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Extrapolation of wire current ratings

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aonomus

Sat Aug 02 2008, 03:01AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Ok well I checked around on google and saw

and plotted out each AWG's amps vs distance and added a logarithmic trend line. What I'm wondering is if it is safe to extrapolate amps vs distance like this, and how I can modify the data at 12V to fit other voltages....

The data at 12V fits fairly well, 18 and 16 AWG are shown. What I'm wondering is if I should switch the wiring inside my 20V 15A power supply from 18 to 16AWG, the internal runs are under a foot each so I'm not sure if the charts even apply now....

...

Sat Aug 02 2008, 03:22AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

There are 2 things that determine the size of the wire you need, you use the one that gives the larger diameter wire.
1. voltage drop from the wire-this is what your charts show and generally only applies for long distances
2. max heating of the wire. This is the one you should be worried about. There is a formula to calculate this value, but you can just look it up in a chart as well.

If you are drawing 20a I would recommend 14awg, but I suppose 16awg would probably work as long as it has decent airflow. I would stay away from 18awg.

aonomus

Sat Aug 02 2008, 03:39AM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

I ask the question cause I'm a bit cheap, I don't want to rewire the entire power supply cause there is already a fair bit of wiring in it... :/

Oh well, 100ft of red and black 16AWG sit unopened next to me, it would be a 15A draw once I'm confident that the pass transistors can take the current with sufficient cooling, until then a 6A fuse will sit on the output... I'll switch it to 16AWG since I have plenty of crimp connectors....

Mads Barnkob

Sat Aug 02 2008, 07:55AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

My advice is you do what we do as electricians, insure you have selectivity in your installation.

You achieve this by insuring that the amperage of the wire or breaker is higher or equal of what it is feeding/protecting.

When you have as little wire as under a foot, voltage drop will never be a problem, heating could if you choose too small of a wire.

European rules suggest AWG14 for a continues use at up to 17.5A with bad to none ventilation

Steve Conner

Sat Aug 02 2008, 11:24AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Ponder this: What AWG are the legs on a "55 amp" TO-220 MOSFET?

HV Enthusiast

Sat Aug 02 2008, 11:31AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Dr. Conner wrote ...

Ponder this: What AWG are the legs on a "55 amp" TO-220 MOSFET?

Ponder this:

What AWG are the internal gold bond wires connecting those "outer legs" to the die in a 55A TO-220 MOSFET!

aonomus

Sat Aug 02 2008, 06:23PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

My mind just exploded at the thought of those tiny hair thin wires, I've seen closeup photos of IGBT dies somewhere around here (read: failed IGBTs) and there are just several of those wires...

As for the legs on a 55amp mosfet, the legs are more or less tied to the heatsink tab and are cooled to some effect, plus, they don't have insulation which can melt/burn off.

I can't afford spools of 14AWG, I'll do double runs of 16AWG if neccessary, but from the looks of it (I did a full load test with a motor) I won't need to as long as I keep my draw under 10A.

Good to know still that I should use higher gauge wire for the future though.

Mads Barnkob

Sat Aug 02 2008, 09:02PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Find some scrap cable at a recycling plant or if you know an electrician, you could get some smaller amounts of larger gauges for such small installations.

But using double wires of a lower gauge is just as fine, we use that alot when we f.ex. cant fit AWG5 into a circuit breaker, we use double AWG10 instead.

Ken M.

Sat Aug 02 2008, 09:03PM

Registered Member #618 Joined: Sat Mar 31 2007, 04:15AM
Location: Us-Great Lakes
Posts: 628

Still though I used a 24AWG jumper wire on a 24v SSTC design and the thing would run but if ran for more then 60secs it would glow RED hot.

Mads Barnkob

Sat Aug 02 2008, 09:08PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Ken M. wrote ...

Still though I used a 24AWG jumper wire on a 24v SSTC design and the thing would run but if ran for more then 60secs it would glow RED hot.

some weeks ago I thought I set my tiny sstc on fire, but it was in fact the supply wires that melted and burned, they were pretty thin and didn't like the 30 amp i forced through :)

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