The Electrical Contractor Network

ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

Books, Tools and Test Equipment for Electrical and Construction Trades

Register Now!

Register Now!

We want your input!

Featured:
   

2017 NEC and Related
2017 NEC
Now Available!

   
Recent Posts
Industrail Control Panel bonding per 409.108
by sparkyinak
Yesterday at 06:29 PM
Calling all Non-US members!! (Non-US only)
by aussie240
12/07/16 02:39 AM
Photo Upload Tutorial
by DanK
12/06/16 11:35 PM
Sprinklered equipment 26-008
by bigpapa
12/02/16 04:24 PM
On Delay Relay with Auto Reset
by Potseal
12/01/16 09:59 AM
New in the Gallery:
12.5A through 0.75mm˛ flex (just out of curiosity)
Shout Box

Top Posters (30 Days)
gfretwell 13
HotLine1 10
sparkyinak 9
Texas_Ranger 8
Potseal 6
Who's Online
0 registered (), 194 Guests and 4 Spiders online.
Key: Admin, Global Mod, Mod
Page 1 of 4 1 2 3 4 >
Topic Options
Rate This Topic
#86628 - 11/17/03 12:03 PM 3-wire subpanel?? I goofed.
brucepirger Offline
Junior Member

Registered: 11/17/03
Posts: 6
I goofed...through a development of plans and intentions, I ran 3 wire service from the disconnect (on a pole) outside into a house (which started as a workshop...)


Because the meter and disconnect are on the pole, I need to wire the panel as a subpanel. But I ran only 2 hots and 1 neutral, no ground.


I have read in the 2000 NEC that it is allowable to have a 3 wire subpanel. I believe I need to drive ground rods at the subpanel and connect the neutral from the main to these rods.


Furthermore, it would be good to drive additional ground rods which I tie to my GROUNDS in the subpanel, which ARE isolated from the neutrals in the panel.


Therefore, there is no "loop" back to the main panel should my neutral ever be broken.


Is this correct?


It's about 170' from the disconnect and service entrance pole to the house. Everything is buried...in conduit, but I know I'll never get another line through the conduit. I could dig again and add the fourth wire, probably the BEST thing. Is it necessary?


Safety is the key, I agree.


Open to all comments and attacks...at least I am saying I goofed! LOL Thanks.

Top
2014 / 2011 NEC & Related Books and Study Guides
#86629 - 11/17/03 01:12 PM Re: 3-wire subpanel?? I goofed.
wayne Offline
Member

Registered: 04/04/01
Posts: 55
Loc: NC
NO , Thats the way we are required to do it now here in fayetteville nc .
wayne

Top
#86630 - 11/17/03 01:33 PM Re: 3-wire subpanel?? I goofed.
resqcapt19 Offline
Member

Registered: 11/10/00
Posts: 2209
Loc: IL
 Quote:
Furthermore, it would be good to drive additional ground rods which I tie to my GROUNDS in the subpanel, which ARE isolated from the neutrals in the panel.

With only a 3 wire feeder the equipment grounding conductors must be bonded to the grounded conductor at the subpanel. If there is not a bond there is no fault clearing path.
Don
_________________________
Don(resqcapt19)

Top
#86631 - 11/17/03 01:42 PM Re: 3-wire subpanel?? I goofed.
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Bruce.

You are correct that if you follow a few rules you may run a 3 wire feeder in from the disconnect to the panel.

Here is the code article that allows this.

 Quote:
250.32(B)(2) Grounded Conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, (2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved, and (3) ground-fault protection of equipment has not been installed on the common ac service, the grounded circuit conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The size of the grounded conductor shall not be smaller than the larger of
(1) That required by 220.22
(2) That required by 250.122


Section 2 of this is very important.

 Quote:
(2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved,


One thing is still wrong according to your post

 Quote:
my GROUNDS in the subpanel, which ARE isolated from the neutrals in the panel.


This is wrong and dangerous, as you are feeding this panel without a grounding conductor you must bond the neutral bar to the enclosure just like it was a main panel.

You are correct that you need a grounding electrode.

When you are allowed to run a 3 wire feeder to a panel you must treat this panel as a service panel, the bonding the electrodes etc.

The way you have it set up a fault from hot to ground will not trip a breaker it will only feed current into one of your ground rods.

Feel free to ask more questions the members here will be glad to help.

Please read the code article I posted carefully and fully.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

Top
#86632 - 11/17/03 01:44 PM Re: 3-wire subpanel?? I goofed.
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I have got to type faster, thank you Don.

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

Top
#86633 - 11/17/03 02:43 PM Re: 3-wire subpanel?? I goofed.
brucepirger Offline
Junior Member

Registered: 11/17/03
Posts: 6
Thanks for the reply.

So what you are saying iwire is that I treat the panel just like a normal "main" service panel. That's what I always intended...

Install my ground rods and tie them to the ground/neutral bus and also install the bonding screw into the panel.

Therefore, my neutrals and grounds ARE bonded in the panel, and I also have ground rods at the panel.

So, exactly as if the service is coming from the pole straight into the house, right?

And this is "OK" for me becuase I do not have any other metallic connection between the building with the disconnect (i.e. pole in my case) and the other building. Therefore, I do not have the possibility for the neutral current to be flowing through the ground wire...becuase there IS no ground wire OR other conducting pathway.

Correct?


Question related to your comment about current being dumped into the ground rods without blowing a breaker...Is this just becuase the resistence is high enough in the ground rods to not blow the breaker typically?

So does that mean that the ground wire would become energized? And then all the 4 wire applicance cases (fridge, stove, dryer, etc. would become HOT to the touch?)

So the neutral, which orignates from the center tap of the transformer, is clearly the "low impedance" return for the current to flow...and with the bonding screw, a fault would cause the breaker to pop becuase the low resistance would create a large current.

Correct?

So what happens if a neutral fails in a standard panel? Seems like the 240 stuff would work fine since they are balanced and the current would flow through the HOTs, but all the single line stuff would not work, as the neutral is not closing the circuit? But the ground rods to the panel, say, are still good...but the resistance is high...so the current is low...and with any load, the voltage will drop.

And that's what causes all the nightmares with neutral faults...some things work fine, some work to some degree, and some not at all.

EDITED: And now with the 4 wire installation of the 240 appliances, the shells of the appliances are tied to the ground, specifically NOT the neutral.

And the purpose of the ground rods is to ensure there is a lower resistance pathway from the ground (and also the bonded neutral) than through, say, me.

So the ground rods are purely a "safety" issue, always hopefully assuring a lower resistance pathway to ground than through a person.

BOTTOM LINE: I will check with my inspector and do what he says. It appears that I fit under this guideline. Wayne, I see you say it is NOT ALLOWED in NC, regardless. I'm in NY, FingerLakes area, so we shall see. Since I don't have any other metallic pathways, and hence (I think) the reasoning for 4 wires and no bonding in the panel (no potential for current flowing through ground), I guess I am no different than if the service came straight to me from the street.
Thanks again folks!


[This message has been edited by brucepirger (edited 11-17-2003).]

[This message has been edited by brucepirger (edited 11-17-2003).]

Top
#86634 - 11/17/03 03:16 PM Re: 3-wire subpanel?? I goofed.
Ryan_J Offline
Moderator

Registered: 08/19/03
Posts: 1355
Loc: West Jordan, Utah, USA
What am I missing here?

I can only think of this being allowed for a detatched accessory structure.

I think you'll have to dig and add an equipment grounding conductor, but I'm usually wrong when I argue Bob and Don.
_________________________
Ryan Jackson,
Salt Lake City

Top
#86635 - 11/17/03 03:18 PM Re: 3-wire subpanel?? I goofed.
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Bruce you got it.

What was worrying me and I think Don too, was your statement that the neutrals and grounds where isolated from each other.

As to the tripping breaker and ground rods.

Say you have 25 ohms of resistance in your ground rod and you have a 120 volt fault to ground.

120 volts / 25 ohms = 4.8 amps

So you will not even be close to tripping even a 15 amp breaker.

Losing a neutral is a bad situation ground rod or not, a grounding connection to a water pipe in a neighborhood with all metal piping may help in that case.

Others can jump in here but from what I have learned the ground rod provides some measure of lighting protection.

But more importantly can help operate the utilities over current protection on the primary side of the transformer and help your electric equipment survive if a high voltage line was to break and land on the bare neutral / ground wire heading to your house.

At that voltage level 25 ohms is much more effective.

13,800 volts / 25 ohms = 552 amps much more than enough to open a POCO cut out.

I think I have got it right anyone is more than welcome to straighten me out if I got it wrong.



[This message has been edited by iwire (edited 11-17-2003).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

Top
#86636 - 11/17/03 03:23 PM Re: 3-wire subpanel?? I goofed.
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Hi Ryan, I see no reason the pole that this service starts at would not be the first "structure"

The article 100 definition of structure is pretty open.

 Quote:
Structure. That which is built or constructed.


Bob

[This message has been edited by iwire (edited 11-17-2003).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

Top
#86637 - 11/17/03 03:27 PM Re: 3-wire subpanel?? I goofed.
brucepirger Offline
Junior Member

Registered: 11/17/03
Posts: 6
Well, I wired the box with the grounds and neutrals isolated...but the bonding screw is in...and the ground rods are too.

If I have to dig and put in the 4th wire...then I remove the bonding screw and I am done. Viola.

If I can wire as a normal panel, than I am OK.

Is there a difference in safety? It seems to me, since there is no other possible conduction pathway from the main disconnect

(on a pole, 170' away, where someday I thought I'd build the real house, but live in the workshop for a few years first...but the workshop has become a mighty fine house over the past couple years...LOL)

then it seems that wiring as the "primary" panel is just fine. If there was another pathway, such as within the same structure, or metal water pipes, etc. then THAT changes everything.

I could never understand why I needed te subpanel...but now I think I do...and since I have no other electrical connections....I think I'm OK.

But the inspector will make the final say. I don't want to try and show him the NEC book and "force" my way. Not at all.

I never realized ground rods were more of a safety cushion...but now I understand. If their resistance is that high...they won't do much against the 120 fault...but a boatload against 4800KV!

Thanks folks.

Top
Page 1 of 4 1 2 3 4 >



ECN Electrical Forums - sponsored by Electrical Contractor Network - Electrical and Code Related Discussion for Electrical Contractors, Electricians, Inspectors, Instructors, Engineers and other related Professionals