Sub Panels
- Thread starter firelitr
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- engineer
firelitr said:
the basic answer is to reduce the flow of current on the egc. if you tie the neutral and egc together at more than one point, you will have some current flow on the egc.
electricman2
Senior Member
- Location
- North Carolina
- Occupation
- Retired Electrical Contractor
As Petersonra said, to avoid a parallel path for neutral current. the grounded conductor and grounding conductors are bonded at the service. Bonding them together at a sub panel would result in part of the neutral current flowing on the EGC. This would be an unsafe condition if a break were to occur on either the grounded conductor or the EGC. 250.24(5) prohibits such a connection for this reason.
al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
Welcome th the Forum, Firelitr!
The image I like to use is about how we work hard to keep all the current (I'm not thinking of voltage - just current) in the hot wire, inside the hot wire until it reaches the load (light, motor, heater, etc.).
The return current needs to be given the same careful attention so it is kept inside a single insulated path. . .
That is, until it reaches the Service Disconnect. At the Service Disconnect, there will be the Main Bonding Jumper that connects the neutral and the equipment ground together, and from there on out to the Power Company transformer, the neutral is not separate. (In fact, in my opinion, the Power Company side of the Main Bonding Jumper is just plain messy. . .but that's not the point of your question).
Good wiring practice will result in the hot conductor current being physically close (same cable or raceway) to the neutral conductor current. The two currents are equal, but traveling in opposite directions. The magnetic field that each current creates is the opposite of the other, but of equal strength, so, they literally cancel each other out. . . .but only if all the current that comes out on the hot is returned only on the neutral.
In a sense, neutrals are constructed like spokes of a wheel. . .a single line from a hub (service disco) to the rim (the connected loads). The equipment grounding is not done this way. The equipment grounding is all interconnected where ever two or more different ones are in a junction box together. . .so the equipment grounds look, not like a spoke, but like a spiders web.
Any current that gets onto the spiders web like equipment ground will spread out and take all available paths back to the Power Company transformer (source), leaving an unbalanced magnetic field around the hot conductor.
That magnetic field can heat ferrous metals by inducing eddy currents into the them.
The image I like to use is about how we work hard to keep all the current (I'm not thinking of voltage - just current) in the hot wire, inside the hot wire until it reaches the load (light, motor, heater, etc.).
The return current needs to be given the same careful attention so it is kept inside a single insulated path. . .
That is, until it reaches the Service Disconnect. At the Service Disconnect, there will be the Main Bonding Jumper that connects the neutral and the equipment ground together, and from there on out to the Power Company transformer, the neutral is not separate. (In fact, in my opinion, the Power Company side of the Main Bonding Jumper is just plain messy. . .but that's not the point of your question).
Good wiring practice will result in the hot conductor current being physically close (same cable or raceway) to the neutral conductor current. The two currents are equal, but traveling in opposite directions. The magnetic field that each current creates is the opposite of the other, but of equal strength, so, they literally cancel each other out. . . .but only if all the current that comes out on the hot is returned only on the neutral.
In a sense, neutrals are constructed like spokes of a wheel. . .a single line from a hub (service disco) to the rim (the connected loads). The equipment grounding is not done this way. The equipment grounding is all interconnected where ever two or more different ones are in a junction box together. . .so the equipment grounds look, not like a spoke, but like a spiders web.
Any current that gets onto the spiders web like equipment ground will spread out and take all available paths back to the Power Company transformer (source), leaving an unbalanced magnetic field around the hot conductor.
That magnetic field can heat ferrous metals by inducing eddy currents into the them.
well put, Al
well put, Al
is main service disconnect , the same as the first disconnect?
the reason for this question is, there is a sds feeding 2 panels,
panel 1 is within sight of the xfmr 75kva 480/208/120 wye 3 ph
panel 2 is 120' away, the first disconnect for panel 2 is fused within sight of the xfmr , the egc and neutral pass thru onto the main panel which is 120' away which is a 100amp 3 ph panel with main.
so the bonding bar, strap or wire or screw would be used here, correct?
if so wouldn't there be a parallel path back the 120' to the xfmr?
well put, Al
al hildenbrand said:
is main service disconnect , the same as the first disconnect?
the reason for this question is, there is a sds feeding 2 panels,
panel 1 is within sight of the xfmr 75kva 480/208/120 wye 3 ph
panel 2 is 120' away, the first disconnect for panel 2 is fused within sight of the xfmr , the egc and neutral pass thru onto the main panel which is 120' away which is a 100amp 3 ph panel with main.
so the bonding bar, strap or wire or screw would be used here, correct?
if so wouldn't there be a parallel path back the 120' to the xfmr?
frizbeedog
Senior Member
- Location
- Oregon
Intentionally controlling the path of the current
Intentionally controlling the path of the current
Al, I'll have to ponder that till my neck straightens out. That was wild.
View attachment 1417
Intentionally controlling the path of the current
al hildenbrand said:
Al, I'll have to ponder that till my neck straightens out. That was wild.
View attachment 1417
Last edited:
CNC
Member
- Location
- Bay Area, CA
"Good wiring practice will result in the hot conductor current being physically close (same cable or raceway) to the neutral conductor current. The two currents are equal, but traveling in opposite directions. The magnetic field that each current creates is the opposite of the other, but of equal strength, so, they literally cancel each other out. . . .but only if all the current that comes out on the hot is returned only on the neutral."
Can you help me understand when the current flowing back to the source, hits the neutral / ground bus in the main panel, how it does not ever leak onto the grounding conductors? Like it would if it were coming from a Sub Panel.
Can you help me understand when the current flowing back to the source, hits the neutral / ground bus in the main panel, how it does not ever leak onto the grounding conductors? Like it would if it were coming from a Sub Panel.
al hildenbrand
Senior Member
- Location
- Minnesota
- Occupation
- Electrical Contractor, Electrical Consultant, Electrical Engineer
CNC, my comments, that you quote, aren't about the PoCo side of the Main Bonding Jumper.
On the PoCo side, any current takes all available paths.al hildenbrand said:
JohnJ0906
Senior Member
- Location
- Baltimore, MD
firelitr said:
Whenever I am asked this question, I tell the person, "The proper question is, why do we put the neutrals and EGCs together at the service entrance?"
Putting them together is really the exception, not the rule. Everywhere else, we keep them seperate, with a few exceptions.
They are connected together at the main service (and SDSs) to provide a path to source for fault current.
- Location
- Bremerton, Washington
Good point at the service entrance on connecting neutrals and EGCs. That connection is done via the main bonding jumper. It is the most important part of the the premise wiring system
peter d
Senior Member
- Location
- New England
tom baker said:
I'll add that the most important connection is back to the source, not the ground, and the MBJ is how we accomplish this.
frizbeedog
Senior Member
- Location
- Oregon
firelitr said:
I've been asked this question from an apprentice or two and my first respose is always another question. "Why would we have gone to all the trouble of pulling two wires, one called a grounded conductor and one called a grounding conductor, the latter of which being usually smaller than the first? And after doing so land them on the same terminal in the sub panel?
This usually gets them a thinking.
Both conductors are designed to carry current but under very different circumstances. The grounded conductor for the connceted loads which are sized accordingly. And we have to assume that they will normally be conducting current and that the current they conduct should be kept containded in the grounded conductor all the way back to the source (transformer).
The grounding conductor, apart from being used to keep all eqiupment at one potential, is designed to conduct current only under fault conditions and for a short period of time. Also sized accordingly. Smaller for a reason.
Now, back at the service the grounding conductor makes a connection to the grounded conductor through the main bonding jumper to provide a direct path back to the source (transformer) which allows the overcurrent device to operate.
The only reason I ask the question is to get them thinking about the flow of current, and where we want it to go, safely. :smile:
Along with the other explainations I hope this one will help. It makes sense to me and it gets you thinking.
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