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#100852 - 01/12/07 08:16 PM subpanel debate  
trekkie76  Offline
Member
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
Hello Guys,

I am working in a commercial building, 200A 120Y/208V service. The meter is on the building, with the service conductors terminating in a 200A MB panel. Their is a 200A sub in the next stud pocket over fed from the main panel.

Here is the controversy. I asked for 5 runs of THHN to feed the subpanel, and the boss told me that it only needed 4 wires.
I asked him how you could make a load side connection to the grounded conductor, and he responded the inspector told him that because the panels where next to each other, the neutral and ground where bonded in the subpanel as well as in the main panel. Could anyone play devils advocate for me and tell me where I have gone astray?
Thanks, Gary

[This message has been edited by trekkie76 (edited 01-12-2007).]


2017 / 2014 NEC & Related Books and Study Guides

#100853 - 01/12/07 09:23 PM Re: subpanel debate  
Ron  Offline
Member
Joined: Mar 2002
Posts: 582
White Plains, NY
Gary, you are correct. It should be a 5 conductor run for 3 phase 4 wire system plus ground. You are not permitted to have a n-g bond downstream of the main bonding jumper unless it is a separately derived source.


Ron

#100854 - 01/13/07 09:32 AM Re: subpanel debate  
JohnJ0906  Offline
Member
Joined: Oct 2006
Posts: 65
Baltimore MD USA
If the wires are in metal conduit, you may not need a EGC per se, bonding would be through the conduit itself. How are the Phase conductors fed in the first panel? Do they come from a breaker or from the main breaker line side?


#100855 - 01/14/07 05:35 PM Re: subpanel debate  
trekkie76  Offline
Member
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
The subpanel is fed from a 3 phase breaker on the load side of the main. I havent done anything yet, I was going to pipe it. If I used an acceptable metallic raceway I would not need an equipment ground, but I would still have to seperate nuetrals and grounds, correct? Can anyone think of a reason why the inspector would tell my boss such a thing?


#100856 - 01/14/07 06:33 PM Re: subpanel debate  
Tom  Offline
Member
Joined: Jan 2001
Posts: 1,044
Shinnston, WV USA
If you run the feeder in a metal raceway, then all you need to do is keep the neutrals insulated from the enclosure and have a separate equipment ground bus.

Surely you have enough imagination to come up with a reason for the inspectors statement. I don't know about your area, but I know two around here that had former careers as CATV installer & the other one was a backhoe operator. With qualifications like that, they will occaisionally make a mis-statement about the NEC.

Tom

[This message has been edited by Tom (edited 01-14-2007).]


Few things are harder to put up with than the annoyance of a good example.

#100857 - 01/15/07 04:18 PM Re: subpanel debate  
trekkie76  Offline
Member
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
Tom, Normally I would agree with you, but this guy has been an inpector for about 35 years!! He is very knowledgable, so something is not right. I am going to discuss this further with my boss tommorrow. Thanks all for your input, Gary


#100858 - 01/15/07 04:40 PM Re: subpanel debate  
renosteinke  Offline
Cat Servant
Member
Joined: Jan 2005
Posts: 5,316
Blue Collar Country
I saw this 'short cut' once.....

As in this example, the panels were next to each other, with but a short nipple connecting them. The nipple was secured properly, with locknuts on both sides of the sheet metal.

Unfortunately, in the thirty years or so since the place was built, the connection was no longer as good as it once was. A little corrosion, a little vibration ... and the resistance of the connection increased. In effect, the nipple became a resistor.

Since the neutral was bonded to the case, the nipple was the only return path for unbalanced current. With the nipple no longer reliable, the "voltage to ground" varied greatly as loads changed during the course of the day.

Finally, one Thanksgiving holiday, the 'swing' in voltage became great enough to fry a large number of small surge suppressors. Looking at the circuits involved, all were powered from the same phase - though several circuits were involved. We looked and looked and looked - then realised what we were NOT seeing was any neutral wire to that panel!

I repaired this, and all was well. Indeed, the IT guys were astounded in all the mysterious power quality issues that went away! They had more fun with all their fancy scopes and meters than a kid on Christmas [Linked Image]

Since then, I've be a lot more focused on keeping my ground path and my neutral separated.


#100859 - 01/15/07 11:14 PM Re: subpanel debate  
EV607797  Offline
Member
Joined: Oct 2006
Posts: 745
Fredericksburg, VA, USA
I think you might be misunderstanding what they are telling you. Your fifth conductor is assumed to be a bonding conductor between the panels. If the panels are connected with the proper metallic nipple, locknuts, bushings, etc., this becomes the fifth (bonding) conductor. The other four become A,B,C and N.

The sub panel's neutral bar IS NOT bonded to the can.

Now, if you are using a non-metallic connection between the panels, then you'll need a fifh conductor for bonding.

The only exceptions to this are a few rare ones when dealing with detatched buildings.


---Ed---

"But the guy at Home Depot said it would work."

#100860 - 01/16/07 12:08 AM Re: subpanel debate  
gfretwell  Offline


Member
Joined: Jul 2004
Posts: 9,142
Estero,Fl,usa
Quote
I repaired this, and all was well. Indeed, the IT guys were astounded in all the mysterious power quality issues that went away! They had more fun with all their fancy scopes and meters than a kid on Christmas


You just had the wrong IT guys or they had the wrong tools.
My Drainitz would have found that bad neutral in a minute or a foot of tape, whichever came first. [Linked Image]


Greg Fretwell

#100861 - 01/17/07 04:35 PM Re: subpanel debate  
trekkie76  Offline
Member
Joined: Jun 2004
Posts: 220
baileyville, maine, usa
A little update, I asked the electrical instructor at the local tech college, who is very knowledgeable, and he said that because the panels are grouped together they are one service. He said it is in 230-70, which pretains to the six disconnect rule. I am unable to see how this applys, can anyone help?


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