Does ground wire need to be same gauge as power conductors?

TroySmith80

In the high voltage DC lines from the array to the inverter, does the ground wire need to be same gauge as power conductors?

I'm making a substantial upgrade to my solar installation (adding a bunch more panels). The array is about 200-250' from the inverter and the wires are run underground in 1" ID black plastic conduit. There's three conductors, a positive, neg and ground. All are insulated 4 AWG wires. When i install the new array, i'm probably going to have to run a new, larger conduit... unless... if i pull the 4awg wire out and replace them with larger positive and negative wires, and a small-ish ground, then i may be able to get enough current through the existing conduit.

mcgivor

 The ground conductor is usually a lighter gauge than the carrying conductors by a trade gauge or less depending on current , example 12/2 nmd90 uses a 14 gauge ground. The ground is there to carry short circuit current for a fraction of a second until the overcurrent  protection opens, it gets tricky when large gauge is used to prevent volt drop but the actual current is far less than the conductors maximum capacity. If the overcurrent protection, fuse/circuit breaker, is undersized then the ground could be sized according to the protection. Note, I use the word "could" cautiously as I do not have an electrical code book to reference.Generally the ground is sized according to the maximum current capacity of the carrying conductors, which would prevent the next person unknowingly assuming the ground is of correct gauge, should a smaller be used.
Edit,  Attached pdf file nec  table 250.122 would indicate tha tthe ground size is determined by the rating or setting of the overcurrent device. The second pdf shows the ground size for given wire gauge nmd 90, which contradicts the NEC table from 12 gauge 20A rating using 14 gauge ground, go figure. The electrical code is always evolving, so there's that to consider. 

Sometimes using a charge controller which allows a higher input say 600 VDC is cheaper than running large gauge conductors a second time, simply increase the series panel count and use the existing home run. Naturally without knowing what it is you currently have in terms of panels and controller, it's impossible to make an informed comment. 

TroySmith80

Thanks for the great response mcgivor. 

More detail on our install. We're already have 2 strings of 13 panels in series (about 400v) from the array to the inverter, so there's not a lot of gain to be had by going higher voltage, but there is some. It's direct grid-tie, no batteries or charge controller.

So it looks like the equipment ground conductor could maybe be sized smaller than the power conductors, but maybe only by one awg step. So if i upgrade to 2awg power conductors, i'd still need the ground to be 4awg.

mcgivor

With 400 volts front the 13 panels in series, the amperage would be low I'm guessing around 9 amps, using these figures with a 250 foot run, the 4awg will result in a volt drop of 0.26%, even double the array or 26 series  panels parrallel, the volt drop would would be 0.56%, which is low, generally a figure of 3% is considered acceptable. Calculation based on copper, but even aluminum would be less than 1% at double the current array, so replacing the run with larger gauge would be a waste, given the present losses would be negligible even with double the array , naturally the ground would be oversized. Below is a link to a volt drop calculator enter your actual figures, if mine are incorrect, or you plan a much larger expansion. Hope this will save a lot of headaches and money.
http://www.calculator.net/voltage-drop-calculator.html?material=aluminum&wiresize=0.8152&voltage=400&phase=dc&noofconductor=1&distance=250&distanceunit=feet&amperes=20&x=80&y=21