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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