(2017) permit modification questions: grounding second panel

[anythingyoucanimagine]

I got a little carried away with home runs in the house so decided to do a garage-only 100A panel. It is an attached garage, service entrance is in the garage and second panel is going be right next to SE panel. Do I need an additional ground rod for the new panel? Can I come out of the second panel and mechanically bond to the existing ground conductors or do I HAVE TO bring over 4 conductors (black, red, white, green) from the main panel to second?


I spoke to inspector yesterday and he said to submit a field change --except I've never done one. I understand the basics, just not specifically how to ground a second panel right next to main.


Before someone says don't bond neutral & ground in second panel, I know...


Thank you in advance!!!

 

[wyliesdiesels]

Panels in attached buildings do not need electrodes.

You need to run 4-wires from main to sub- 2 hots neutral and ground. The ground will bond the 2 panels together and provide a fault current pathway.

 

[alfredeneuman]

No additional grounding electrode will be necessary for a panel in the same structure.
Unless you use rigid conduit, intermediate metal conduit, or EMT for the run between the 2 panels the green grounding wire will need to be run.

EDIT: If the conduit goes through concentric knockouts, you'll need to install grounding bushings on the ends of the conduit.

 

[anythingyoucanimagine]

Panels in attached buildings do not need electrodes.

You need to run 4-wires from main to sub- 2 hots neutral and ground. The ground will bond the 2 panels together and provide a fault current pathway.

EDIT: If the conduit goes through concentric knockouts, you'll need to install grounding bushings on the ends of the conduit.

Thank you both. Working with split phase is killing me. I'm over-thinking everything. Back home I have to worry about balancing neutral, three legs, etc. This seems very straightforward but also I don't want to be an idiot and make a huge mistake. Thanks again.

 

[alfredeneuman]

I should amend my statement to read metal conduits only are required to use grounding bushings.

 

[anythingyoucanimagine]

I should amend my statement to read metal conduits only are required to use grounding bushings.

Understood. I assumed you meant metal... Then I got curious and Googled. If you are into nerdy type things, this is a pretty cool article/paper I found about five highly conductive plastics: (article is actually LED and lighting related)


https://www.smithersrapra.com/SmithersRapra/media/Sample-Chapters/XPolymer_Electronics_Chpt5.pdf

 

[wyliesdiesels]

No additional grounding electrode will be necessary for a panel in the same structure.
Unless you use rigid conduit, intermediate metal conduit, or EMT for the run between the 2 panels the green grounding wire will need to be run.

EDIT: If the conduit goes through concentric knockouts, you'll need to install grounding bushings on the ends of the conduit.

If my memory serves me correct, that only applies to 480v systems...

 

[alfredeneuman]

Oops, you're right.
It only applies to voltages 250 to ground and above, and service conduits.

 

[anythingyoucanimagine]

You guys know way too much! I'm just lucky I don't get zapped! It's like the phrase painters say... "I've breathed more paint than you'll ever spray". Thanks for the great resource!!

 

[wanderer]

Looks like you've got the best advice above. Four conductors, three of which are current carrying conductors. Maximum unbalanced load is the breaker size, so all three the same size. The ground is the fourth.

 

[anythingyoucanimagine]

I'm being an idiot... For two days I've been second guessing myself about overloading the neutral. In main panel and sub both breakers will be 100A 2-pole. Neutral does not go through a breaker. It's my understanding about 2-pole breakers that they are essentially two single-pole breakers tied together. So couldn't you load up say 85% on each leg and be fine for constant run devices but then you'd also be loading up neutral to 170%? What am I missing?

 

[wyliesdiesels]

Neutral only carries the imbalance.

If you have 100a on one leg and none on the other the neutral would carry 100a.

Obviously if the second leg instead had current on it, the neutral would carry less than 100a...

 

[alfredeneuman]

You're missing that the legs are 180 degrees out of phase (time) with each other, and so they pulse at different times of the 60 cycles per second. They in effect cancel each other according to the loads on them.
If the 2 legs were equally loaded then you'd have 0 amps on the neutral.

 

[anythingyoucanimagine]

They balance each other out even in the neutral? Sorry --this is where having only ever worked with Wye makes me look split phase dumb. Yes I get that each leg of split phase is 180* off therefore they cancel --but I thought that only worked on 240V?

Actually, Thinking about it and doing the math... You guys are right. In addition to having Wye at home, I do a lot of datacenter/computer work and most of the drops into the racks are 208Y. In the "old days" we would actually have to pay attention and balance (servers use different loads at different times so log the use, monitor, adjust, dual power supply so no down-time, etc.) These days the networked PDU's do it all automagically.

Thanks for the kick in the pants. Split phase is weird. What is it? Just a center tap off one can on a pole?

 

[alfredeneuman]

Yes, the transformer is center tapped.

The ratio figure works for linear loads (heaters, incandescent lights, etc.)
As far as the neutral carrying current, nonlinear loads, such as motors, lighting ballasts, and computer power supplies modify the ratio figure.

 

[sberry]

I was in a panel recently that was really contested and had a **** load of tandems. The handyman called when he saw some 12/3 on 15 on some of them. Reason this 12 was there was likely what the master that did this had on the truck. There were limited spaces on the N bar and a couple other things so the guy had ran some 3 conductor out of the panel and used some fixture boxes in the utility room as a j box where he combined a couple wires to multi wire on light circuits. The wires he connected to were 14 at that point. It was a busy box, I did manage to find one he had both feeds on same leg,,, it wasn't a big deal about neutral overload,,, not a recept circuit but some low light loads. Would have never caused a problem. Wasn't cause the guy didn't know, it was a minor error that didn't matter in this case.

 

[anythingyoucanimagine]

You guys would probably have a heart attack if you saw how our house was when we bought it. Whole neighborhood was built back in the 50's with 208 Wye. We had FOUR panels on the wall: One main panel for service entrance with all four wires (edit: when I said ground I meant "the important wires, not that PoCo supplied ground) then three "sub panels" but each of those three panels only had one leg plus neutral. Grounds all came out of the bottom of the boxes and went to main ground conductor going to outside ground electrode.

Think about that ****-show in a residential home with sketchy DIY homeowner ****. All of the 208V stuff (we didn't have any 208Y in the house at that point) came out of the main panel where the service came in. There were 120V circuits in the main panel, only 208V. All of the 120V stuff was randomly run out of each of the three 120V sub panels.


You wouldn't believe the **** I found in that house. Was all that old cloth wrapped 2 conductor wire (no ground). People didn't understand that they used to run line to the device (like a light) and then run the same (two wire, no ground, one black, one white) down to a switch. And then over the years people had reversed polarity here & there because they couldn't tell the difference between silver and gold. Now you'd have a white wire that was hot all the time and a black wire that was hot only some of the time (switched)... Eventually they started stripping back the (cloth) jackets on random sections of wires just to find a hot that was hot all the time. All of this was done without wire nuts or boxes, just twist, tape & pray... then bury it back in the insulation. I pulled so many browned/burnt white wires out of that house it was amazing it hadn't burned down. And all those hots/neutrals coming/going from different legs out of different panels. Did complete rewire to nec2009 (homeowner permit). Was a fair amount of drywall work but thank god for slider boxes (screw in new work boxes).


I thought this house would be same/similar but this split phase 120/240 stuff makes me feel like an idiot This house & garage should be easy too since we don't live there and I can leave the walls open for as long as I need.


Anyone ever do this before? I hope I don't regret it: I'm doing all adjustable new work boxes (with screws so you can adjust box depth). It's more expensive but the drywall guy is charging almost nothing. Take good pictures/measurements then screw the boxes back to flush with the studs. All drywall guy has to worry about is windows & doors. I'll come back later and cut open all the box holes. (like I said, hope I don't regret this)

 

[TRWham]

...One main panel for service entrance with all four wires (five with ground)...

Should still be only 4 wires from the PoCo with 3 phase service (sometimes just 3 if the building has it's own transformer). The ground conductor goes to your grounding electrode(s). The neutral is grounded on the PoCo end.

 

[anythingyoucanimagine]

Should still be only 4 wires from the PoCo with 3 phase service (sometimes just 3 if the building has it's own transformer). The ground conductor goes to your grounding electrode(s). The neutral is grounded on the PoCo end.

Sorry, edited. In my head I was thinking "the five important wires" (including ground) and didn't mean to imply that PoCo supplied ground.