# Wire ampacity for pulsed current

Discussion in 'General Electronics Discussion' started by MarkWilliams, Nov 18, 2009.

1. ### MarkWilliams

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0
Nov 18, 2009
I'm designing a 120VAC capacitor clearing machine. It uses a transformer connected across a metalized film capacitor via a SSR. It should produce a pulse about 60A rms < 2 sec. I'm looking for a formula to calculate the awg for the wire at 20% duty cycle. Any help would be appreciated.

Thank You
Mark Williams

2. ### Resqueline

2,848
2
Jul 31, 2009
The theory is simple, using ohms law.
For instance; doubling the current means also doubling the voltage drop giving four times the power loss - thus the duty cycle has to be lowered to 25%.
20% is not far off from that so I suggest you can use half the normal (60A) wire area - giving you a small safety margin.
But remember that the "normal" wire area depends highly upon how the wire is laid. A single wire in free air can carry a lot more than the standard tables say - they are usually meant for conduit (house) wiring.

What is a capacitor clearing machine anyway?

3. ### MarkWilliams

3
0
Nov 18, 2009

The capacitors we make are metalized film capacitors. They are polypropylene film with zinc layer on one side. This film is wound on a core with two layers offset. The two ends are sprayed with zinc for good connections. This type of capacitor is said to be self healing. That means that if there are any impurities or flaws in the film they will burn out under voltage without destroying the whole capacitor. Our capacitors are mostly used for high power laser and in defibrillators, 100uF to 9000uF typical at 2050VDC to 250000VDC. The clearing machine clears out most of these flaws at a lower voltage. If this type of clearing happens at higher voltage with a large capacitor fully charged there would be a lot of energy available and might damage the capacitor.

4. ### Resqueline

2,848
2
Jul 31, 2009
Oh, manufacturing.. Yes, I know about & have observed those self-healing properties, have seen the shiny films & matte ends, but didn't know the metal layers were zinc. I also work with laser & defib caps so I appreciate the (destructive) power they pack at full voltage but didn't know they used that technology & would clear at such low voltages.
Seriously, 250kV? That one needs a forklift I guess..

Btw.; I guess you are going to wind the transformer yourself. The area formula is sq.root of duty cycle. So you use 45% of the normal wire area for 20% duty. You'll find tables describing wire gage & area on the net. The so-called conservative rating is meant for transformer windings.