Residential Wiring Offset Voltage

Does the neutral line in North American electrical utilities have a specified tolerance specified what voltage it can float above ground?

NEC 210.19 has a limit of 5% total voltage drop for sizing conductors on branch circuits. 5% of 120VAC is 6V or 3V then for neutral. At the load (outlet), neutral goes above ground because it's 1/2 way in series with the cable run. Example 10A load 0.2R of (black) conductor resistance and 0.2R neutral (white) you would see a 2V difference to earth ground, at the outlet.

Neutral and earth ground are bonded at the service, so at that point no difference in potential. Even though you might have a poor, high resistance ground for the building. Your house incoming neutral connects to all homes on the secondary of the distribution transformer.

Max mains voltage can be 126V of which 5% is 6.3V which seems the neutral can float to 6.3V above ground.

The question can be asked in two regards.
- What's the maximum voltage expected in normal operation?
- What's the maximum voltage to be considered for safe circuit design, e.g. insulation strength?

As for the second question, the answer is: full mains voltage multiply overvoltage category factor. Safety design must not rely on correct neutral grounding.

Zak28 said:

Max mains voltage can be 126V of which 5% is 6.3V which seems the neutral can float to 6.3V above ground.

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It's specified as a total voltage drop, for both of the hot and neutral conductors, from the point of view of efficiency. You don't want large voltage drops due to thin wire. Residential mains cable is two conductors plus ground.

With your numbers, 126VAC mains -5% gives 119.7VAC at the appliance, for 6.3VAC lost and half that is in the neutral conductor, so 3.15VAC is what would be expected. This assumes the load is a constant resistance, as motors and SMPS etc. will use less current at 126VAC than 117VAC. Note that for a 10A load the cable is dissipating 63W for the run, about 125 ft./38m run of NMD90 14ga.

In practice, neutral has a lot of mains hum and RF noise. Just put your multimeter on ACV and measure N-GND at an outlet. With no load, it should be zero but never is. Surge-protected powerbars have a MOV between N-GND which has never made much sense to me.

You design electronics assuming Line and Neutral are reversed, that either or both can be hazardous live.
No cheating connecting a MCU to Neutral and assuming that is close to GND. This is unsafe practice that can arise when making an energy metering design or phase-controller etc. You have to assume the MCU and circuitry (switches, pots, LED's) are hazardous live and have appropriate insulation for the human touching them. This is required and tested in the electrical safety standards.

Is there any method to null out any floating voltage on the floating neutral in the industrial or residential mains but without any transformers?

Shouldn't the powerbars have a pair of MOVs 1 across line to ground and the other for neutral to ground?

Electrical codes require neutral to be bonded to earth ground, after any (distribution) transformer and at the service entrance, breaker panel. A distribution transformer is step-down from say 15kV to 240VAC.
Insulation breakdown, leakage currents in the transformer windings, could raise the secondary 240VAC up to 15kV if the secondary side was allowed to float. This is unsafe and brings the Code requirement to ground a transformer's secondary as a neutral.

I'm not sure how any neutral voltage is causing problems for you. What is your circuit?
If you are doing voltage sense, the divider resistors are between line and neutral if your circuit is tied to neutral. A potential transformer PT or instrumentation amp is used otherwise.

Industrial electrical distribution is 3-phase and neutral is approached differently there. In some Delta configurations there is no neutral connection.

Surge-protected powerbars usually have one or three MOV's. Line-Neutral (most important), Line-Ground, Neutral-Ground.

The trouble is 3v offset at neutral can set off Vgs its fine to just use earth conduit for fullwave?

Rest of circuit is omitted.

FvM said:

TAs for the second question, the answer is: full mains voltage multiply overvoltage category factor. Safety design must not rely on correct neutral grounding.

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Neutral is grounded at utility posts in North Amirica not residential breaker panels otherwise there would be lots of ground currents.

If safety doesn't rely on proper outlet connections then why bother with differently shaped prongs having different blade widths?

Zak28 said:

The trouble is 3v offset at neutral can set off Vgs its fine to just use earth conduit for fullwave?

Rest of circuit is omitted.

Neutral is grounded at utility posts in North Amirica not residential breaker panels otherwise there would be lots of ground currents.

If safety doesn't rely on proper outlet connections then why bother with differently shaped prongs having different blade widths?

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Are you sure about that? I have a 8' copper ground rod that is connected at service on side my house. I believe in some installation they connect to rebar through foundation.

If safety doesn't rely on proper outlet connections then why bother with differently shaped prongs having different blade widths?

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No, safety must not rely on polarized connectors and correct connection of neutral and phase. Safety relies on safe isolation of mains circuit. Polarized connectors can at best improve safety.

Important related to this thread, grounded neutral and polarized connectors don't imply that it's legal to carry operational current through protective earth terminal.

Several safety rules are strictly against it. E.g.
- Equipment is required to pass a Hipot test PE against N and L
- Leakage current through PE must be limited to a few mA

FvM said:

No, safety must not rely on polarized connectors and correct connection of neutral and phase. Safety relies on safe isolation of mains circuit. Polarized connectors can at best improve safety.

Important related to this thread, grounded neutral and polarized connectors don't imply that it's legal to carry operational current through protective earth terminal.

Several safety rules are strictly against it. E.g.
- Equipment is required to pass a Hipot test PE against N and L
- Leakage current through PE must be limited to a few mA

Click to expand...

But this is only because the industry really likes to isolate mains conduits from the rest of the circuit using transformers. Nothing is really mains powered, its transformer powered. Transformers reduce available power, increase heat and reduce available space.