As an engineer, I don't see inspectors (AHJ) reviewing the installed product and enforcing the spec. They seem to enforce only the code.
What is funny though, I send them drawings to approve (plan review and approval) and they return them stating something like they have reviewed them, but they have not reviewed them for code compliance. LOL Cushy job, huh?
Seems that my job is to review their install and enforce the spec (and the code if I have to).
I have seen plenty of installs where the Contractor installed to code and didn't come close to the spec. I've also seen them not come close to code either.
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We always over design and give a quality product, but if they send in an RFI and propose a cost saving method, I will consider it and may even let them do it. If they just do it without asking and then try to 'talk me down to code' then by golly they're going to rip it out and install what was designed...even if its overkill.
Same thing on submitted products. I may not care what brand of wire, lug, condulet, color paint, bolt, nut, widget you submit and install, but if it isn't what was submitted (or its all different types) then we have problems.