shower mixing valve, metal or not?

A question came up by email from a student in one of my inspection classes. I am an electrician and electrical inspector as well as an electrical instructor and have little or no knowledge of the plumbing codes so any help would be greatly appreciated.

Is there a code or federal law that mandates that the mixing valve on a shower be made from some sort of metal?

Yes this question will relate back to the electrical code and the student asking the question will be a multi-trade inspector when he finishes all his community college classes.

And yes I have searched the NC plumbing code to no avail.

They can be made of anything the manufacturer wants to use. There are "all plastic" shower valves although they are seldom used in houses. WIth the current "lead free" hysteria sweeping the industry, we are seeing more and more 'plastic" items being produced.

Thanks my friend, I thought this but wasn't sure.
The question had to do with continuity between hot and cold. My student thought that there was a federal law mandating that all shower mixing valves be made of brass.