Non Separately Derived Emergency Generator Neutral

[dnrawson]

I had installed a non separately derived generator setup which plugs into a 50amp plug with interlock to my 200amp main panel. My generator has been unbonded for use with this setup and maintains a floating neutral (neutral is not switched with a third pole). Not applicable in my case, but curiosity raised the question of why a 4th conductor for neutral was really required between the generator and panel since it's not bonded at the generator. Can anyone explain the need for this conductor if the system uses the homes neutral/ground setup? Thank you!!!

 

[Little Bill]

I am allowing this as only a technical question is being asked. Please, no "how to " answers, just answer the question as asked.

 

[wwhitney]

uriosity raised the question of why a 4th conductor for neutral was really required between the generator and panel

Current always returns to the source of the voltage difference that is driving the current. When on utility power, that source is the utility. When on generator power, that source is the generator. So you need a neutral conductor to the generator so that the generator can power 120V line-neutral loads. That neutral conductor to the generator will carry whatever unbalance you have in the current on the two line conductors.

In other words, your question, as you've framed it, is equivalent to the question of "why do we need a neutral conductor from the utility?" As that seems not to be something you've questioned, I suspect that you have some related misapprehension. E.g., the myth that current returns to ground--this is only true to the extent that one of the source conductors is connected to ground, and that ground therefore becomes a path back to the source.

Cheers, Wayne

 

[infinity]

Not applicable in my case, but curiosity raised the question of why a 4th conductor for neutral was really required between the generator and panel since it's not bonded at the generator.

The fourth conductor would be the EGC which is used to bond the metal parts of the generator.

 

[dnrawson]

Thanks for the willingness to accept this post and explain. I overlooked the fact 120v loads need a path back to source and not just ground/neutral bus in the panel. Quite simple now that I think about it. I've since noticed these beginner type questions are frowned upon here and the forum is designated mostly for contractors, electrical engineers etc. I am a certified ISO category III Vibration Analyst. In my career, I diagnose industrial electrical motor faults via vibration analysis and occasionally supplement with motor current signature analysis (with support of E&I) to verify findings in vibe data so I do indirectly work in the field, but in a very specialized area. Thanks again

 

[__dan]

Not applicable in my case, but curiosity raised the question of why a 4th conductor for neutral was really required between the generator and panel since it's not bonded at the generator. Can anyone explain the need for this conductor if the system uses the homes neutral/ground setup? Thank you!!!

Two reasons. The neutral is a fully insulated (grounded) current carrying conductor for your regular 120 V loads as noted above. It's a regular insulated load conductor.

The non separately derived neutral is also fault carrying for shorts to ground via the main bonding jumper in the service panel. It's an essential part of the protective system

Shorts to ground are picked up by the equipment ground, then to the service panel main bonding jumper, then back to the generator on the neutral (the neutral is referenced to ground at the MBJ). The low impedance of the entire short circuit path creates a sufficiently large fault current, necessary for fast clearing, opening of the supply breaker.

Without the neutral reference to ground, the solid bolted connection, the generator supply would be floating and shorts to ground could happen invisibly and unhindered.