I went to a service call lately to a medical office to add some isolated ground circuits for computers. I was shocked to find that the majority of the wiring, more than thirty 110V circuits, was done in 16ga MTW/THHN in EMT on 15 amp breakers. The service is 200 amp single phase and there are no problems with the installation of the service equipment or the load center. The building is less than 10 years old. It is in an area where there are no inspections or licensing required. I was taught to never use less than 12ga wire in commercial buildings and that is what I have always done but I can't find the code to back it up. I want some really solid ground to approach the owners with to convince them to do something about it. Can anyone help me with code references? Thanks!

WW,

Although 12 ga minimum is a good policy to go by I don't know of any rules requiring that. There is one though that specifies 14 ga minimum see 310.5

Bill

Bill, I saw that and my interpetation was that anything smaller than 14ga could only be used in fixtures and control panels but not for fixed wiring in a building. The ampacity of the 16ga THHN is 18 amps, so I guess it is not overloaded, but it still seems very strange to me. The 12ga minimum size for commercial buildings may have been a local code in places I have worked over the years and it just stuck with me. Thanks for the reply.

Your statement is correct. 16 ga should not be used for the wiring in this building. In Table 310.16 that 18a you see is under the 90C column and that could only be used if wiring was connected to terminals with a rating of 90C. Breakers are either 60C or 75C (some older ones even 40C) If you notice, there is no ampacity under the 60C and 75C columns for 16ga wire.

Bill

[This message has been edited by Bill Addiss (edited 03-11-2002).]

240.4(D) limits 15 ampere max. overcurrent protection for #14 awg.