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I have big power transformer that's used in an old-style arc welder; it is just this huge thing, which can be connected to 110VAC or 220VAC and has four output tabs: one is ground, the others (I got told) are "low", "mid" and "high" output. But it is a home made transformer, so it doesn't have any specs.

Question: is it possible to find out the Amps it would yield, without running it?

Remember it is an arc welder, I guess the amps are like 40 Amps up, right? It works nice, I'm just curious what are the real approximate amps output, so perhaps I can pick the right rods.

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  • \$\begingroup\$ What does the person who made it say? Seriously, you've given us no useful information here. What are the core dimensions? What gauges of wire are used for the windings? Without this kind of information, there's no way to even guess what its capabilities might be. The only alternative is to operate the transformer while measuring various voltages and currents, with and without a load. \$\endgroup\$
    – Dave Tweed
    Dec 25, 2016 at 18:10
  • \$\begingroup\$ So measuring with a multimeter between tabs won't tell me anything? Sorry I know very little electrics, was expecting perhaps that impedance somehow would tell me anything. This is a home made power transformer the guy sells as arc welder, very heavy and big...i put some pics \$\endgroup\$
    – Aram Alvarez
    Dec 25, 2016 at 23:00

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The weight of a transformer can be a good indication of its power capability, it will probably be in the region of 25 Watts/lb (total weight including copper as well as iron).

Are you supplying 60Hz or 50Hz? - 60Hz has a higher capability for a given weight.

I would power it up and measure the secondary voltage. The combination of the power capability and the output voltage should give an indication of maximum current output.

The gauge of the wire on the secondary should also give an indication of the current capability - there are tables of wire current capability versus gauge on the internet.

I found this discussion about this subject on another websiteAssessing an unknown transformer's capabilities.

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  • \$\begingroup\$ I can supply 110v or 220v 60Hz. The thing weights about 60 pounds. \$\endgroup\$
    – Aram Alvarez
    Dec 26, 2016 at 2:20

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