4 wire vs. 3 wire safety

[JoeNorm]

Is a 4 wire feeder inherently more safe that a 3 wire in normal conditions? Assume wiring is attached to an OCPD.

If so can you please explain how?

 

[winnie]

I assume you are talking about single phase circuits with 2 'hots', 1 'neutral' and bonding via EGC (4 wire) or bonding via neutral (3 wire).

In _normal_ conditions, a 4 wire feeder is _slightly_ safer than a 3 wire feeder. In normal conditions, current will be flowing on the neutral conductor, which will result in a neutral-earth voltage due to voltage drop. This voltage will be experienced by anyone touching both bonded metal and earth or earthed conductors.

In normal conditions this voltage will be small. This is exactly the sort of elevated neutral to earth voltage which many will find acceptable in utility power distribution systems. But this voltage is _not_ zero and can be problematic.

-Jonathan

 

[junkhound]

Dryer circuit example: 24 A, 10 AWG.
120V return current say about 3 A on neutral.
3 wire: 3% voltage drop of 3.6 V, so R = 3.6/24 round trip = 150 mohms; so 75 mohm neutral EGC, volt drop = volts on cabinet = 3*.075= about 1/4 volt. 4th wire with isolated neutral cabinet = near zero volts.
Wet person say 1 kohm, current = e/r = .023/1k = still well below GFCI trip.
No problem.

Biggest marginal improvement in safety is due to redundancy in case of double failure (short of case to line plus open neutral -which is grounded to case in 3 wire system), case would be 60V or less with separate ECG vs. 120V touch hazard.

 

[Tulsa Electrician]

Is a 4 wire feeder inherently more safe that a 3 wire in normal conditions? Assume wiring is attached to an OCPD.

If so can you please explain how?

This question could a few different directions. How about giving us an example to work with.

If both are done in compliance with the NEC they we can look at each under there own merits.

An example:
A three wire feeder sized correctly for a load is safer than an under sized four wire feeder for the same load.

In the example above the three wire feeder could be three phase three wire load using the EMT as the EGC.

Or a three phase three wire load with a wire type EGC.

So the answer to the question would be. The proper sized three wire feeder using EMT as an EGC running under normal conditions.

The under sized four wire feeder could cause conductor damage with the load operating under normal conditions.

When I say under sized it could be wire size for the feeder based on the load and calculated per NEC.

So make something up and we will run with it.

 

[ptonsparky]

This question could a few different directions. How about giving us an example to work with.

If both are done in compliance with the NEC they we can look at each under there own merits.

An example:
A three wire feeder sized correctly for a load is safer than an under sized four wire feeder for the same load.

In the example above the three wire feeder could be three phase three wire load using the EMT as the EGC.

Or a three phase three wire load with a wire type EGC.

So the answer to the question would be. The proper sized three wire feeder using EMT as an EGC running under normal conditions.

The under sized four wire feeder could cause conductor damage with the load operating under normal conditions.

When I say under sized it could be wire size for the feeder based on the load and calculated per NEC.

So make something up and we will run with it.

Yes, he needs to make something up. My image was different than yours.

 

[petersonra]

A three wire feeder sized correctly for a load is safer than an under sized four wire feeder for the same load

The only reason it might be less safe is if the ocpd was under sized. The proper application of ocpd is what provides safety, not how many wires.

It does not make an iota of difference what the load is, the wire size is, or the number of wires. As long as the wires that are there are properly protected by ocpd, they are equally safe.

 

[kwired]

If using the grounded (often a neutral) conductor for equipment grounding as well, then you are raising the voltage to true ground on objects connected to said EGC by however much voltage drop you have on the feeder grounded conductor as well as giving it an opportunity to seek other parallel pathways back to the source if they are present.

Resulting stray voltages likely remain rather harmless most the time, but when some failure occurs is when they become more dangerous. A failed EGC can have same dangers but a properly done EGC shouldn't be subject to carrying current or a rise in voltage associated with carrying current during normal operations either.