Sub-panels
- Thread starter Jherb
- Start date
- Status
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Here is an explanation that I wrote some time ago:
Why grounds and neutrals are tied only at the main service, and not at a subpanel.
Reference: NEC article 250.42.
The reasons that there is a difference are (1) That current is always seeking a path back to its source, and (2) That current will take every available path.
The function of the equipment grounding conductors (EGC), that ones that connect to the ground bar in the panel, is to carry fault current. If a fault occurs with a piece of equipment, such that a hot conductor comes into contact with the case or other external metal part, any person who touches that equipment is going to get a shock. The shock can be enough to kill, but the current will not be high enough to cause the breaker to trip.
However, with the EGC creating a path from the case back to the ground bar, then via the ground screw or bonding jumper to the neutral bar, the current in this path will be high enough to trip the breaker. This will terminate the event before the person can receive a fatal shock. That is why the ground and neutral buses are connected at the main service disconnecting means ? to complete the current path from the fault point back to the source. In this context, I am treating the main panel as the "source." Once the current gets to that point, it has nowhere else go.
If you also connect the ground and neutral at a subpanel, then there will be two paths for current to flow back to the source during normal operation. Current will be flowing in the neutral most of the time (unless the loads running at the moment are perfectly balanced among the phases). But with the ground and neutral tied together both at the main panel and at the subpanel, the EGC will be in parallel with the neutral wire. Therefore, the EGC will carry current. This will cause the external metal parts of each and every component that has an EGC its to become energized. You could not safely touch anything in the facility.
Why grounds and neutrals are tied only at the main service, and not at a subpanel.
Reference: NEC article 250.42.
The reasons that there is a difference are (1) That current is always seeking a path back to its source, and (2) That current will take every available path.
The function of the equipment grounding conductors (EGC), that ones that connect to the ground bar in the panel, is to carry fault current. If a fault occurs with a piece of equipment, such that a hot conductor comes into contact with the case or other external metal part, any person who touches that equipment is going to get a shock. The shock can be enough to kill, but the current will not be high enough to cause the breaker to trip.
However, with the EGC creating a path from the case back to the ground bar, then via the ground screw or bonding jumper to the neutral bar, the current in this path will be high enough to trip the breaker. This will terminate the event before the person can receive a fatal shock. That is why the ground and neutral buses are connected at the main service disconnecting means ? to complete the current path from the fault point back to the source. In this context, I am treating the main panel as the "source." Once the current gets to that point, it has nowhere else go.
If you also connect the ground and neutral at a subpanel, then there will be two paths for current to flow back to the source during normal operation. Current will be flowing in the neutral most of the time (unless the loads running at the moment are perfectly balanced among the phases). But with the ground and neutral tied together both at the main panel and at the subpanel, the EGC will be in parallel with the neutral wire. Therefore, the EGC will carry current. This will cause the external metal parts of each and every component that has an EGC its to become energized. You could not safely touch anything in the facility.
weressl
Esteemed Member
charlie b said:
The last sentence is false.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I only wrote the thing. You don't actually expect me to have read it too?weressl said:
:roll: There is nothing false about it. It's just not complete or precise. That is the price of writing at a non-technically intensive level. One must, on occasion, over-simplify. So, OK, I will admit. It may be safe to touch the "safety first" sign in the break room.
weressl
Esteemed Member
I was wrong, it is the sentence before that is false.charlie b said:
I only wrote the thing. You don't actually expect me to have read it too?
There is nothing false about it. It's just not complete or precise. That is the price of writing at a non-technically intensive level. One must, on occasion, over-simplify. So, OK, I will admit. It may be safe to touch the "safety first" sign in the break room.
- Location
- Massachusetts
weressl said:
You feel this is false?
I would say that it is a true statement, even though it will be in most cases an insignificant difference of potential from 'earth' but there will be some difference of potential.
- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
Yea. What he said.iwire said:
weressl
Esteemed Member
iwire said:
There is potential difference all the time, all over, but for practical purposes the connecting of the EGC will not create a hazardous touch voltage condition as it was stated, therefore the last two sentences were false, misleading and confusing in understanding of what is actually happening. It does not help to educate.
- Location
- Massachusetts
weressl said:
See the thing is if you choose to be so precise it will come back to haunt you.:wink:
You had said that the second to last sentence was false.
weressl said:
Then
weressl said:
Well the second to last sentence is true.
If your gonna 'bust em' you better be accurate or expect questions.
Lee Honeycutt
Member
- Location
- Round Rock, TX
What about a seperate building. A 4-wire feeder going out to a panel in an outbuilding. Should the nuetral be bonded to the seperate Grounding Electrode?
rcarroll
Senior Member
- Location
- Fort Collins, Colorado
No, 250.32(B)(1)Lee Honeycutt said:
JohnJ0906
Senior Member
- Location
- Baltimore, MD
Lee Honeycutt said:
rcarroll said:
Assuming 4-wire feeder is 2 hots, neutral and EGC, I agree.
stickboy1375
Senior Member
- Location
- Litchfield, CT
Lee Honeycutt said:
no..............your only allowed to bond the grounded conductor to the grounding conductor at the first disconnect, anything after that will create parallel paths...
weressl
Esteemed Member
iwire said:See the thing is if you choose to be so precise it will come back to haunt you.:wink:
You had said that the second to last sentence was false.
Then
Well the second to last sentence is true.
If your gonna 'bust em' you better be accurate or expect questions.
I am not "busting" 'them' the facts are. You defending schemantics not that the connection of the EGC in the subpanel does not change the hazard level ONE IOTTA.
jwelectric
Senior Member
- Location
- North Carolina
weressl said:
I am always trying to learn so based on your statement the equipment grounding conductor serves no purpose at all and we can use the grounded (neutral) in place of the equipment grounding conductor.
Also based on your comment should the equipment grounding conductor and the grounded (neutral) be bonded together on the load side of the service in branch circuits and feeders where the conductors are less than a 1/0 then there would be no violation of 310.4?
If there is not ONE IOTTA of hazard in the bonding of the two in a branch circuit then it would be alright to just jump from the grounding terminal of a three wire receptacle to the grounded (neutral) terminal in order to have a fault path for the receptacle.
Please help me to understand this. Am I correct in my understanding of what you are saying?
M. D.
Senior Member
And what about 250.6 ,.. if the neutral is connected to the equipment ground at a sub panel the grounding of the system is not arranged to prevent objectionable current from flowing over grounding conductors or other grounding paths,indeed it would be encouraged to do just that.
- Location
- Massachusetts
weressl said:
You where busting them, maybe you don't know that you are but it seems to be your SOP here.
But that aside which one of my posts in this thread is not entirely factual?
Thats odd as the NEC is about safety and the NEC prohibits that connection.
Why do you imagine the NEC prohibits that connection if it has no possible effect on safety?
weressl
Esteemed Member
jwelectric said:
What I said is that bonding the neutral to the EGC at the main AND the subpanel does not change the level of hazard.
Should we choose not to employ a separate EGC then we would need to bond the Grounded conductor to the casing at every single location. The danger here is that if the neutral is open the case would be at line voltage compared to the ground. To avoid that you would need to have a bonding connection to ground at every location where you have connected the grounded conductor to the enclosure.
Purpose of the grounding path, weather it is a separate groudning conductor or the conduit, is to provide a relaively low impedance fault current path in case the ungrounded conductor inadvertently came into contact with non-energized conductive parts of the electrical apparatus. Should the grounded conductor (neutral) accidentaly contact to the same parts since the case is already grounded via the EGC and the grounded conductor is bonded at the service origin.
In case of a ground fault close to the equipment the fault current would take the following paths:
a - Fault to case
b - case to EGC,
c - Case to earth(1),
d - EGC to grounded conductor in the subpanel,(2)
e - (2) subpanel grounded and EGC will proportionately split the current between them,
f - both EGC and grounded conductor(neutral) carries the fault current back to the WYE point of the source to comnplete the fault circuit,
g - (1) there will be a high impedance component of the fault current that will flow though the earth - however good the incidental or intentional grounding of the equipment case at the use point - back to the WYE point of the source.
All these parallel and series paths will complete the circuit, but the flow of the current only goes through one protective device on the ungrounded conmductor of the circuit. The additional parallel paths on the grounded side reduce the overall total impedance of the circuit when comnpared to the pre-fault segment of the wiring so that the fault can open the breaker.
Should there be loose grounding connections or corroded or long conduit path providing high impedance path to complete the circuit it is possible that the fault current will remain insufficient to open the protective device.
In addition, if the equipment casing itself does not have a ground connection at it's location and somehow its mounting is insulated, it is possible to have line voltage on that device's casing. In that case if one is to touch the device casing and another item that is effectively grounded, such as a waterpipe, a gas-line or a metallic structure that is grounded throught connections to the foundation a "shocking" experience is to be expected. ;-) If the case is connected to the surrounding grounded structure, even if the impedance path back to the source ground bonding point is to high to operate the protective device there is no potential difference between the equipment and its surroundings therefore no electrocution danger is present. We can talk about step voltages, but again in low voltage sitiations - under 1000V - it is insignificant.
The tricky thing about EGC's and grounding connections is that they are "passive" connections, eg. they do not normally carry currents - not quite true, but insignificant to the argument - so you only discover their defects when you need them!
I wish somebody install a spellchecker on this!
Last edited:
weressl
Esteemed Member
M. D. said:
Current will flow on the grounding conductor?? yes?
It is not allowed . Yes?
Current continously flows on grounding conductors, you can't stop physics.
You can't teach anything by just saying it is not allowed.
Current inteded for the grounded conductor(neutral) may not be allowed to flow on the grounding(EGC) conductor and that's the intent of the NEC. Yet the power company forces you to do that EVERY day! So why don't they stop them? If it would be unsafe, they can take it up with OSHA. Yet they don't, because? Because they loose on the science.
I did not argue that it was in accordance with the NEC or not, I maintained that it poses no additional or increased hazard. If you wish to teach me and others, please explain where I am wrong.
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