RD35
Something has been bothering me for a long time now on residential 1P3W 120/240V services and I thought I would post it up and see what you all think.  There is, what seems to be, a huge market right now for breaker-interlock transfer switch set-ups.  This is due to their easy installation and low cost.  I also see lots of 2-pole transfer switches and these little generator panels that have double throw switches in them being installed for stand-by power.  In all of these cases the installation is an non-separately derived system.   In almost every case, the system gets installed with the generator connection being a 4-wire SO cord and an L14-20 or L14-30 plug (depending on the generator receptacle).  Every one of these generators that I have seen is bonded internally (neutral to ground).  So, when the homeowner plugs his transfer switch into the generator receptacle outlet does this not create a code violation since part of the return current that normally travels on the grounded conductor is now traveling on the green grounding conductor wire as well?  Thoughts?
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Birken Vogt
Technically it is a code violation but usually the green conductor is the same gauge as the white and they both land the same place so nothing to overheat.
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Brian Baughman
RD35 wrote:
Something has been bothering me for a long time now on residential 1P3W 120/240V services and I thought I would post it up and see what you all think.  There is, what seems to be, a huge market right now for breaker-interlock transfer switch set-ups.  This is due to their easy installation and low cost.  I also see lots of 2-pole transfer switches and these little generator panels that have double throw switches in them being installed for stand-by power.  In all of these cases the installation is an non-separately derived system.   In almost every case, the system gets installed with the generator connection being a 4-wire SO cord and an L14-20 or L14-30 plug (depending on the generator receptacle).  Every one of these generators that I have seen is bonded internally (neutral to ground).  So, when the homeowner plugs his transfer switch into the generator receptacle outlet does this not create a code violation since part of the return current that normally travels on the grounded conductor is now traveling on the green grounding conductor wire as well?  Thoughts?


You are correct, it is a code violation as an objectionable current path will be created, and the generator frame could become energized as well during use.  702.7(C) in the NEC requires that the power inlet box be labeled for the type of portable generator connection, separate or nonseparately derived.  

2 pole MTS's are only legally installed where a floating neutral portable generator is connected to the premise wiring system.  If the portable generator has a bonded neutral, the only legal installation to a premise wiring system is through a 3 pole MTS, like the Generac Homelink MTS.  In addition the NEC requires that a grounding electrode system be connected to the generator frame to reestablish the grounded conductor(neutral) at the generator.  This is why there are grounding lugs on portable generators.
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RD35
Thank-you Birken and Brian!  Now I’m going to check both of my portable generators to see if they have removable bonding jumpers....just for the sake of my own curiosity!  I have 3-pole transfer switches at my own place just so that I can use my portables as-is with no modifications.  

So, while on this subject, if a person has an optional standby system that is non-separately derived....is there a legal way to connect a bonded portable generator to it...like maybe omitting the grounding conductor from the power cord....or not connecting the grounding conductor at some point?
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Cobranut
RD35 wrote:
Thank-you Birken and Brian!  Now I’m going to check both of my portable generators to see if they have removable bonding jumpers....just for the sake of my own curiosity!  I have 3-pole transfer switches at my own place just so that I can use my portables as-is with no modifications.  

So, while on this subject, if a person has an optional standby system that is non-separately derived....is there a legal way to connect a bonded portable generator to it...like maybe omitting the grounding conductor from the power cord....or not connecting the grounding conductor at some point?


Only way that is legal and safe is to remove the bonding jumper from the genset and use a 4-conducter cable. 
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