I have to raise a ten foot 2-1/2 IMC riser about three feet. Building is going from a flat roof to a trussed one.
Can I use a 3 foot piece of PVC between the meter base and the 10 foot IMC without bonding issues

No.

how did you get your 8 ft above the flat roof if you don't have the clearance over the trussed roof? you would be isolating the IMC from the grounding in the meter base.

You could bond the riser at the top end to the neutral if you want.

Personally I would only extend this with the same type of raceway.

Bob

I know around here you can't use anything for mast except steel RMC. Even though it isn't part of the immediate load bearing portion of the service entrance, I'd think it would still be subject to torsion from wind, etc, and I'd use IMC like the original section.

Check with your AHJ, but they'll probably insist on the use of IMC.

Don't leave out the serving electric utility since it will be their service cable that will be hitting the service mast. If it doesn't meet their requirements, you may not get hooked up.

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Charlie Eldridge, Indianapolis, Utility Power Guy