Electrical Subpanel Question

[bob_moody]

I need some help in making me understand the concept of the current code regarding subpanels. Fortunately where I live, "code" isnt a real issue. I can pretty much do whatever I want.

With my display expanding, I need to expand my service. I have a box with 8ea 15amp breakers and I need 10. So I purchased a new service box that will actually support 12 breakers.

Take a look at the attached drawing that I downloaded from somewhere about how to wire this panel. Let me state that the subpanel is electically 11' from the main panel. The main panel is in our bedroom closet and the sub is located on the outside wall. (The 11' is wire going from the panel, up to the attic, out and down the wall.

Let me also add that I have a 50amp, outdoor rated RV type service plug attached to the end. This is to allow me to make a portable service panel that I can disconnect at the end of the season leaving only the 50 Amp plug (50Amps @ 220V)

Looking at the drawing, one thing to me makes no sense whats so ever. The Neutral and the ground, originate from the SAME pont are at this point electrically identical.

In the subpanel, the neutral and ground are isolated. This I understand but still do not see the point, but I digress.

In the brand new panel I just bought (GE panel from Home Depot) there is only ONE buss bar out side the mains (L1 and L2) and this is labled NEUTRAL. There is NO seperate ground bus. At first thought I said okay, they mean for you take it to the box itself but it seems that could lead to a "shocking" revelation. Since Neutral and Ground are electrically identical at this point, you would be putting neutral on the neutral bus and neutral (ground) on the chassis. You walk up and touch the box and if there is a difference in potential from where your standing... ZAP .. you will feel the current on Neutral.

Until 2005, I think, there were only 3 wires needed. L1, L2 and Neutral. Now there is a forth and if I had the ground bus bar, you are effectively lifting ground.

I had a question when I started typing, but now I have only confusion.

1. What is the problem with my new box?

2. Do I need to take it back or just wire it per 2005 code (actually thats what has been there all along)

Each plug coming from the breakers is a GFI plug. I have never been shocked or felt a tingle and with the exception of a rainy Christmas Eve, I've never even tripped a GFI.

The new code makes no sense to me what so ever, but since I'm redoing part of it, I thought what the heck.

Any one have a pearl of wisdom other than that is just the code? Looks to me like the only people winnng in this set up (No.. NOT Charile Sheen) is the people selling wire.

I just really want to understand the concept, AND know what to do with what I have.

Long post .. I know .. but I sincerely appreciate any feedback ...

Bob






Attached files

[Gary Levelius]

Bob, this is exactly the same way I had to wire the 100 amp service sub panel I installed in the storage barn I put in a couple years ago. Our house was rewired when the new main panel was put in in 2000. This was before the four wire configuration was required. I believe the separation of the nuetral and ground wires was designed so that the "ground" potential of the two service panels would stay the same regardless of load. On the old system (3-wire), if the loads on each phase were different, you could end up with a difference in potential between the two panels due to the uneven loading creating a new voltage drop in the neutral wire. This could lead to a shock hazard if contact was made between the two panels which would be possible in cases where one display element was run from the original panel and another next to it from the add-in panel. It would also make GFCI trips more likely as well.

-Gary-

[bob_moody]

Gary Levelius wrote:

Bob, this is exactly the same way I had to wire the 100 amp service sub panel I installed in the storage barn I put in a couple years ago. Our house was rewired when the new main panel was put in in 2000. This was before the four wire configuration was required. I believe the separation of the nuetral and ground wires was designed so that the "ground" potential of the two service panels would stay the same regardless of load. On the old system (3-wire), if the loads on each phase were different, you could end up with a difference in potential between the two panels due to the uneven loading creating a new voltage drop in the neutral wire. This could lead to a shock hazard if contact was made between the two panels which would be possible in cases where one display element was run from the original panel and another next to it from the add-in panel. It would also make GFCI trips more likely as well.

-Gary-

Gary, Thank you for the quick reply. The first thing that caught my eye in your reply was the service panel in a storage barn. I am assuming this would be a seperate dwelling (building.. detached from the house or location of the mains)?

Thats what throws me, the attchement I included states for "Subpanel in same dwelling with service equipment"

Now, I do understand where your going with the load balancing on neutral ... I think. Neutral is a current carrying service. With all the load going back through L1-> Neutral (110) and L2 -> Neutral (110), the ground (in this 4 wire system) is a non-current carrying service that can conduct the potential difference in the event of a failure at the load ***** ASSUMING THE LOAD HAS A GROUND !!! ****** and can drain through there.

Am I even close ??

of course the BIG cavaeat to all of this is .... Christmas lights dont carry a ground ..

[-klb-]

Yes, generally, the idea is that ground shall stay at ground potential, unless a fault occurs, and it is carrying current. Neutral may move several volts either way depending on circumstances.

In some cases at the service entry, it may be permissible to use the same buss bar for ground and neutral, so they sell the panels with only one, and it bonded to case ground. However, most places I have been, for as long as I have been paying attention, have required separate ground and neutral buss bars, even if they are both tied to the case. The reason is that some bars allow more than one wire under each screw, and if you think you are loosening a ground, but in reality are loosening a ground and a neutral, you will now have 120V loose in the panel, if there is any load on the circuit at all.

There is probably a kit you can get that will add a new ground bar to your case, and a screw to remove from the existing one that removes the bonding to ground. Take a look and see if it is mentioned in any of the reference material for your panel.

[Gary Levelius]

You are correct in the load balancing assumption. The same hazard could exist with lights even though there is no ground. In the three wire system, the voltage drop across the neutral/ground wire could develop between neutral wires on a loads being from different panels since their neutral wires would come from two different sources.

And yes, this would be correct on two different buildings although you could potentially have two different display devices next to each other, one being attached to the house system and one to the bard/building sub-panel in the three wire configuration.

-Gary-

[bob_moody]

-klb- wrote:

Yes, generally, the idea is that ground shall stay at ground potential, unless a fault occurs, and it is carrying current. Neutral may move several volts either way depending on circumstances.

In some cases at the service entry, it may be permissible to use the same buss bar for ground and neutral, so they sell the panels with only one, and it bonded to case ground. However, most places I have been, for as long as I have been paying attention, have required separate ground and neutral buss bars, even if they are both tied to the case. The reason is that some bars allow more than one wire under each screw, and if you think you are loosening a ground, but in reality are loosening a ground and a neutral, you will now have 120V loose in the panel, if there is any load on the circuit at all.

There is probably a kit you can get that will add a new ground bar to your case, and a screw to remove from the existing one that removes the bonding to ground. Take a look and see if it is mentioned in any of the reference material for your panel.

KLB,

Thank you for the reply. I did go pull the paperwork that came with the GE panel and sure enough you are 100% on the money. The paper work refers to an "Optional" Equipment Grounding bar kit (TGL2). I measured and verified the nuetral bar is NOT bonded to the case.

Talking this out has REALLY helped me and I want to thank you and Gary for the replies. It was just one of those things that at first glance looked redundant.

I have seen this discussion for years and it always came down when asked about sub panels is that you need a 4 wire system AND becasue ... its code. I never really saw or understood the reasoning.

I think I do now. THANK YOU ..

Bob

[bob_moody]

Gary Levelius wrote:

You are correct in the load balancing assumption. The same hazard could exist with lights even though there is no ground. In the three wire system, the voltage drop across the neutral/ground wire could develop between neutral wires on a loads being from different panels since their neutral wires would come from two different sources.

And yes, this would be correct on two different buildings although you could potentially have two different display devices next to each other, one being attached to the house system and one to the bard/building sub-panel in the three wire configuration.

-Gary-

Gary,

Thank you very much for you insight into this issue I was having. I will say that all of my display is run from this one single panel. I do not use any electrical source from any other part of the house.

As I mentioned to KLB, there is a kit for my panel which I will try to get from Home Depot tomorrow (Yeah right .. like they will have it stock .. LOL .. I hope I eat crow on this statement .. LOL)

Thank you so very much again. You guys have been great.

Bob

[PMC]

If your interested in learning the rules for electrical codes

Or if you feel like having a copy of the NEC Rules, enjoy

Or watch some Videos on this topic

-Paul

[bob_moody]

Paul, Thanks for the reply. My main panel has its own grounding rod and conductor. Also, refering to the drawing, the subpanel does not have a seperate grounding rod or any source to ground other than back to the main service panel. I think we are all on the right track. I appreciate your feedback. Bob

[Steven]

Here's another way to look at the requirement. The general concept is that a single failure should not result in a hazardous situation.

If neutral and ground were over a single wire, which would have to be connected to the metal case, then what would happen if this wire developed a loose connection? You would have current flow from the hot, through the load (LOR controller), to the neutral bar, and would then energize the metal case. Not good!