I have a possible job, where I will be adding a sub to a Zinsco main breaker. I see three different ways to do this:

1) I can double-lug the main panel feed (with the appropriate lugs) and have my 'main breaker' for the sub-panel in the new panel;

2) I can buy a new ETL (non-UL) Chinese breaker; or,

3) I can buy a 'reconditioned' breaker on-line.

What would your choice be? Why?

... A Zinsco panelboard serving the house now?

If you can make a compliant tap, I would go that way.
The new panel needs to suitable as service equipment ... but you know that

If it was my house I would install a panel big enough for all my loads and swing them over as time permitted or just get rid of the Zinsco right then.

The Non-listed breaker is probably not an "approved" device to your AHJ.

The "reconditioned" breaker is someone else's trash with a bath. It may be fine or it may have been replaced for a problem. How do you know?

All good points - and the reason behind this thread.

I had not considered using a panel large enough to replace the Zinsco .... then continue with the Zinsco as just a meter box and junction box (this is a Zinsco 'all in one.') An interesting idea.

I was under the impression that ETL is just as valid a listing as CSA, UL, CE among others. At least thats the way it here in Canada. Any NRTL can certify a product to national standars.

If anything I would be more worried about a fake approval mark from any testing agency on a Chinese part.

I have found only one use for the old Zinsco panels. The cans (with the addition of a few screws) make good Jboxes.
Forget the Chinese breaker ... been there, done that. It lasted about 8 months, tripped and wouldn't reset. The "Boneyard" in Alexandria, VA sells "reconditioned" Zinscos (if you want the telco, drop me an email). I believe their definition of "reconditioned" is, as gfretwell says, washed and dried.
My vote would be to get one large panel and sell the Zinsco guts on eBay.

I have had good luck with a company from California, www.relectric.com.
They have Zinscos, reconditioned with a 1 year warranty. I bought some
discontinued/reconditioned Westinghouse breakers from them, been in for
several years now with no trouble. They claim that the recon. breakers
are tested, but I have no idea about the testing parameters.

I've seen those claims as well: "tested to industry standards." The claim is pure BS. Part of the problem us that there is NO way to effectively test a breaker.

Oh, there was once an ANSI standard that had something to do with testing breakers. The problem was, the standard only addressed certain aspects, and was quite specific in stating that it was NOT intended to be used ro check used or reconditioned breakers. Over at UL, it is necessary to destroy a number of breakers in the testing. After a breaker is 'listed,' quality control tests at the factory are complimented by on-going destructive tests.

Whether ETL is as relaible as UL is another story; considering the shennanigans that Chinese manufacturers seem to delight in, I suspect that there's a reason they are avoiding the UL follow-up testing.

It occurs to me that a currently available breaker costs only $8, while the Zinsco-type replacement costs ten times as much. Do, I think I will combine approaches: get a 'bargain' used Zinsco off the internet, and back-feed a breaker at the panel. That way, even if the obsolete breaker fails, I still have the panel protected.

Reno,

I may have a couple Zinsco 2 Pole devices in Storage, which I could let go for a good price.
They are from past Improvement Projects, where the Panelboards and / or Gear was removed, and replaced with more current equipment... likely due to layout revisions, etc.

The largest Panelboard Device I would have (a Device which would fit on a standard Zinsco < 225 Amp 240V Bus Kit), is 100 Amp (I think...).
Might be 70 Amp, but it seems like there was a 100 Amp 2 Pole Device in my Archives.

If the use of a non-tested Device may cause Nocturnal Horrors, the best option would be to perform a "Sub-Feed Tap", as you have mentioned.
That may be the most cost-effective choice overall.

I also agree with the option of ditching the Zinsco Panelboard, and installing something modern.

Scott