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Old 01-19-2011, 10:30 PM   #1
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Subpanel next to Main panel wired the same in 1990


Electricity: If a sub panel was wired just like the main panel (ground and neutral bonded) what is the danger?
In 1990 I back fed a sub panel (located next to the main panel) with a breaker from the main panel, then I bonded both the neutral and ground wires on the (one) bar in the sub panel, then I attached the bar in the sub panel to the bar in the main panel with appropriate sized wire (both bars with green screws attaching the bars to respected breaker cases).

-- The inspector approved it in 1990, but now I read that the new codes require that the sub panel must isolate the ground and neutral wires in the sub panel.

What is the danger of my situation?


Added question: If I unscrew the green screw bonding the one bar in the sub panel, what will ground the the sub panel case? I hope I do not have to pull everything out and float the neutral bar and add a ground bar to the sub panel .

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Old 01-19-2011, 11:43 PM   #2
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Subpanel next to Main panel wired the same in 1990


yes there is a danger. you will have your grounds become hot. they will be carrying current now. you need to separate all grounds and neutrals with one and ONLY one exception that is in the main panel they MUST be bonded

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Old 01-19-2011, 11:43 PM   #3
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Subpanel next to Main panel wired the same in 1990


You dont need to rewire anything more than likely as it is grandfathered in.

Heres the basics of why.

They want to have a separate system to create a Grounded path and a CURRENT return path (Neutral)

They also changed the naming of wiring

Ungrounded conductors are commonly called "Hots" or Hot legs
Grounded conductors are Neutrals and Grounds
Current Carrying conductors are HOTS AND NEUTRALS

You need to understand the terminology then everything will make sense to you. the NEC is going on a Bonding and Grounding kick meaning EVERYTHING is Grounded, Bonded and Grounded/Bonded again for redundancy.

It used to be that these systems were interchangeable but now the thinking is to have a completely isolated grounding system.

Your main panel is bonded together to give it 3 ways to ground/return. One is your Service Drop neutral/messenger wire, the second is your water pipe ground wire and the other is your ground wire that runs to your electrode/ground rod.

On a subpanel you do not ground them to anything external other than the 4th wire coming in(when in the same structure) except when in a detached structure you ground to 1 (usually a ground rod) AND run an isolated ground back to the main, 4th wire (giving you a redundant system)

to answer your second question, If you unscrew the bonding strap, one of the Bars should be detached from the case (Neutral) and the other should be connected at somepoint (ground bar) You need to make sure you have the right Branch Circuit wires running to it AND you have 4 wires coming in from the Main panel. Basically, when a panel is acting as a Main Breaker Panel, you dont have a "Ground Bar" you just have 2 neutral bars. When it is a sub panel you have a Neutral and a Ground bar.

DO NOT UNDER ANY CIRCUMSTANCE ALLOW THAT SUBPANEL CASE TO BE UNGROUNDED......


I hope this helps.
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Old 01-20-2011, 12:43 AM   #4
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Subpanel next to Main panel wired the same in 1990


This drawing sums up the danger of bonding neutral and ground in a 4 fire feeder serving a panel load side of the service equipment. You never want system current (neutral current) to use the equipment ground to return to the source. Equipment ground never has current on it unless there is a ground fault and then very briefly until the circuit breaker opens. Second drawing is the correct bonding configuration. You simply add an additional ground bar bonded to the case and remove the bonding means (green screw in this case) installed the neutral bar.
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Subpanel next to Main panel wired the same in 1990-4-wire-improper-bonding.jpg   Subpanel next to Main panel wired the same in 1990-4-wire-proper-bonding-1.jpg  
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Last edited by Stubbie; 01-20-2011 at 07:30 AM.
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