Is cable size dependent on watts or amps?

[viewer]

Ignoring the tit-for-tat pecks between posters and the OP; there are a few questions the posters might have asked which may be useful.

To the OP:_ Without querying your knowledge about Ohms (, - are the figures correct? Are you building it yourself (I fancy doing that)?
That is one huge electric cycle. I've seen pictures of electric scramble bikes in the USA which may be high powered, but 1500W seems maximum in UK and most are below 1000W, with voltages up to 48. Once above 50V there start to be implications for electrical safety, so your voltage is much higher than expected.
Having said that - is this your aim? -http://www.youtube.com/watch?v=ZzvtY5TzqKQ

Why do you want 3 core flex? Normally for high currents such as you expect (as found in automotive applications) the cable is a single unit either a thick cable or a bar connector (one for positive, the other negative/earth. Relays are used to limit the amount of such thick cable.
Electric vehicles are a specialist area and I'm more interested than knowledgeable so your comments on my questions would be useful to me

 

[pupenhausen]

Ignoring the tit-for-tat pecks between posters and the OP; there are a few questions the posters might have asked which may be useful.

To the OP:_ Without querying your knowledge about Ohms (, - are the figures correct? Are you building it yourself (I fancy doing that)?
That is one huge electric cycle. I've seen pictures of electric scramble bikes in the USA which may be high powered, but 1500W seems maximum in UK and most are below 1000W, with voltages up to 48. Once above 50V there start to be implications for electrical safety, so your voltage is much higher than expected. Why do you want 3 core flex? Normally for high currents such as you expect (as found in automotive applications) the cable is a single unit either a thick cable or a bar connector (one for positive, the other negative/earth. Relays are used to limit the amount of such thick cable.
Electric vehicles are a specialist area and I'm more interested than knowledgeable so your comments on my questions would be useful to me

Hi, I started 6 weeks ago with a Goldenmotor Magic Pie 3 wheel set, a 48v lifepo4 battery pack and charger. Been doing 25-26mph on it on the flat, downhill my fastest was 30.1mph and uphill it went 22-23mph. I have a huge off-road cycling path by mine and wanted to go faster on the straights and have more power uphill so I decided to buy a high power controller, a cycle analyst, another 8 cells, a 24cell bms (battery management system) a new throttle and 72v charger. It has been a massive learning curve as I only found out about ebikes through a mis-click on a YouTube video and now I'm hooked! I have spent over £1000 now but that's it now. I got it working yesterday and went 46mph on it (off-road, ahem!) I have fitted a 3 pole switch so now I can set it at 15mph (legal limit) 30mph and wide open throttle whenever I choose. I was asking the question as a lot of experienced ebike hobbyists use alpha ecowire 12awg which is around 2.5 mm to link the 3 phase wires from the motor to the controller, but only available in 1metre lengths in America, they are using it up to 125v at 80 amps!!!. I ended up going for broke anyway and used some 2.5mm flex. I went on a 15 minute ride averaging 3800watts and peaking at 4900watts at between 78-85 volts due to voltage sag, my max current draw was 67amps. After the ride I felt the wires and they were still cold all the way up to the controller. And this was at max speed all the way, up and downhill. So I know I'll have no problems with the medium speed setting which I use to ride to work, off-road! Any questions just ask or go on endless sphere forums, you will be amazed at the power and speed some people put into their bikes. I have spent the last 2 months sleeping and eating ebike knowledge and have now built one which looks, and performs as good as experienced ebiker's builds. Here is a pic of it nearly finished, I still have to connect the regen brakes and the cruise control. //media.diynot.com/191000_190652_48243_12619475_thumb.jpg

 

[viewer]

Wow, that's a mean machine! Many thanks for the reply and info. I'll keep looking and learning. Best of luck with your project.

To go back to the original question and your findings in the real world, I'd suggest that given 2.5mm 3 core like this - http://www.tlc-direct.co.uk/Product...w=2.5mm+flex&gclid=CL33tNXunrECFYEXzQodnH-VXA
- which has a nominal rating of 25 A, then, that rating is for continuous use over an infinite length where heat dissipation is the limiting factor. This probably doesn't apply to your situation where voltage drop is the biggest problem. The lost volts/power must disappear as heat, but the cable can get rid of this small amount of heat easily as the cable is short and the motor and other connections act as "radiators". If you've space, perhaps it would be worth doubling up the 2.5 and see if the reduced resistance resulted in the expected improvement in reducing voltage drop.
Good luck.

 

[EFLImpudence]

.
Only three questions in the original post:

2.5mm cable, but its only rated for 20 amps. My bike runs at 87.6v x 45amps= 3942 watts, would it still be ok to use?

I answered that. I was correct.

What I don't understand is why all cable is rated in amps and not watts,

I answered that. I was correct.

does this mean if I were powering a 300w mini amp at 3volts I'd need 100amp rated cable?

I answered that. I was correct.

I don't get it.

It appeared that you did. I was wrong.


Oh. well.
.

 

[pupenhausen]

.
Only three questions in the original post:

2.5mm cable, but its only rated for 20 amps. My bike runs at 87.6v x 45amps= 3942 watts, would it still be ok to use?

I answered that. I was correct.

What I don't understand is why all cable is rated in amps and not watts,

I answered that. I was correct.

does this mean if I were powering a 300w mini amp at 3volts I'd need 100amp rated cable?

I answered that. I was correct.

I don't get it.

It appeared that you did. I was wrong.


Oh. well.
.

Well done, I'm made up for you.

 

[ban-all-sheds]

a lot of experienced ebike hobbyists use alpha ecowire 12awg

Possible reasons for that:

1) It's smaller & lighter than PVC insulated cables, which would be an obvious attraction.

2) It can run a lot hotter than PVC insulated cables, and so it has a higher current carrying capacity.

3) They fear it catching fire and want LSZH insulation.

which is around 2.5 mm to link the 3 phase wires from the motor to the controller, but only available in 1metre lengths in America, they are using it up to 125v at 80 amps!!!.

80A per phase?

I ended up going for broke anyway and used some 2.5mm flex. I went on a 15 minute ride averaging 3800watts and peaking at 4900watts at between 78-85 volts due to voltage sag, my max current draw was 67amps.

4900W at 78-85V is 57.6 - 62.8A.

3800W is 44.7 - 48.7.

After the ride I felt the wires and they were still cold all the way up to the controller.

They were in free air, with forced air cooling a lot of the time, and I doubt they would have been at 40-60A for long periods.

And this was at max speed all the way, up and downhill. So I know I'll have no problems with the medium speed setting which I use to ride to work, off-road!

You will or you won't.

If the conductors are migrating through the PVC where the cable is bent it will be a gradual process, and if you are going to get a fault it could well take some time. I'd advise some fuse protection, just in case.

 

[dormermike]

Interesting discussion, pity about the tattle - feel a bit sorry for EFLImpudence trying to help and can't see what he did wrong.

I have a couple of alternators here that i'm going to convert to motors to put into a buggy, so the youtube video link was interesting and led onto other things. Also the discussion around current capacity was helpful too - in my early experiments I was surprised to relatively thick cables smoking.. my suspicions confirmed. Cheers.