I have an application where I am running 115VAC from the secondary of a local transformer 50 feet to a sub panel at a remote site. At the remote site, the same 115VAC will then run through contactors on pressure and other limit switches. With contact closure of these limit and pressure switches, the same 115VAC will then make the return 50 feet trip back to the local sub panel, where it will energize relays controlling input signals to a PLC.
At the remote site the run of 115VAC will energize a 70W crankcase heater, 3 contactors, and a solenoid. The current should be below 3A.
I want to keep voltage drop below 3%. Using #12AWG will do this. I am supplying the remote site with two separate conductors coming from the secondary lines of my transformer. One side of my secondary is grounded. Would I need to size both of these runs of conductors with the #12AWG, or can I use #16AWG for the grounded secondary phase?
I hope my question is clear and I appreciate the help.
Even if you drove a ground rod at the remote site and bonded your secondary return at both ends (which is not allowed by code), the resistance would likely be somewhere between 5 and 25 ohms, so that will not help your voltage drop problem in any way.
With that out of the way, the voltage drop caused by the I*R of the return wire is going to be just as large as it would be for the same wire in the positive side. If we assume that your crankcase heater and the contactors and solenoids draw the 3A limit you mention, then the voltage drop on the #16 would be roughly twice as great as the voltage drop on the #12.
My calculations, however, indicate that even with #16 for both conductors you will not exceed your self-imposed 3% VD limit.
Just to set some background:
1. The voltage difference on the hot lead at the remote site will be caused by the current actually consumed by the heater, solenoid and contactors plus the current which will be going back to the relays at the PLC end.
2. The voltage difference on the grounded conductor will be just that of the heater, solenoid and contactors, since the return current of the PLC-end relays will not be going through that conductor.
3. The total voltage drop at the hot lead of each PLC-end relay will be the voltage drop calculated in #1 plus the voltage drop in the wire to that relay from the remote site (which you can probably ignore.) There will be no offset in the voltage at the low side of those relay coils since they will be returning directly to the secondary of the transformer. (You can think of that as a bunch of switch legs, not involving the neutral.)
4. Your only restriction on minimum wire size, either grounded or ungrounded conductor, will be set by the OCPD feeding the hot lead. This could either be on the primary or the secondary side of your transformer, as required by Code. If the transformer has an output current capability of 15A, then you would not be allowed to use your #16 wire for either conductor. If the crankcase heater is plugged into one half of a duplex receptacle, then you would have to wire the circuit at the full current of the OCPD, not just the connected load, independent of the voltage drop issue.
