[kwaropay]

I'm installing a 100A sub-panel 300ft away from the main breaker. I plan on using #1 AWG copper wire for both the 2 hots and neutral to accommodate Vdrop for up to 70 solar PV micro inverters that I am using. What size ground should I use? Not accounting for distance I know I can use #8 AWG copper. Does distance/Vdrop matter for the ground? What size ground do I need for a 100AMP sub panel 300ft away?

The design will be:
1. 100A breaker in MAIN
2. 100A Outdoor Circuit Breaker enclosure acting as service disconnect next to Meter
3. 100A Sub-panel with 6 double-pole 15A breakers 300ft away
4. Max AMPs for all inverters would be (.8A per inverter X 70 inverters/solar panels) * 1.25 = 70A

More specifically, I would like to know what the "minimum" size ground wire is in this scenario and also discuss any "recommended" size as well and any reasons for the sizing.

Note, I am familiar with non-bonding neutral and ground and using a grounding rod out by the sub-panel.

Thanks!

[Missouri Bound]

You want the good news or the bad news? That #1 would be OK for the ground... but you should be using 2/0 for the feeds.

[kwaropay]

Hi Missouri Bound,

I'm very curious what you base your answer on? I've done my voltage drop calcs based on the Enphase Microinverter documents and my own knowledge and I know I could use #1 AWG COPPER for the hots. See my calc below. If I am wrong I really would like to know why because I've put a lot of thought into it. I even reached out to Enphase Support and had them correct their Application notes because they were wrong. Enphase acknowledged me and another member on their forums for doing so. Enphase's 208V three phase calcs were wrong but were just corrected this month (11/2010).

Vdrop of 10 Enphase microinverters (center-fed) = .44Vdrop
Vdrop of #6AWG copper from Sub-panel to Inverter junction boxes = (.8A * 10 inverters) * (0.4023/1000 ohms * 45ft) = .1448Vdrop * 2 hots = .2896Vdrop
(.8A * 70 inverters) * (.1295/1000ft ohms * 300) = 2.1756Vdrop * 2 hots = 4.25Vdrop.
TOTAL: .44 + .2896 + 4.25 = 4.98Vdrop

That's within range of the 5.4Vdrop that Enphase recommends and also less the NEC 3% max Vdrop allowed. I've also got some "fudge factor" room because I rounded up my distances (300ft and 45ft are longer than what I will be actually running). If I was "end-feeding" all my inverters at 10 per string (10 inverters "end-fed" is 1.76Vdrop), then I would have a problem that you are suggesting. I'm center-feeding 5 per string and connecting two strings at each junction-box which greatly reduces the Vdrop at the inverter portion of the circuit (.44Vdrop for center-fed).

I'm of the opinion that sizing for Vdrop on the ground doesn't matter since it is not a hot wire. I know my Neutral needs to be sized accordingly. I reviewed table 250-122 of the NEC code regarding grounds and I know a #8AWG is good for 100 Amps. I think that's what I can use but I just wanted to see if anyone else had another opinion

Thanks!

[kwaropay]

I also should've mentioned that my calc above is for 240V Vdrop Calcs for the Enphase M190 Inverters.

[Missouri Bound]

I use a website which is a wire size calculator for all projects. It is spot on with the NEC requirements. 404page.html

[kwaropay]

Hi Missouri,

OK, I'm familiar with csg. I'm only sizing it for a 70A load. Sorry for the confusion. #1 AWG is all I need for 300ft. I just need to confirm if #8 ground is adequate or if circuit length affects that as well. I don't think it does.

[tkrussell]

When the feeder size is increased for voltage drop, the equipment ground shall increase in size proportionally.

As per the NEC 2008 Edition,

Section 250.122 (B) Increased in Size.
Where ungrounded conductors are increased in size, equipment grounding conductors, where installed, shall be increased in size proportionately according to the circular mil area of the ungrounded conductors.

#4 copper should be sufficient.

[kwaropay]

Thanks.