pipe and tubing sizes

I was updating my stock size list and noticed that the ID of EMT tubing matches the ID of the same nominal size of water pipe, while
chain link fence posts are made with the same ODs as water pipe, though their trade sizes are closer to the actual dimension, for example 1" pipe has the same 1.315" OD as 1-3/8" fence tubing. This means EMT should telescope into fence tubes, and water pipe can be machined to splice either of them. http://www.afsfence.com/resources/chain-link.html
Is there some general rule of thumb for the minimum length of a tubing splice to maintain strength? The slip joints in TV antenna mast tubing are much longer than the lugs on bicycle frames.
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