Wire Guages and amp limits.

T

theRealFury

10 kW
Joined
Jan 26, 2011
Messages
535
Location
West Yorkshire, UK
Guys,

Im in the process of designing the wiring for my new setup and realised that i have no real idea as to what amperage you can safely run on what guage of wire. I have had a look arround but seem to find conflicting numbers on sites. Is there a rule of thumb or a defined way of working this out? I have a lot of 12awg silicone wire sat arround here but no idea weather it would cope with my intended setup of 12s lipo and 50amps controller, i suspect not but just having some figures to go on would help me alot. Any of you guys have the knowledge or a link to somewhere that can answer these questions?

Cheers :)
 
Yea i have seen that but that is confusing me as other places i have seen up to 55amp given for 12awg wire. Also looking at some of the beastly bikes here running 100-650amps they sure dont have 1awg or fatter wire on them. According to that list id need 5awg or 6awg wire to handle my 50 amps without noticable temperature rise... it just didnt sound right. Maybe im just a awg noob lol
 
This chart gives limit values for specific insulation materials:
http://www.seas.gwu.edu/~ecelabs/appnotes/PDF/techdat/CCCofCC.pdf

http://www.oilfield.de/download/techtabs/mechanic/mech001.pdf :)
 
Standard gauge & amps rated:

14ga = 15 amps
12ga = 20 amps
10ga = 30 amps
8ga = 50 amps

This is for UL listed electrical applications.
These ga wires are required to be "fused-breakered" at the listed amperage.

Of course, at this rating, the wire should never get, even, warm.
In application, these amperages can be greatly exceeded, especially for brief periods.

Note: The EZip battery pack is built with 14ga wire (15amps), but fused for 40amps - close to 3 times what it is rated at!
 
That wiki chart is for solid wire and constant current, 24/7/365, so I think it's a bit conservative....if you're only pushing 50a for 30 seconds then you can probably get away w/ 12awg or so.

That said, I'm not an expert electrical dude so if you spontaneously combust it's not my fault. :lol:
 
REdiculous said:
That wiki chart is for solid wire and constant current, 24/7/365, so I think it's a bit conservative....if you're only pushing 50a for 30 seconds then you can probably get away w/ 12awg or so.

That said, I'm not an expert electrical dude so if you spontaneously combust it's not my fault. :lol:
Click to expand...

Hhaha, almost managed that a couple of times already so another one or two close calls wont bother me much :)

Thanks for the replies guys. I think i have come to the conclusion that the awg of wire really doesnt mean all that much, its more about the quality of it and the internal composition. Ill try it with 12awg for starters and keep my eye on it when i finally get it all set up. If it starts getting warm to the touch then im wasting watts so ill double it up and go from there. Im currently running 25 amp controller on 12awg and wire doesnt even get slightly warm so it must be capable of way more than that.
 
Look at the battery wires going into your controller and use at least the same size or larger. 14 awg is more than adequate for 50A up to about 10'. 12 awg certainly won't hurt.
http://www.csgnetwork.com/wiresizecalc.html
 
Well, part of the problem is length of the wire matters a lot too for DC. Since Ebikes rarely have much more than 6 feet of wire anywhere, and often less than 3 feet, you can cheat a bit.

My death race bike ran 110 volts of 40 amps. Entirely wired with 12 guage wire. Motor melted, wires got mildly warm. Got away with it because the wire was short, and duration of the ride was expected to be short, like 30 min. Any bad connector though, would have gotten hot fast with 3000+ watts on it.

Many who build similar bikes are using 10 guage at least.
 
It's not so much the wires "limits" that you need to worry about on an ebike. Well, of course it CAN be an issue, but the bigger issue IMHO is wasted power. Batteries are expensive, and we are limited to what we can carry! It doesn't make sense to me to spend $800 on a battery and have %5 of it go to heating your wires!

A stock 16 gauge 6 feet wiring harness run at 30a will waste ~50 watts of power. The thicker the wire the better. I run 8 gauge or 10 gauge. Some people here do double runs of 8 gauge.
I made this spreadsheet to show how much power you can waste in wire.
 
theRealFury said:
i have no real idea as to what amperage you can safely run on what guage of wire. I have had a look arround but seem to find conflicting numbers on sites.
Click to expand...
Wire gauge vs. current isn't a simple pass-or-fail concept. Within specific contexts, there are some general regulatory rules laid out (eg: in the US, the national electric code says to use 14 AWG for a branch circuit protected by a 15A breaker), but there's also a degree of nuance when it comes to low-voltage, high-current stuff which isn't in the codes.

To wit: voltage drop.

Let's say that you have a 18 AWG solid copper conductor which is ten feet long. Now, in terms of safety, you could probably pass a couple of hundred amps through this cable (for a short period of time) without having it melt or catch on fire. But there's a difference between it not exploding and it being efficient.

Cable has resistance. If it didn't, then there'd be no need for large cable sizes. The resistance is quite small, but measurable. 18 AWG solid copper, for instance, has a nominal resistance of 10.15 milliohms per foot, so a ten foot length of it would be equivalent to a 0.1015 ohm resistor. Now, let's say that you are going to try to pass 50 amps through that wire. You will have a "voltage drop" of about 5.1 volts, so if you're putting 36 volts into the wire at one end, you'll only get 31 volts out the other end. Assuming this is a closed-circuit, you need a return lead too, and if it's also 10 feet of 18 AWG, then the drop will double and your usable voltage will fall to about 26 volts.

Now, this is obviously a very exaggerated example, but it conveys the point quite nicely.

So even in an environment where you are using a cable which is entirely "adequate" and "safe" for the application, there is still, in theory, some small gain to be had from upsizing the conductors beyond what some might consider reasonable, to get that extra little drop of efficiency out of the system.
 
I think one of our main limitation is the insulation of the wires. Yes when running smaller wires there is higher resistance and loss in the wires at higher amperages but my concern is when the loss of power is wasted as heat and is allowed to compromise the integrity of the jacket of insulation. Running bigger wires keeps the temp down on the conduit leaving less of a risk of it melting off and shorting to neighboring wires. I personally still run stock wires on my 9C. But I pulled the outer sheat off and put another layer of insulation/heat shrink on the phase wires and they are also cut outside the axle so its barely 8 inches of wire going to the windings to the outside of the motor. The Thin PTFE insulation on the original phase wires Mechanically degrade at 500F and physically melt at 621 degrees. The heatshrink I put on the outside of the wires is good up to 275 degrees. I run moderate to high power and Ive never had it fail at the motor. Ive had wires fuse together at the controller but now I heatshrink those wires and never had an issue with them doing the twizzler pull and peel again. Dont bundle or wrap high current wires together this derates the insulation and cable more because heat is allowed to build up and its not allowed to passively be air cooled.. I run 12G to my motor from my controller. Ive never had a problem with the common motors but Ive had problems with high turn motors like the 5303 and the 9C 1606.

Common automotive wire is good to 176 degrees. Ive made this wire tacky at high amp continuous duty rinding 5kw+ continuous like a motor cycle. I think as long as we protect the integrity of the jacket it will solve alot of problems but in the end nothing replaces having the correct gauge wire going to your components which should be 8G or better. I think a layer of heatshrink on all wires is a good alternative for most low end to moderate setups but all higher end builds need 10G Silcone wire(Very soft) or 10G PTFE (which is very hard and abrasive resistant).
 
Lot's of good replies above. Oftentimes we must design for allowable voltage drop at current. That will typically oversize the conductor and make heating irrelevant.

Below is a chart the ol' guy uses to get into the ballpark. It is pretty self explanatory. Enclosed current carrying capability is when your harness is all bundled up and stuck in a layer of fiberglass insulation in the wall of your house. The table is a little less conservative than the national electrical code. The free air chassis requirement is like the name implies. Wires going point to point in open air where convection can occur. These tables are for more common, non exotic wire insulation and temperature rises.

Fusing current is sort of neat. This is the current it takes to "blow" the wire up. I sometimes use short jumpers of #30 on a PCB to get a poor man's 10 amp catastrophic event fuse in a circuit that usually draws no more than 1 amp or so.
View attachment FusingCurrents.pdf

Since we are on the subject, if you ever wondered what the rule of thumb is for PCB trace currents here it is below.
PCBtraceCurrents.gif

Here is a paper by ESSEX on motor winding wire temperature determinations:
View attachment emcwa-nema_magnet-thermal-class-ratings.pdf
 
Cheers guys for all the replies. Its beginning to sound like the 12awg wire i have isnt really the best for the job, i guess ill have to double it up or buy some 8 or 10 awg wire i think (not that it costs alot, just a shame when i have about 30 feet of 12awg sat in my cupbord). It shows how much i know about wires, i never even thought about voltage drop over distances... i have 6 feet of 12awg wire between my battery and my cotroller(with a watts up meter in the middle) just because i already had connectors on the end and couldnt be bothered at the time to shorten the wire and resolder the connector.... i wonder if thats having a negative effect on my setup lol :S

Just goes to show choose how much you think you learn about this business, you can never get it all right :)

Thanks again guys... off to Ebay i go for some wire :)
 
I was meaning to get into free air and enclosed current capacity and how it changes but Moose hit it on the nose pretty much.
 
Hrm --- 12 gauge might be perfectly adequate for 12s lipo if your doing 20-30a.... making it shorter would probably just as effective as using a thicker gauge.


As for voltage drop - how does that affect the motor and the controller? Is everything just slower because of the lower voltage?
 
Your battery maybe 50v. You controller may put out 50v but if you have like front motor and a long run of wire that is to small it can rob 5-6v so that your motor may see 45v. Its an over exaggeration but the higher amps you push the higher the volt drop will be on an inadequate wire. The lower voltage will reduce your tops speed by a few mph.
 
Also don't be afraid to double up conductors. The bundle will be more flexible. My load bank has 5 or 6 #12's braided together. It was fun having the wifey teach me how to braid ... :)
 
Overbuilding is always good, and wiring makes no exception. 8ga silver wire, 75A Anderson connectors soldered and crimped, silicone tape weatherproofing, braided nylon jacket... Gives you reliability, efficiency, and power at will.
 
Short runs don't make undersized wire a bit more reliable.

It's the same amount of heating per inch and thermal disipation per inch for any given current value, no matter the length or voltage etc.

Exotic wire insulations are a fools solution. Size your wire right, and you don't have to waste the pack energy heating your wires.

I run double 4awg battery leads, and 0awg motor leads. The double 4awg leads get warmer than I would like.
 
liveforphysics said:
Short runs don't make undersized wire a bit more reliable.

It's the same amount of heating per inch and thermal disipation per inch for any given current value, no matter the length or voltage etc.

Exotic wire insulations are a fools solution. Size your wire right, and you don't have to waste the pack energy heating your wires.

I run double 4awg battery leads, and 0awg motor leads. The double 4awg leads get warmer than I would like.
Click to expand...

Agreed. By the way, what current warms your double 4awg?
 
Copper is getting expensive...and I'm sure the purity and quality will vary widely from one manufacturer to the next. Especially with the way one company will buy out another with a good name...and then begin cutting corners with quality.

The crappier the quality of the wire, the fatter it needs to be so its adequate for the job...
 
liveforphysics said:
I run double 4awg battery leads, and 0awg motor leads. The double 4awg leads get warmer than I would like.
Click to expand...
That must be on your mad Agni BMX. :twisted:
For one would melt a hubmotor, feeding enough power to warm this wire harness. :wink:
 
Haha, yes its gotta be the BMX, i know it was running 650amps but not sure of the voltage... Loved the video of the guy trying it.... it literally blew his shoes off right before his a$$ hit the ground. Priceless :)

On the topic of doubling up on wire runs, if i use double 12awg wire is tha the equivelant of using 6awg wire? and if i use 4 runs of 12awg wire is that then equivalent of using 3awg wire?

As for connectors, im currently using genuine XT60 connectors which i beleive are rated at 60-70amp constant. So hopefully these will be fine for my build. I prefer the XT60 to the deans connectors as there is a bit more body to them to grip when separating them and also its much easier to solder the wires onto an XT60 due to the wire cups on the ends.
 
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