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Electrical Sub-Panels – Why MUST the Neutrals and Grounds be Separated

As home inspectors we know that the grounds and neutrals must be separated in any sub panel. We also know that a sub panel should be supplied from another electric panel with 4 conductors. I have had “electricians” tell me that that code is overkill and unnecessary. I have heard different explanations on the reasons for this. I have also heard “electricians” say the neutral does not carry current. CLICK HERE for a great video showing a live demonstration. Let’s start with that neutral wire, which is actually a current carrying conductor.  Because the neutral conductor appears to have no voltage because there is no resistance. However if a person becomes part of the circuit, the voltage will form across their body. In a electrical circuit all of the current from the hot conductors will travel on the neutral conductors back to the service panel. If we bond the ground wire to the neutral in the sub panel, current will flow on both the neutral and ground. If the ground and neutral conductors are not kept separated, current will flow on the ground wires back to the panel supplying the sub panel. In addition to the dangers (shock hazards) of current flowing on the ground conductors, it could cause equipment interference, cause issues with GFCI’s. Another issues is magnetic fields not cancelling themselves out because the return current has multiples paths, the current remaining in the neutral will not counterbalance the current in the hot wire.  The resulting imbalance creates a magnetic field that can interfere with sensitive electrical equipment. If metal conduit is included, the imbalance will induce current into the conduit, and could cause the conduit to overheat.  According to the NEC (National Electric Code), you can only have one connection between the ground system and the neutral system, and that must be at the Main Service Panel. This is also important for the proper operation of Over Current Protection Devices.

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