When installing a subpanel off a service panel lets say a 60 amp 120/240 volt you would run two hot one neutral and one ground . what are the dangers if you do not run the ground and wire it like a service panel? ( the subpanel wires run in pvc or emt any difference) the ground and neutral only touch at the service but in this case they would touch also in the subpanel.
SUBPANEL
- Thread starter domnic
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- Licensed Electrician
You must have an Equipment Grounding Conductor along with the hots and noodle. EMT counts for an EGC..... PVC pipe you have to pull a green. All EGS in the sub panel must be isolated from the neutral.
I think I misread the question. Are you talking about mounting a sub panel right on the service panel?
I think I misread the question. Are you talking about mounting a sub panel right on the service panel?
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electrofelon
Senior Member
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- Cherry Valley NY, Seattle, WA
If both the main and sub are bonded to neutral, their cabinets will be at different potential because of the voltage drop on the neutral conductor. Of course this will be very small and imperceptible during normal operation but in some rare circumstances could be a shock hazard, for example during a fault (high current flowing=how voltage drop).
Also if the interconnecting raceway were metal, than you would have neutral current flowing on the raceway and for the same reason as above you could have different potentials between it and other metal stuff which could be a shock or fire hazard. I once fed a bunch of panels in a building with MC cable. The existing panels had been temped up with 2+ground cable so all the panels were bonded. I typically hooked the Mc up to the panel and worked it back to the service, and I would frequently get slight arcing as the Mc was pulled over/around other conduits pipes and ducts - some of the neutral current in those panels was coming back on the MC armor. So that illustrates the point well I think.
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- Licensed Electrician
Along with what electrofellon said.....
We used to do ranges and dryers this way up until the '90s. There is no danger as long as you don't loose your ground potential. If you do, touching the metal at the sub panel and something grounded would, at a minimum, give you a nasty shock.
If you have ever been shocked by an open, loaded, neutral, that's what the danger is.
One of my workmates did this while working on a 277V lighting circuit. The lights on the floor went out, his elbow touched the ceiling grid, the lights came back on for a second, he hollered, "OUCH, #$&@!", the lights went out. "I thought a neutral didn't shock you", he said.
iceworm
Curmudgeon still using printed IEEE Color Books
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I've been struggling with this for a lot of years. I can't find any diffinitive answers.
In addition to that already posted, consider, on our side on the meter, one and only one NG bond is safe and legal.
On the utility side of the meter, grounding the neutral often is safe and legal.
Only one of many things that baffle me today.
ice
electrofelon
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- Cherry Valley NY, Seattle, WA
Granted the chance of a shock or fire from a properly neutral grounded enclosure is quite low, however keeping neutral current off of raceways and enclosures is safer - I dont think anyone can debate that. Utilities dont have many non current carrying metal parts that are accessible to other than highly trained people. Further most utility installations are outside where there is much less fire risk. Finally, Im sure the NEC would like to see a 4 wire service drop, but that is too big for them to tackle
If I am to understand you correctly what you are referring to is to have stall the subpnl as you would the SE with the neutral and ground bonded together at both panels by only running aa single grounded neutral conductor.
Remember that the neutral is a current carrying conductor and when you use it as a ground for the subpanel that neutral current then becomes available on all of the ground conductors at the subpanel and and device that relies on the ground conductor as a ground.
As you may recall the old 3w method of wiring electric stoves and driers used a single grounded neutral conductor as a combined duty in that it provided a return path for neutral current as well as it is also bonded to the enclosure to ground the enclosure. Such installations today are not code complient today as the wiring must consist of an insulated and isolated neutral conductor and a separate equipment grounding conductor.
When equipment relies on a grounded neutral as a ground should the connection of the neutral back to the sourse beome compromised in anyway the neutral current will seek another path back to the sorce via and anything that uses the neutral for a ground which presents a shock hazard.
Fast__Eddie
Member
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- So. FLA
Take a look at the Soars Book of Grounding, this may help answer you questions.
iceworm
Curmudgeon still using printed IEEE Color Books
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- North of the 65 parallel
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- EE (Field - as little design as possible)
Granted the chance of a shock or fire from a properly neutral grounded enclosure is quite low, however keeping neutral current off of raceways and enclosures is safer - I dont think anyone can debate that. Utilities dont have many non current carrying metal parts that are accessible to other than highly trained people. Further most utility installations are outside where there is much less fire risk. Finally, Im sure the NEC would like to see a 4 wire service drop, but that is too big for them to tackle
Absolutely. I agree. I just haven't nailed down the physics that makes this true.
There are a bunch of merlin-gerin (sp?) papers covering this subject (or skirting it maybe) I'll find the links. Probably time for me to figure this out. Although if I stall for a few more years - I may not have to.
ice
jumper
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1) You get red tagged.
2) You don't get paid.
3) Someone gets shocked and you get fired.
4) See above.
It is somewhat humbling when one considers the minute amount of current that can cause a lethal electric shock. When an EGC is exposed to a neutral current that neutral current can be a source of asnd electric shock. Considering that a GFCI has a 5,ma sensitivity it wouldn't take much to make 5ma available on an EGC by a person coming into contact between a device that is grounded with an EGC and a actual ground such as a faucet or copper water pipe, bare feet on a wet floor or pool of water..
winnie
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- Springfield, MA, USA
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- Electric motor research
On this question you will not get a black and white answer because it is one of those areas where you can trade off safety for cost.
It is entirely reasonable to feed a circuit using a single 'hot' circuit conductor and any available bonded metal. This is a common approach used in automobiles (12V circuits), and my understanding is that some World War 2 era wiring in Europe used a single 'hot' conductor and the steel conduit.
If your bonded metal path opens, then you have a severe shock hazard, and in normal operation you have a slight shock hazard because of voltage drop on your bonded metal.
For building wiring, it is clearly _safer_ to separate your bonded metal (egc, conduit, etc) from your grounded current carrying conductor, because open connections won't automatically put dangerous voltage on exposed metal. IMHO this is not 'safe versus unsafe', because the 'combined neutral and ground' installation is safe enough when nothing goes wrong. But the separate ground and neutral is much safer when something does go wrong.
For outside power distribution wiring, you have the additional factor of trying to protect systems from lightning imposed voltages and from downed HV wires. I believe that the combined neutral and grounded conductor provides a safety benefit versus these 'high voltage insults', but if you look at reports on 'stray voltage' it is clear that POCO combined ground and neutral does present problems.
The physics won't give you a definitive answer because this is not an absolute issue. Rather the physics will only tell you the magnitude of the voltages and currents that you may be dealing with in any particular situation.
-Jon
It is entirely reasonable to feed a circuit using a single 'hot' circuit conductor and any available bonded metal. This is a common approach used in automobiles (12V circuits), and my understanding is that some World War 2 era wiring in Europe used a single 'hot' conductor and the steel conduit.
If your bonded metal path opens, then you have a severe shock hazard, and in normal operation you have a slight shock hazard because of voltage drop on your bonded metal.
For building wiring, it is clearly _safer_ to separate your bonded metal (egc, conduit, etc) from your grounded current carrying conductor, because open connections won't automatically put dangerous voltage on exposed metal. IMHO this is not 'safe versus unsafe', because the 'combined neutral and ground' installation is safe enough when nothing goes wrong. But the separate ground and neutral is much safer when something does go wrong.
For outside power distribution wiring, you have the additional factor of trying to protect systems from lightning imposed voltages and from downed HV wires. I believe that the combined neutral and grounded conductor provides a safety benefit versus these 'high voltage insults', but if you look at reports on 'stray voltage' it is clear that POCO combined ground and neutral does present problems.
The physics won't give you a definitive answer because this is not an absolute issue. Rather the physics will only tell you the magnitude of the voltages and currents that you may be dealing with in any particular situation.
-Jon
- Location
- Tennessee NEC:2017
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- Semi-Retired Electrician
Yes, and along with that, you wouldn't have to lose a neutral to be shocked. Current will flow on both the neutral and any metal it is connected to. So if you touched the EMT and something else that is grounded it will shock you. That is if the neutral and ground were bonded again at the subpanel.
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- Licensed Electrician
Assuming that you are now talking about a set up where the neutral is isolated isolated at the sub panel then it is safer. With the neutral isolated from at the sub panel the EMT is a non-current carrying part of the the electrical system. Loose the the neutral and all you have is an open circuit.
Now if you have a bonded neutral at the service and at the sub panel with EMT between them then the EMT and the neutral share the job of carrying current. That's not good. Loose the real neutral and the EMT becomes the neutral. That's even worse. The idea is to keep all the current on the insulated wires and off of the exposed, uninsulated metal.
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- Licensed Electrician
The post that winnie made holds true again in your question here. There is no direct yes or no answers, but the short answer is you are less likely to get shock at the service panel no matter what the condition of the neutral because the connection to earth is right there and you have the requirements for all the metal raceways to be connected with more than a simple lock nut.
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