Re: cable amp ratings - answers to wrong questions

["Dave Davidson" <davetex99@hotmail.com>]

Generally, wire is rated at a continuous current.  If you run more current 
through it than its rating, it will generate more heat than it can dissipate 
and will get hotter than the insulation is rated for - IF you run the 
current through it long enough.  If the insulation gets too hot, it will 
melt and can cause a short or catch fire (or both).  The more current you 
run through it, the faster it will reach it's maximum temperature.

You can exceed the rating for the wire but shut the current off before it 
gets too hot and be fine (same as running a motor above it's rated 
capacity).  However, remember that the heat you're generating is power 
wasted.  That may be acceptable if you just need to move a lot of amps for a 
short period of time. That's a judgement call you have to make.

When wire is put into a cable assembly, it's harder to get rid of the heat 
generated, so the current rating is reduced.  If you look at the various 
tables in the National Electric Code, you'll see different ratings for the 
same size wire depending on it's insulation and if it's in a cable assembly, 
conduit, etc.

Dave Davidson
Laurel, Maryland
1993 Dodge TEVan




>From: Victor Tikhonov <vtikhono@lsil.com>
>Reply-To: ev@listproc.sjsu.edu
>To: ev@listproc.sjsu.edu
>Subject: cable amp ratings - answers to wrong questions
>Date: Thu, 30 May 2002 12:19:51 -0700
>
>The trouble with all the current ratings found in these books is that
>it's pointless.
>
>What means  max current for #4 is 95A? At 96A it blows up?
>
>All it means that at above 95A the temp is more than rated value
>manufacturer chooses as safe one. And safe rated value listed by
>most manufacturers is dictated by liability reasons, not technical.
>So if I'm willing to go over this rated temp (especially for a few
>seconds while accelerating), 100A, 150A and may be 200A is ok
>*for my application* for 95A "rated" cable.
>
>This all has to be calculated based on voltage drop, power dissipation
>and insulation property allowing the heat to escape. Then variables
>like if it's in the open air or bundled and buried under carpet
>come to play. So, just looking up info in household reference books
>is almost useless.
>
>Other day I went to home depot to get a wire to run 240 to other side
>of my garage there ai plug my EV. For fun I asked what wire size
>would I need for 40A load. Sales woman said #6. I asked, can I use
>thinner #8? No, it's rated for 30A she said. So, I asked, what exactly
>happens if I run 40A through #8 wire?
>
>Reply was "we were told to inform the customers that this is unsafe
>and they are not suppose to do it".
>
>Me: I understand this position, but this wasn't my question.
>THe question was what happens to the wire.
>
>She: I don't know and not doing to tell you that "nothing happens" -
>if you burn down your house, you'll come back after me.
>
>
>So, who do you think writing these references books?
>
>Victor
>
>
>
>Mark Hanson wrote:
> >
> > My Howard Sams book of Electronic Tables shows:
> > #4 is .2485 ohms per 1000' 95A max
> > #2 is .1563 ohms per 1000' 150A max
> > #1 is .1239 ohms per 1000' 190A max
> > 1/0 is .09825 ohms per 1000' 260A max
> > 2/0 is .07793 ohms per 1000' 300A max
> >
> > So number 2 cable is fine for a 120V 2000-3000lb vehicle with a ADC 8 or 
>9"
> > motor. The motor to controller cables need to be twice as thick 2/0 (2X
> > amps) and be as short as possible for back emf spiking and I2R losses at
> > pulsed current. The motor and power cables must be ty-wrapped together 
>if
> > you like listening to AM.
> >
> > -----Original Message-----
> > From: Victor Tikhonov [mailto:vtikhono@lsil.com]
> > Sent: Tuesday, May 28, 2002 9:41 PM
> > To: ev
> > Subject: Welding cable amp ratings
> >
> > Listers,
> >
> > Does anyone know a (web) source for reliable info about a welding
> > cable current handling capability? Net search didn't yields anything
> > meaningful to me. I want to know the voltage drop on X feet
> > per Y amps (whatever data available) for standard gauge sizes,
> > (gauge 4, 2, 1 and 1/0 will be enough).
> >
> > Thanks in advance for leads.
> >
> > Victor
> > '91 ACRX




_________________________________________________________________
MSN Photos is the easiest way to share and print your photos: 
http://photos.msn.com/support/worldwide.aspx