A multi-family building had an addition added where the existing switchgear did not have enough space to include additional air conditioning circuits. The remedy was to add an additional exterior main breaker panel fed from the same pad mount xformer, metered through the existing CT, to feed the new A/C condensers (3ph 208v). This results in (2)seperately grounded services from the same xformer to the same building. Question: Should the GEC's be bonded, and if so, won't this create parallel neutral currents flowing through the line sets between the respective furnaces and their connections to the original service?
Bonding two services @ the same building
- Thread starter photon
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david
Senior Member
- Location
- Pennsylvania
Re: Bonding two services @ the same building
Assuming this set up has been pre approved by the inspector,
the short answer is yes both services have to use all available grounding electrodes to form a grounding electrode system.
Yes the grounding electrodes for both services have to be bonded together.
Assuming this set up has been pre approved by the inspector,
the short answer is yes both services have to use all available grounding electrodes to form a grounding electrode system.
Yes the grounding electrodes for both services have to be bonded together.
bphgravity
Senior Member
- Location
- Florida
Re: Bonding two services @ the same building
Yes, this causes a severe problem. It should be specified that the grounded (neutral) connections at each separate service should be inspected and checked periodically. The opening of one service neutral puts the flow of normal operational current on the grounding electrod system.
I have still not quite understood the logic of interconnecting service neutrals through the use of the metallic parts of the building? It just doesnt make any sense to me.
Yes, this causes a severe problem. It should be specified that the grounded (neutral) connections at each separate service should be inspected and checked periodically. The opening of one service neutral puts the flow of normal operational current on the grounding electrod system.
I have still not quite understood the logic of interconnecting service neutrals through the use of the metallic parts of the building? It just doesnt make any sense to me.
- Location
- Illinois
- Occupation
- retired electrician
Re: Bonding two services @ the same building
How did you comply with 230.72?
Don
How did you comply with 230.72?
Don
Re: Bonding two services @ the same building
Some clarification: Bryan, my concern was parallel paths created from the neutral/ground bonding jumpers at both services. Because these services are fed from the same transformer, the air handlers are fed from service #1, and the A/C's are fed from service #2, the interconnecting metal parts, along with the GEC, I would think create parallel paths for neutral flow between the 2 main disconnects.
Don, I believe the installing contractor probably dodged 230.72 with 230.2(C)(3).
Some clarification: Bryan, my concern was parallel paths created from the neutral/ground bonding jumpers at both services. Because these services are fed from the same transformer, the air handlers are fed from service #1, and the A/C's are fed from service #2, the interconnecting metal parts, along with the GEC, I would think create parallel paths for neutral flow between the 2 main disconnects.
Don, I believe the installing contractor probably dodged 230.72 with 230.2(C)(3).
bphgravity
Senior Member
- Location
- Florida
Re: Bonding two services @ the same building
Photon:
I understand exactly what you are saying. Now imagine if one of the services gets an open nuetral from the transformer. Where is the current going to flow? On the grounding electrode system!
Photon:
I understand exactly what you are saying. Now imagine if one of the services gets an open nuetral from the transformer. Where is the current going to flow? On the grounding electrode system!
karl riley
Senior Member
Re: Bonding two services @ the same building
BPH, it will flow on the bond to the other serv ice entrance neutral bus and back to the transformer on that neutral, no?
I observed this problem at a shopping center where a service fed a supermarket and another service fed the shops. The required bonding to the common metal water pipes for the buildings resulted in a lot of neutral current flowing through the pipes. This created high magnetic fields in the shops.
All you need is a better connection for one of the neutrals and current will flow that way. I saw no way to remedy the situation other than break the electrical continuity of the water pipe between the buildings. (of course building steel may take over).It was not done since I was not called in professionally; I was just a customer in the coffee shop walking around with a gaussmeter.
But I don't know of a Code article that requires two service grounding electrode systems to be bonded together. Can anyone cite?
Karl
BPH, it will flow on the bond to the other serv ice entrance neutral bus and back to the transformer on that neutral, no?
I observed this problem at a shopping center where a service fed a supermarket and another service fed the shops. The required bonding to the common metal water pipes for the buildings resulted in a lot of neutral current flowing through the pipes. This created high magnetic fields in the shops.
All you need is a better connection for one of the neutrals and current will flow that way. I saw no way to remedy the situation other than break the electrical continuity of the water pipe between the buildings. (of course building steel may take over).It was not done since I was not called in professionally; I was just a customer in the coffee shop walking around with a gaussmeter.
But I don't know of a Code article that requires two service grounding electrode systems to be bonded together. Can anyone cite?
Karl
karl riley
Senior Member
Re: Bonding two services @ the same building
Thanks, eprice. Now as I remember it, the supermarket and the shops were actually separate buildings. I forget whether they were physically connected or not. What would be the definition of separate buildings? They were sharing the same water system.
Karl
Thanks, eprice. Now as I remember it, the supermarket and the shops were actually separate buildings. I forget whether they were physically connected or not. What would be the definition of separate buildings? They were sharing the same water system.
Karl
arneykaner
Member
- Location
- Illinois
Re: Bonding two services @ the same building
The definition of separate structures would come from a Building Code - whatever is adopted in that
municipality. Usually, it deals with fire separation issue and the most common case is three or four hour of fire separation rating.
Karl,
The definition of separate structures would come from a Building Code - whatever is adopted in that
municipality. Usually, it deals with fire separation issue and the most common case is three or four hour of fire separation rating.
arneykaner
Member
- Location
- Illinois
Re: Bonding two services @ the same building
Also, some Fire Departments have a rule, that occupancies at the same address shall be treated as the same building - City of Chicago is pretty notorious for that.
Karl,
Also, some Fire Departments have a rule, that occupancies at the same address shall be treated as the same building - City of Chicago is pretty notorious for that.
brian john
Senior Member
- Location
- Leesburg, VA
Re: Bonding two services @ the same building
If I understand what Pho is asking, will there be some current flow under normal day to day operation of the two separate services on the GEC, the answer most likely is yes. Some current will flow on the GEC from one service through the other GEC back to the other service. We have encountered this problem several times.
What is usually done (in or area) is to set a master ground bar (MGB) in the main electric room and run a both service GEC’s to this MGB, with a single GEC run to the grounding electrode. While some may question this practice from a NEC standpoint, it does minimize circulating currents and the result EMF that can and does cause PQ issues, i.e. screen shake with PC monitors.
If I understand what Pho is asking, will there be some current flow under normal day to day operation of the two separate services on the GEC, the answer most likely is yes. Some current will flow on the GEC from one service through the other GEC back to the other service. We have encountered this problem several times.
What is usually done (in or area) is to set a master ground bar (MGB) in the main electric room and run a both service GEC’s to this MGB, with a single GEC run to the grounding electrode. While some may question this practice from a NEC standpoint, it does minimize circulating currents and the result EMF that can and does cause PQ issues, i.e. screen shake with PC monitors.
karl riley
Senior Member
Re: Bonding two services @ the same building
Brian, are you saying that you don't bond to the water pipe?
Karl
Brian, are you saying that you don't bond to the water pipe?
Karl
arneykaner
Member
- Location
- Illinois
Re: Bonding two services @ the same building
If each service neutral is grounded immediately
at the service transformer, the difference betwen two neutrals' potentials would be only the difference between voltage drops from neutral current in each neutral conductor.
Certainly, using "main ground bus" is a very good idea, because it visualizes the grounding connections and provides quality connections of all grounding electrodes, thus simplifying the maintenance.
But I don't see a problem with "loop currents"
in connecting the grounding electrode conductors.
The current in a conductor is proportional to the voltage applied. Under the normal conditions (no fault to ground, no open phase) the diffrence in potential (voltage to drive the current) between parts of grounding system would be within 2% of voltage to ground (2% of 277V). What kind of current and what magnitude of EMF could it possibly create?
If each service neutral is grounded immediately
at the service transformer, the difference betwen two neutrals' potentials would be only the difference between voltage drops from neutral current in each neutral conductor.
Certainly, using "main ground bus" is a very good idea, because it visualizes the grounding connections and provides quality connections of all grounding electrodes, thus simplifying the maintenance.
But I don't see a problem with "loop currents"
in connecting the grounding electrode conductors.
brian john
Senior Member
- Location
- Leesburg, VA
Re: Bonding two services @ the same building
Arny: The level of milligauss that affects PC monitors is around 14 milligauss. This weekend we did an investigation with the exact situation discussed. We had 18 amps of circulating current. Oh and Pho this is not a parallel path, but a direct connection of the two grounded conductors.
Karl: No all electrodes as described in the NEC are connected to the services at the MGB.
Arny: The level of milligauss that affects PC monitors is around 14 milligauss. This weekend we did an investigation with the exact situation discussed. We had 18 amps of circulating current. Oh and Pho this is not a parallel path, but a direct connection of the two grounded conductors.
Karl: No all electrodes as described in the NEC are connected to the services at the MGB.
arneykaner
Member
- Location
- Illinois
Re: Bonding two services @ the same building
18 amps were circulating - between neutrals of different services, or just in a neutral conductor of a feeder? It is not unusual to have current in the feeder neutral conductor up to
100% of phase current (up to a maximum of 1.73x 100%), if you have an ubalanced 3Ph feeder with a lot of 1Ph load.
Could you, please, give some details?
Thank you
I am sorry, Brian, but I did not understand where
18 amps were circulating - between neutrals of different services, or just in a neutral conductor of a feeder? It is not unusual to have current in the feeder neutral conductor up to
100% of phase current (up to a maximum of 1.73x 100%), if you have an ubalanced 3Ph feeder with a lot of 1Ph load.
Could you, please, give some details?
Thank you
arneykaner
Member
- Location
- Illinois
Re: Bonding two services @ the same building
Again, were those 18 amps in an Equipment Grounding Conducor, that connects a piece of equipment to an upstream panel, or EGC between two panels, or in Grounding Electrode Conductor between main switchboard (disconnect) and Grounding Electrode? or you found 18 amps in the link that interconnects two services' grounding buses?
My concern is, that to generate 18 amps, there should be substantial difference in potential between two (supposedly) grounded points - and such condition shall not exist.
Brian,
Again, were those 18 amps in an Equipment Grounding Conducor, that connects a piece of equipment to an upstream panel, or EGC between two panels, or in Grounding Electrode Conductor between main switchboard (disconnect) and Grounding Electrode? or you found 18 amps in the link that interconnects two services' grounding buses?
My concern is, that to generate 18 amps, there should be substantial difference in potential between two (supposedly) grounded points - and such condition shall not exist.
- Location
- Illinois
- Occupation
- retired electrician
Re: Bonding two services @ the same building
Arney,
Sometimes the grounding system results in a parallel path for the grounded conductor current and in these cases there can be a significnt amount of current on the grounding system. A good example is when you have a number of houses that are connected to a common utility transformer and a common metal under ground water pipe system. It would not be unusual to find that 15+% of the grounded conductor current is flowing on the water system.
Don
Arney,
Sometimes the grounding system results in a parallel path for the grounded conductor current and in these cases there can be a significnt amount of current on the grounding system. A good example is when you have a number of houses that are connected to a common utility transformer and a common metal under ground water pipe system. It would not be unusual to find that 15+% of the grounded conductor current is flowing on the water system.
Don
brian john
Senior Member
- Location
- Leesburg, VA
Re: Bonding two services @ the same building
Arney:
The 18 amps were measured on grounding electrode conductors (actually 22.5 amps see below). These conductors originated in two separate services at the line side neutral connection. The services are fed from a common utility distribution network system. The conductors run approximately 50 feet to the building incoming water pipe where they are connected by means of a UL listed pipe clamp. The current is present on both conductors, though not exactly 18 amps per GEC. If you put the current probe around both conductors you read 3.5 amps. We have isolated the 3.5 amps to the emergency distribution system.
Arney:
The 18 amps were measured on grounding electrode conductors (actually 22.5 amps see below). These conductors originated in two separate services at the line side neutral connection. The services are fed from a common utility distribution network system. The conductors run approximately 50 feet to the building incoming water pipe where they are connected by means of a UL listed pipe clamp. The current is present on both conductors, though not exactly 18 amps per GEC. If you put the current probe around both conductors you read 3.5 amps. We have isolated the 3.5 amps to the emergency distribution system.
arneykaner
Member
- Location
- Illinois
Re: Bonding two services @ the same building
Brian,
If we are talking about 480V (or lower) system, 18A in a grounding electrode conductor is abnormal(too high). Also, the fact that the difference in currents, flowing through GECs is about 20% of each current (you measure the difference, when put clamp meter around both conductors) indicatates, that the currents flow in opposite directions.
My guess:
1. There is an ungrounded system somewhere downstream of one of services, and it has a constant fault to the ground, which is not detected. Or, a ground is mistakenly used instead of the neutral with some 1Ph load. That creates about 18-20A in GEC-1.
2. If the water pipe is the single means of grounding (no driven electrodes, no Ufer ground,etc) the pipe itself may have a difference
of potentials with the ground, and some of the current from GEC-1 branches into GEC-2, instead of dissipating into ground.
If my guess is correct, then additional grounding electrodes, as close as possible to each service neutral termination should solve the problem.
Now I understand your concern in general about interconnecting GECs of two services. But I still think that the cure for this is troubleshooting and:
1. eliminating the cause(s) of currents in a GEC
2. providing effective grounding for each service with a driven electrode or a Ufer ground, etc., and bond it to water pipe, assuring that pipe is at near zero volts to ground.
The main reason for interconnecting the services' gounding conductors is equalizing the point of zero potential. For some reason, it did not work in your case.
Brian,
If we are talking about 480V (or lower) system, 18A in a grounding electrode conductor is abnormal(too high). Also, the fact that the difference in currents, flowing through GECs is about 20% of each current (you measure the difference, when put clamp meter around both conductors) indicatates, that the currents flow in opposite directions.
My guess:
1. There is an ungrounded system somewhere downstream of one of services, and it has a constant fault to the ground, which is not detected. Or, a ground is mistakenly used instead of the neutral with some 1Ph load. That creates about 18-20A in GEC-1.
2. If the water pipe is the single means of grounding (no driven electrodes, no Ufer ground,etc) the pipe itself may have a difference
of potentials with the ground, and some of the current from GEC-1 branches into GEC-2, instead of dissipating into ground.
If my guess is correct, then additional grounding electrodes, as close as possible to each service neutral termination should solve the problem.
Now I understand your concern in general about interconnecting GECs of two services. But I still think that the cure for this is troubleshooting and:
1. eliminating the cause(s) of currents in a GEC
2. providing effective grounding for each service with a driven electrode or a Ufer ground, etc., and bond it to water pipe, assuring that pipe is at near zero volts to ground.
The main reason for interconnecting the services' gounding conductors is equalizing the point of zero potential. For some reason, it did not work in your case.
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