Sid you said;
I screwed up my panel to the studs and mast up the wall.
This leads me to believe you are talking about an installation exposed to weather.
In my opinion that would be a violation of 300.6
300.6 Protection Against Corrosion.
Metal raceways, cable trays, cablebus, auxiliary gutters, cable armor, boxes, cable sheathing, cabinets, elbows, couplings, fittings, supports, and support hardware shall be of materials suitable for the environment in which they are to be installed.
If the drywall screws where not of the galvanized type I think the inspector was correct.
In my opinion the above is clear cut.
The use of drywall screws (even if galvanized) to support a mast is more subject to interpretation.
There are two code articles I can think of relating to mast and this question.
230.28 Service Masts as Supports.
Where a service mast is used for the support of service-drop conductors, it shall be of adequate strength or be supported by braces or guys to withstand safely the strain imposed by the service drop. Where raceway-type service masts are used, all raceway fittings shall be identified for use with service masts. Only power service-drop conductors shall be permitted to be attached to a service mast.
344.30 Securing and Supporting.
RMC shall be installed as a complete system as provided in Article 300 and shall be securely fastened in place and supported in accordance with 344.30(A) and (B).
The typical drywall screws that come to my mind are fine threaded bugle head #6s.
In my opinion they are to brittle and to thin to be strong enough for a mast, also the threads are to fine to get a good grip on the lumber.
For a mast I would use at least 1/4" galvanized lags.
All this is really up to the inspector to determine the suitable of the fastener for the particular application.
I sometimes use drywall screws to mount 4" square boxes this is a far cry from a mast and I would rather use #10 screws for almost any small equipment.
[This message has been edited by iwire (edited 04-16-2005).]