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.