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 Post subject: Wire Sizing and Fusing
 Post Posted: Fri Feb 19, 2010 10:10 am 
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Joined: Mon Jun 01, 2009 3:30 pm
Posts: 17
Location: Element One: Lawrence Tech University
Hello,

Appendix G lists a max continuous current for various sizes of wire. Does this table overwrite a manufacturer rating? I only ask because we have a commercial hybrid wire that is rated for a higher ampacity than the value listed in the table.


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 Post Posted: Wed Feb 24, 2010 9:36 am 
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Joined: Tue Oct 30, 2007 6:50 pm
Posts: 78
Location: Illinois Tech
I would say that your best bet is to contact the organizers and see if you can get an answer faster than waiting for them to read the forums and answer the questions. Out of curiousity can you give us a link to the wires weba page?


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 Post subject:
 Post Posted: Wed Feb 24, 2010 11:29 am 
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Joined: Fri Jan 29, 2010 11:56 am
Posts: 44
Location: St. Cloud State University
Would the commerical hybrid wire be any cheaper/lighter/less resistant per foot for it's rated amperage than an equivalent wie in appendix G? One thing that surprised me when I took apart a prius battery pack was how small the wiring looked. Not sure if it is commerical hybrid wire but it does have around 270V and probably not much more than 200A so it doesn't need to be very big. It also had orange insulation (no big shock there).

Sounds like a link to a webpage would be good idea!


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 Post subject: Re: Wire Sizing and Fusing
 Post Posted: Wed Feb 24, 2010 10:43 pm 
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Joined: Wed Jun 03, 2009 1:29 am
Posts: 117
Location: Texas A&M
E1-Dan wrote:
Hello,

Appendix G lists a max continuous current for various sizes of wire. Does this table overwrite a manufacturer rating? I only ask because we have a commercial hybrid wire that is rated for a higher ampacity than the value listed in the table.


At the competition last year in 2009, the table overrode everything. If you didn't meet the table, you did NOT pass electrical tech (a lot of teams had troubles with this). I would still ask for an official ruling, but I strongly suspect they still force you to stick to the table.

_________________
- Texas A&M 2009 FH Crew Chief
- 1st place FH '09: 981 pts


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 Post subject: Don't forget "time delay"
 Post Posted: Thu Feb 25, 2010 1:47 pm 
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Joined: Sat Apr 18, 2009 9:33 pm
Posts: 40
Location: Cal Poly San Luis Obispo
Don't forget that the ratings in appendix G are for max continuous ampacity ratings, which means you can take advantage of time delay or slow blow fuses for "short bursts" over the maximum continuous fuse rating. For example, the LPJ series fuses can handle current above its ampacity for a period of time before it blows (http://www.cooperbussmann.com/pdf/d9dc1f65-ef8e-44a3-8729-193e641be8c9.PDF) so you can push your car hard during the straits without blowing your fuse right away (but it will blow at extreme currents or sustained high currents). Using these kinds of fuses, I suspect, will allow you to achieve currents closer to your cable's rating (for short times, not continuously) without violating the rules.

I remember specifically we had trouble at last year's competition because we used fast-acting fuses rated for our maximum current draw, but one of the inspectors actually suggested using a time delay fuse (I think it was Charlie, but don't quote me on that).

Regardless, if anyone does run into trouble with fusing at competition, our team found that the best/quickest place to get a proper fuse in a pinch is the closest Graybar to the race track (its about 33 miles away from the speedway, just google "Graybar near Loudon, NH"). Of course, we wouldn't recommend going into comp planning on buying fusing there, but it's there if you need it.


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 Post subject:
 Post Posted: Wed Mar 03, 2010 2:59 pm 
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Joined: Mon Jun 05, 2006 4:05 pm
Posts: 111
Location: Rules Committee
I can confirm two key points that people have made above, and also make some other suggestions.

1) We go by the table not by what the manufacturer says, and

2) You can run much higher current than that for short times, using a time delay fuse. In some cases you might want two fuses--for example if your wire is 150 A on the chart in the rules, and you want to run 500 amps in short bursts for acceleration, you are fine by the rules if you get a slow 150 A fuse. However, you might also want a fast 600 A fuse--for example the slow fuse might not protect your motor controller, and some motor controllers specify that you should use a fast fuse with them. With both fuses in series, you meet the rules and protect your expensive motor controller.

The downside to that is that you'll need to do a bunch of analysis and/or testing to make sure that your fuse doesn't blow in a race. That's your engineering challenge.

You can attempt to convince the rules committee that your design is safe with a higher fuse current and the special wire, but the fact that it is listed in a catalog with a higher current next to it won't sway the committee. The question they would have is what is it about that wire that makes it OK to run higher current? Unless it's solid silver, it won't have any lower resistance than copper, so the heat generated by 500 A running through it will be the same as if you used any other wire. So the only thing I can think of that might be different is the insulation temperature rating. Maybe it's 180 degree C insulation. If that's the case, then you can run a larger current through it without the insulation melting--but having a 180 degree wire in your car might melt other things (e.g., conduit), burn crew members or otherwise cause problems. So you'd need to examine all those issues and prepare an argument that shows you've addressed all those issues. And even after you do that, it's likely the committee would decide, for the sake of a fair competition, to stick with the chart.

I also wonder why you want to use smaller wire. You'll get more voltage drop and thus decrease your acceleration capability and efficiency. If you are trying to save weight, consider aluminum wire. For a given resistance or current capability, you need a bigger wire, but even with the bigger wire it's much lighter. The only downside is you have to learn about how to make reliable connections to aluminum.

Of course the best design change is to use a layout with short wire runs. That decreases weight and resistance!


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 Post subject:
 Post Posted: Sat Mar 06, 2010 2:23 pm 
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Joined: Fri Jan 29, 2010 11:56 am
Posts: 44
Location: St. Cloud State University
is Silver stranded wiring available anywhere? I'm sure it'd be on the order of thousands of dollars per ft. but just in case it's around that'd be pretty cool.


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 Post subject:
 Post Posted: Sun Mar 07, 2010 8:53 pm 
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Joined: Mon Jun 05, 2006 4:05 pm
Posts: 111
Location: Rules Committee
I don't know about stranded, but here's some AWG 10 solid silver wire for $28/ft, with teflon insulation. Not as high a price as you'd imagined, but the gain over copper is small, and it's still worse than Al on a per-weight basis. If you actually got serious about using this you'd need to address the problem that it doesn't list a temperature or voltage rating. Teflon is excellent for both voltage and temperature, so it would probably be OK, but you'd need to address that with the rules committee.

http://www.homegrownaudio.com/bulk_wire.htm


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 Post subject:
 Post Posted: Mon Mar 08, 2010 11:48 pm 
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Joined: Fri Jan 29, 2010 11:56 am
Posts: 44
Location: St. Cloud State University
My estimation was a little overinflated! for stationary applications where weight is absolutely no consideration, these silver wires would probably be the way to go, but obviously the formula-hybrid car is no trailer queen.


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