Is NRTL certification mandatory in the US ?

Hello,

We would like to register our electrical stimulator in the US and I have the following question :
Is the NRTL certification/marking mandatory for the US?

I can't seem to find a define answer on the internet.
Thank you !

From the OSHA page:

OSHA's standards contain requirements for NRTL product testing and certification for 39 product types. For example, in 29 CFR 1910.303, OSHA requires NRTL approval for many kinds of electrical equipment when they are used in the workplace. OSHA’s website contains a listing of the type of products requiring NRTL approval.

OSHA regulations (from where the NRTL definition comes from) is applied to employers, not manufacturers. Because of that, your customer will dictate whether something that is purchased from you is Listed. Typically, if powered from line voltage, most customers will want a NRTL Listing on the product that they buy.