rating. If that alternator produces 3 phase AC current and the rectifier is remote from the alternator housing, you already have at least three conductors carrying output - 1/3 in each conductor.

14 AWG would be a poor design choice to carry 90 amp of current from 12 volt source just because of voltage drop.
 

RichB

Senior Member
Location
Tacoma, Wa
Occupation
Electrician/Electrical Inspector
Think DC first, or remember it is

Ohm's Law

V=IR

12= 90(R)

R= 7.5

Opps, no home work problems! :)

Even on the high side or average 14 V (what one might see on the dash)

14=90(R)

R= 6.42 even better

Huh??:huh: I think you missed in the math dominic, unless I am confused again:?

12=90(R)
12/90=R
.133=R

or
14=90(R)
14/90=R
.155=R
 

Besoeker

Senior Member
Location
UK
So, put 120V in the equation, that would make things exciting!

Your forgetting that your only at 12V!
With respect, that's an irrelevance.
The conductor size has to carry the current regardless of the voltage.
90A is 90A

If the op is correct with his guess that is #14 AWG, then 90A would be beyond what that could normally be capable of.
We do SI here and #14 AWG is about 2mm2.
Our nearest standard size is 2.5mm2.
That, according to "the regs", BS7671, is good for about 33Adc in free air.

But, and there did have to be a but in there didn't there? :p
I can think of a couple of possibly mitigating circumstances.
The conductor insulation may be thinner given that it has to be rated for 12V or thereabouts so, even if it looks like #14 from the outside, the actual conductor cross sectional area could be significantly greater than #14 for general purpose use.

The other point is that the alternator is quite unlikely to be called upon to put out 90A continuously - that's over 1kW at 12V. That being the case, the 90A is short term and the conductor size might take that into account.

And, given that the alternator is in the engine bay, you'd think that the wiring insulation would have to be rated for elevated temperatures thus making higher currents acceptable/permissible.
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
I just put a new alternator in my truck it is rated at 90 amps the wires coming from the alternator look like #14 wire how can this put out 90 amps?

It's probably bigger than #14, but is still smaller than 'we electricians' would use for 90 amps because of a couple reasons. A common size is a #10.

First, the length will be short. Second, the insulation on wiring in an engine compartment is VERY high temp and as such the conductors can handle larger current.

Also, 90A is the peak output. There really isn't anything in a regular vehicle that will present a continuous 90 amp load for three hours.

If you want to see small wiring for a large load, check out the starter wiring on older Chryslers. A starter draws around 125 - 175 amps and is carried by a #6.

Automotive electrical standards are very different than premises standards.
 

Smart $

Esteemed Member
Location
Ohio
...

And, given that the alternator is in the engine bay, you'd think that the wiring insulation would have to be rated for elevated temperatures thus making higher currents acceptable/permissible.
Several automotive wiring spec's (this side of pond; SAE, mainly; Ford; Chrysler) state the max' insulation temperature rating of engine compartment wire at 125?C (257?F). AFAIK, there is no automotive spec' similar to 110.14(C) of the NEC.

I am wondering whether the OP is referring to B+ wire(s), or "control" wires...???
 

rlundsrud

Senior Member
Location
chicago, il, USA
Automotive wiring is somewhat different in that the conductor lengths are relatively short and the wiring is designed for much higher temperatures as has been mentioned previously. The typical conductor sizing requirement for a 90 amp alternator is #8 if the conductor length is less than 5 feet and #6 if greater. I am quite certain that the wire is not a #14, it would not meet the manufacturer's specs for that size alternator.

Bob
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
Automotive wiring is somewhat different in that the conductor lengths are relatively short and the wiring is designed for much higher temperatures as has been mentioned previously. The typical conductor sizing requirement for a 90 amp alternator is #8 if the conductor length is less than 5 feet and #6 if greater. I am quite certain that the wire is not a #14, it would not meet the manufacturer's specs for that size alternator.

Bob

Wire sizes vary from one make to another. I owned and operated an automotive electrical repair facility for over a decade and never could figure out the reason for that.

Below is a chart I found for wire size. It is generic, but as you can see, length is a factor. According to the chart, #12 is good for 100 amps if it's not too long.

http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm

Sometimes other factors like flexibility are figured in. Not always correctly, I might add. For instance, GM oversized the wiring in the door jambs for power windows and the like. They were huge, finely stranded and had thick insulation. Ford and Chrysler just used the same size as the rest of the circuit.

Well, GM's engineers botched the job. I repaired hundreds of broken door jamb wires in GM's over the years and almost none from any other maker.
 

kwired

Electron manager
Location
NE Nebraska
Wire sizes vary from one make to another. I owned and operated an automotive electrical repair facility for over a decade and never could figure out the reason for that.

Below is a chart I found for wire size. It is generic, but as you can see, length is a factor. According to the chart, #12 is good for 100 amps if it's not too long.

http://www.offroaders.com/tech/12-volt-wire-gauge-amps.htm

Sometimes other factors like flexibility are figured in. Not always correctly, I might add. For instance, GM oversized the wiring in the door jambs for power windows and the like. They were huge, finely stranded and had thick insulation. Ford and Chrysler just used the same size as the rest of the circuit.

Well, GM's engineers botched the job. I repaired hundreds of broken door jamb wires in GM's over the years and almost none from any other maker.

I have a 2004 RAM truck that I have repaired the door jamb wires many times - but only on one door and it isn't even the driver door that gets the most flexing. I just butt splice them together when they break but have done so several times and more than once on same conductor in some instances.:(
 

K8MHZ

Senior Member
Location
Michigan. It's a beautiful peninsula, I've looked
Occupation
Electrician
I have a 2004 RAM truck that I have repaired the door jamb wires many times - but only on one door and it isn't even the driver door that gets the most flexing. I just butt splice them together when they break but have done so several times and more than once on same conductor in some instances.:(

When you get ready to pull an entirely new conductor in, PM me and I will tell you how to do it. You have to remove the door and kick panel trim, but not any of the wiring harness.
 

kwired

Electron manager
Location
NE Nebraska
When you get ready to pull an entirely new conductor in, PM me and I will tell you how to do it. You have to remove the door and kick panel trim, but not any of the wiring harness.
Exactly why I have been using butt splices - don't want to take all that apart. This is rear door of a 4 door vehicle - kind of a pain when door lock quits or power window quits - especially if not all the way closed when it quits. Sometimes have lost speaker in that door.
 
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