The 170' run had a 1250 watt heater last year with a sensored light that had two 250 watt bulbs. Then from that location another circuit was run with a shared neutral 220 more feet. It had the same load.
Ok so you have a shared neutral circuit that starts at the sub-panel goes 170' to a receptacle then you carry one of the hot legs and the shared neutral another 220' to a second receptacle.
Ok Cyndi let me give you some guidelines on what I use to calculate the size of heaters for a given outside average winter air temp and water tank
First always figure O degrees and then get colder to size a 120 volt water heater.
Tank size 5 gal at 0 F needs a 250 watt heater these generally are your plastic buckets with a heater built into the bottom.
Tank size 5 gal at -20F needs a 500 watt heater
Tank size 25 gal at 0 F needs 500 watt heater
Tank size 25 gal at -20 F needs 1000 watt heater
Tank size 50 gal at 0 F needs 1000 watt heater
Tank size 50 gal at -20 F needs 1500 watt heater
So what tank size do you have?
I think the first thing to do is get the heaters sized properly or down size the tank so we can use a smaller heater on that 390 foot run.
Then we will take a look at wire sizes and voltage drops to get the most out of the heaters and heat lamps.