Tehben Dean wrote:
> Before I actually purchase, would there be any benefit to
> using #2 if I found deal?
I suppose that depends on the cost of the #2 terminals you would need vs
the #4 you could use. A deal on the cable could actually be offset by
the difference in terminal cost!
The #2 will run a bit cooler than the #4, but that's really the only
benefit.
Quoted below is a gem from Bill Dube explaining the wire size issue more
clearly.
Cheers,
Roger.
> -----Original Message-----
> From:
[email protected] [mailto:
[email protected]]
> Sent: Tuesday, June 04, 2002 6:11 AM
> To:
[email protected]
> Subject: Re: cable amp ratings - The Facts
>
> >Arthur Matteson wrote:
> >> So do the wire rules not apply to cars?
> >
> >Actually they still apply, but use the degrees C rise for cross
> >section, and current flow equation. Then take into account how
> >fast you can disipate the heat away from the conductor bundle.
> >>From that you then can figure out the minimum wire size needed
> >then double to quadruple it to handle individual strand breakage
> >and unexpected conditions. No, I don't know the actual equations
> >off the top of my head and those books are packed.
>
> It is all about heat transfer.
>
> First, you determine how hot the insulation can get
> before it is damaged.
> This can be 60 C (or less) for thermoplastic (like vinyl)
> insulation. (An example is the pretty stereo wire that
> turns to goo when the wires get even slightly warm.) Some
> of the Silicone or Teflon insulated wiring can take as
> much as 200 C without a problem. It is just as pretty as
> stereo wire, but costs quite a bit more. Welding wire is
> typically 75 C or 90 C insulation.
>
> There are two operating regimes, intermittent and
> continuous. The continuous rating is what you will find
> in the National Electrical Code, Section 70, Tables 310-16
> through 310-19. For DC use, the formula is
>
> TC-TA
> I = Squareroot( ----------------)
> RDC * RCA
>
> where
> TC = conductor temperature, C (what the insulation will
> withstand)
> TA = The ambient temperature, C
> RDC = The DC resistance of the conductor at TC
> RCA = Effective thermal resistance between the
> conductor and it's surroundings. (Typically
> 450 to 650 C-cm/watt)
>
> 2/0 welding cable works out to be 200 amps continuous.
>
> The intermittent rating is what seems to confuse most
> folks. From Beeman's "Industrial Power Systems Handbook"
> page 183, we have the formula for copper conductors:
>
> 1 (T2 + 234)
> t = ----------------- log10 (---------------)
> I (T1 + 234)
> (33) * (-------) ^2
> CM
>
> where
> t = time, seconds
> I = RMS current, amperes
> CM = Conductor cross section, circular mils
> T1 = Initial conductor current, Celsius
> T2 = Final conductor temperature, Celsius
>
> Eye-balled from the chart on page 184,
>
> 2/0, 75 C rise,
>
> RMS Amps t, seconds
> 8,000 1
> 3,300 5
> 2,200 10
> 1,400 30
> 900 60
>
> 30 second duration for 75 C rise (This would be for a "peppy" EV)
>
> Gauge RMS Amps
> 10 120
> 8 170
> 6 280
> 4 530
> 2 700
> 1 800
> 1/0 1100
> 2/0 1300
> 3/0 1700
> 4/0 2100
>
> This formula assumes that all the heat is absorbed by
> the copper and none is transferred to the air. This is a
> pretty good assumption for short time intermittent loads
> like this. This assumed temperature rise (75 C) would be
> good for welding wire type insulation, but way off for goofy
> vinyl insulation.
>
> You can see that there are no simple answers.
>
> The keys are the "cruising" amperage and the duration
> and magnitude of the intermittent amperage. If you have a
> very light EV and a huge controller, the duration of maximum
> current will be very low, perhaps 15 seconds or so.
> A heavy EV with a smallish controller will have max current
> durations of perhaps 10 minutes or more.
>
> The KillaCycle has a max motor current of 1400 amps
> that lasts about 10 seconds. I use #2/0, but #1/0 welding
> cable would more than thick enough for the motor leads.
> #1 might get a little warm.
>
> Since the battery current is essentially a ramp from
> zero with a peak of 1400 amps, the RMS current during that
> time is 808 amps. (To find the RMS value of a ramp, divide
> by the square root of 3.) This mean that the minimum wire
> size would be about #3 (from the chart.) I use two #6 flat
> copper braids in parallel for battery interconnects and they
> work just fine.
>
> These calculations will give you the minimum wire
> diameter based on allowable insulation temperature. It turns
> out that for acceleration performance, like on the drag strip,
> the optimal wire diameter typically comes out a little smaller!
>
> I haven't done the calculation for optimal range, but
> it probably close to the NEC size for the cruise current.
> You still have to do the temperature-limit calculation above
> for the max current to make sure that you don't damage the
> wire during max throttle accelerations.
>
> _ /| Bill "Wisenheimer" Dube'
> \'
' <
[email protected]>
> =(___)=
> U
> Check out the bike -> http://www.KillaCycle.com
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