Is there a general rule when a spice is needed for large feedersizes? I have a 600’ run of #500. I think the reels only come in 500’ but I believeI would be able to have a reel made for a 600’ run, correct? Would you splice arun for #500 at 600’ or just install pull boxes? Thanks.
First off you need to make sure the cable is long enough to reach to where ever it's to be terminated. Don't try to figure it down to the last foot, there will need to be a little extra. Then inside of a 90 degree bend is shorter than the outside.
You should be able to order the cable in the length required.
The general rule is that you don't splice unless you have to.
As strathead mentions, if there are not to many bends in the conduit you may not even need pull boxes. If this is underground and nothing but a couple of 90s and a few degrees of bend in the middle a 600 ft pull shouldn't be that hard. Long sweeps are easier to deal with than short 90s when it comes to bigger cable.