Do I need a ELD if running interstate under 116 miles

https://www.fmcsa.dot.gov/sites/fmcsa.dot.gov/files/docs/Drivers Guide to HOS 2015_508.pdf

 

To answer the specific question...

Interstate vs. intrastate doesn't matter. Well not unless you have a state exemption/difference (Cali, Texas, ag. etc.)

If you run paper logs you need to use ELD, unless you use paper logs 8 or less days out of 30.
If you use the time card exemption, you do not need a paper log.
To qualify for a time card exemption to logging all three need to be true.
1. No more than 12 hours on duty/driving per shift.
2. Start/stop at the same location.
3. 100 air miles (115 statutory mile) radius from the yard.

Now if you are right at 115 miles, remember that is a radius from starting. You can drive 600 miles inside that radius. You can go to Radius Around a Point on a Map put in 115 miles, and the starting location to draw a circle. If you breach it, you must log. If you stay inside it all day, you are good with a time card.