enireh
Senior Member
- Location
- Canyon Lake,TX
can you explain the theory on why separate grounds & neutrals at the sub panel but not at the main panel where the meter is?
ground neutral
- Thread starter enireh
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MattS87
Senior Member
- Location
- Yakima, WA
Someone can probably do a much better job at explaining, but it is to not allow a parallel path to ground. For example, 120V loads have current flowing through the nuetral. If you have all the nuetrals/grounds tied together, you now give the current a second option on where to return to the source (metal conduit/boxes) which causes shock potential. By floating your nuetral, all current must travel back to the main panel via the grounded conductor.
- Occupation
- Licensed Electrician
Have you ever been shocked by an open neutral? Think about that happening all over metal parts of the electrical system because of neutral current flowing on them because of downstream neutral to ground bonds.
Even though the equipment grounds are landed at the same spot as neutral where power comes into the building they are not doing the same thing. One is carrying normal current the other should only carry fault current.
- Location
- Illinois
- Occupation
- retired electrician
That only happens if the actual neutral conductor has a very high impedance or is open. If the neutral is in good shape, the only voltage available to drive a shock between two sections of a raceway is the voltage drop on the neutral conductor.
Carultch
Senior Member
- Location
- Massachusetts
The current under normal circumstances should be travelling in wires. Not in conduit walls, enclosure cans, or building steel.
Neutral exists to carry the current under normal circumstances, where it is applicable to have return current on a single phase-to-neutral circuit, or imbalanced current on an all-phase circuit.
Grounding conductors exist to carry fault current, when unintended continuity arises between a live wire and a metal part that isn't intended to carry current. Grounding conductors are bonded to all non-current-carrying metal throughout the system.
If you bond N-G in multiple places, then the normal circumstance current isn't exclusive to the neutral as it should be. EGCs are not sized to carry the full load continuously, and neither are other metal parts in the eqpt grounding system.
K8MHZ
Senior Member
- Occupation
- Electrician
I still don't see the question as being answered. OP wants to know why 3 wire 120/240 is OK on the line side of the service, but 4 wire is required on the load side (where a sub-panel would be fed from), if I understand the question correctly.
IOW, why is a mult-grounded neutral acceptable on the POCO side, but not on the 'customer' side?
IOW, why is a mult-grounded neutral acceptable on the POCO side, but not on the 'customer' side?
Carultch
Senior Member
- Location
- Massachusetts
Other than that the utility is not governed by the NEC, there really isn't a reason.
If the rule were applied consistent with the electrical theory to both the POCO and customers, the neutral-to-ground bond would only occur at each transformer secondary, and at the main generators.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
How do you figure that? If the neutral and ECG are the same size the voltage on them would be the same at the point of contact. If the unbalanced return current on the neutral is high enough the voltage drop can be enough so that you can certainly get a shock from it out on a branch.
- Location
- Illinois
- Occupation
- retired electrician
If the voltage drop on the grounded conductor is high enough to get a shock from, the circuit is not likely functioning correctly.
It remains my opinion that if the neutral has been installed per the code rules, and it and its connections have not been compromised, it is unlikely that the voltage drop would rise to the level of a shock hazard.
rlundsrud
Senior Member
- Location
- chicago, il, USA
It isn't just a shock hazard, it is also a fire hazard. Current will flow through the ground if the neutral is bonded at multiple points. This can cause a small arc to form at poorly linked points in the ground system like at a loose connector. The arc once formed can exist for a significant time before reaching a high enough temperature to combust any flammable materials that might exist. This is occurrence is rare, but has been documented.
If I understand the question correctly, bonding the ground and neutral at both (two) locations would be analogous to running parallel neutrals. You would be energizing the ground cable connected in between the two points.
If they are only bonded together at the main panel, there is no paralleling effect back up the pole because the neutral is the only return path up the pole.
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- Location
- Lockport, IL
- Occupation
- Semi-Retired Electrical Engineer
I agree with Don (happens sometimes :happyyes: ). The vast majority of the voltage drop takes place within the load. There is very little VD along the ungrounded conductor from the source to the load, or along the grounded conductor from the load back to the source. Even if the current is high, the resistance of the wire is very low, so the VD will be low.
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
I see that and stand corrected. I ran some Vd calcs and saw something like 4V on the neutral out on the end of a fully loaded 100' 20A branch.
But audio equipment usually ties the shield to the EGC. If there is 60Hz 4VAC on the shield it seems to me that would screw with high gain unbalanced audio circuits.
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