This is what UL says:
Hi George,
GTO is not Listed for use in wet locations. See Gas Tube Sign Cable (ZJQX) located on page 446 in the White Book. GTO has to be used in Listed signs and in conduit/raceways. Inside a outdoor Listed sign would be considered a damp location. There is integrally sleeved GTO that has been UL Classified for use with specific neon boots or splice combinations that when used together can have a damp or wet location rating as identified on the Classification Mark. The product category is Sign Components Classified for use with Specified Equipment (UYTA), located on page 380 in the 2010 UL White Book. You may also find some of the cable and boot enclosure combinations made by the same manufacturer that may be suitable for wet or damp locations Listed under Sign Accessories (UYMR).
I hope that helps.
Best Regards,
Tom Lichtenstein
Senior Regulatory Engineer
Regulatory Services
Underwriters Laboratories Inc.
Northbrook, IL
