Panel, subpanel, & grounding

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Old 01-21-08, 10:16 AM
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Panel, subpanel, & grounding

Greetings, good people!

I hope you can help me with a grounding question. We're installing solar panels. We'll need to replace/upgrade both our main panel and the subpanel in the detatched garage where the solar panels will mount.

The solar installer says they will need an "unbroken" ground from the inverter to the main panel under new code provisions. I'm trying to figure out if I need to run new wires to the garage. I'm assuming that "unbroken" means, basically, an all copper wire path (as opposed to using conduit).

Currently (all puns intended), the subpanel is supplied through a 50amp breaker via a plastic sheathed cable with 3 #4 wires and a fourth bare conductor. The bare wire is much smaller, maybe #12 or #10. I suspect this was installed in the mid 1980s.

I don't think I could get another wire through the existing conduit, so it would mean some trenching if I have to upgrade the wiring.

I can't find any grounding of the existing main panel, so I'm betting we'll need to add some grounding rods. What is it, 6' apart connected with #2? I understand that there should be only one ground for the AC system, and it should only join with neutral at the main panel.

Anyway, my main question is, will I have to run new larger copper grounding to the garage? If so, what size?

If it matters, we're in Mountain View, CA, single family detached house, planning on using a 200amp main panel, 100 amp sub protected with 50 amp breaker, and the solar will be a 4.9kW system connected through a 30 amp breaker.

Thanks!
-- Rich
 
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Old 01-22-08, 09:27 AM
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Originally Posted by RichA View Post
The solar installer says they will need an "unbroken" ground from the inverter to the main panel under new code provisions. I'm trying to figure out if I need to run new wires to the garage.
The code article that covers PV systems is 690, part 5 covers the grounding requirements. I just read over it, and all I see is a requirement that there be a continuous ground to the grounding electrode system. If you have ground rods driven at the garage subpanel (they should be there), this requirement is met in my opinion.

via a plastic sheathed cable with 3 #4 wires and a fourth bare conductor.
This is probably UF 6/3g cable which is very typical of a 50A subpanel installation.

I can't find any grounding of the existing main panel, so I'm betting we'll need to add some grounding rods.
The primary grounding electrode should be the water service line if metal. The steel rebar in the concrete foundation may also be used. Unless your house is quite old, it's probable that your main panel already uses one of these methods. Look for a thick bare copper wire connecting to the pipes near your water meter.

Anyway, my main question is, will I have to run new larger copper grounding to the garage? If so, what size?
Because of the special nature of your project I would lean toward recommending that you hire an electrician who has experience with solar systems. I don't have that experience, and I don't think any of the regulars on this board do either. Grid-tied PV systems are really new on the scene so the number of contractors who know how to do them correctly and in compliance with local codes is still fairly small.
 
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Old 01-22-08, 11:43 AM
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Originally Posted by ibpooks View Post
The code article that covers PV systems is 690, part 5 covers the grounding requirements. I just read over it, and all I see is a requirement that there be a continuous ground to the grounding electrode system. If you have ground rods driven at the garage subpanel (they should be there), this requirement is met in my opinion.
Thank you! I'm pretty sure there is no grounding at the subpanel, at least not with a wire bigger than 12g. That will need to be fixed.

I thought it was no longer typical to put in a separate grounding electrode for subpanels in detached buildings. Have I gotten the wrong idea?

Originally Posted by ibpooks View Post
This is probably UF 6/3g cable which is very typical of a 50A subpanel installation.
"4 AWG" is printed on one of the wires. This panel serves another panel in a 3'rd building, so the total run is about 100 feet or maybe a bit more, so 4g wire may have been needed for the distance.

But the bigger question (even leaving out solar) is, does the small ground wire serve the grounding purpose, or should a larger one be installed? Or a grounding electrode near the subpanel with a larger wire run to it?

Originally Posted by ibpooks View Post
The primary grounding electrode should be the water service line if metal. The steel rebar in the concrete foundation may also be used. Unless your house is quite old, it's probable that your main panel already uses one of these methods. Look for a thick bare copper wire connecting to the pipes near your water meter.
Gosh, I can't find any sign of a ground conductor bigger than 12g (or maybe 10g) leaving the main panel either. There are a couple of conduit runs; could they serve? I doubt it, but ... There isn't even a separate ground buss in the panel as in newer panels. One of many reasons to replace it, I guess.

The water meter is in the ground at the other end of the lot. No signs of a wire there. The only ground I find on the pipes is from the phone co's box.

Very odd.

Assuming there isn't grounding and I have to add it, what will I need for a 200 amp service? 2 stakes 6' apart with 2/0g wire from the panel?

Originally Posted by ibpooks View Post
Because of the special nature of your project I would lean toward recommending that you hire an electrician who has experience with solar systems. I don't have that experience, and I don't think any of the regulars on this board do either. Grid-tied PV systems are really new on the scene so the number of contractors who know how to do them correctly and in compliance with local codes is still fairly small.
OK, will do, but I hope you'll still help me be better informed about some of the questions that are more about bringing the system up to snuff for safety and code, independant of solar.

Thanks!
 
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Old 01-22-08, 12:59 PM
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Originally Posted by RichA View Post
I thought it was no longer typical to put in a separate grounding electrode for subpanels in detached buildings. Have I gotten the wrong idea?
Grounding electrode(s) are required at all outbuildings with subpanels. If you do not have one now, an 8' x 5/8" copper-clad rod driven flush to the earth is adequate. The rod should be connected to the subpanel ground bar using bare #6 copper wire to a brass acorn clamp on the rod.

4g wire may have been needed for the distance.
That sounds like a reasonable design.

But the bigger question (even leaving out solar) is, does the small ground wire serve the grounding purpose, or should a larger one be installed? Or a grounding electrode near the subpanel with a larger wire run to it?
Both. There are two distinct types of grounding: equipment grounding and grounding electrodes. The bare wire between panels is the equipment ground, and a #10 copper ground is sufficient for up to 60A hots. The grounding electrode conductor runs from the rod(s) to the panel; this should be #6 on the subpanel, #4 on the main 200A panel.

Gosh, I can't find any sign of a ground conductor bigger than 12g (or maybe 10g) leaving the main panel either. There are a couple of conduit runs; could they serve?
Some jurisdictions and power companies install the grounding electrode conductor in the meter box instead of the main panel, so it could be there. This is certainly something to have the electrician check out.

There isn't even a separate ground buss in the panel as in newer panels.
Ground and neutrals may share a bus in a main panel.

Assuming there isn't grounding and I have to add it, what will I need for a 200 amp service? 2 stakes 6' apart with 2/0g wire from the panel?
Two rods, six feet apart, #4 bare copper. Loop the wire through the acorn clamp on rod #1 so that it is unbroken to rod #2.
 
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Old 01-22-08, 10:33 PM
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Originally Posted by ibpooks View Post
Grounding electrode(s) are required at all outbuildings with subpanels. If you do not have one now, an 8' x 5/8" copper-clad rod driven flush to the earth is adequate. The rod should be connected to the subpanel ground bar using bare #6 copper wire to a brass acorn clamp on the rod.
That subpanel has FPE breakers, no grounding buss, and no cuttoff, so it's slated for replacement anyway, and now adding a grounding rod.

Originally Posted by ibpooks View Post
Both. There are two distinct types of grounding: equipment grounding and grounding electrodes. The bare wire between panels is the equipment ground, and a #10 copper ground is sufficient for up to 60A hots. The grounding electrode conductor runs from the rod(s) to the panel; this should be #6 on the subpanel, #4 on the main 200A panel.
That's great info. Not having to trench is good!

BTW, further examination in brighter light shows I was wrong about the #4 wire to the box. It's #4 going to the 3'rd building (presumably to compensate for the distance), but #6 cable going to the subpanel in the garage, with a smaller "equipment ground", just as you predicted. Still should be plenty for the solar.

Originally Posted by ibpooks View Post
Some jurisdictions and power companies install the grounding electrode conductor in the meter box instead of the main panel, so it could be there. This is certainly something to have the electrician check out.
...
Two rods, six feet apart, #4 bare copper. Loop the wire through the acorn clamp on rod #1 so that it is unbroken to rod #2.
I talked with a local electrician today and what he has said matches well with what you have said, which gives me more confidence about it!

No grounding electrodes at all, nor connected to water pipes. It's a combo meter and panel. The electrician said he'd connect the main panel to *both* the water (within 5 feet of entry) and an electrode.

My current inclination is to contract the main panel upgrade, and take care of the subpanel upgrade on my own.

Thanks again so much for your comments.

-- Rich
 
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