On a four wire, automatic transfer switch that is fed from an ocpd that already has a main bonding jumper properly installed, is it required to again bond the neutral on the load side of the transfer switch? This would seem to be a code violation to me, however I was recently told it was required in this application.
main bonding jumper at ATS
- Thread starter bullryder
- Start date
- Status
- Location
- Massachusetts
If you switch the neutral you must bond the generator output somewhere between the genset and the transfer switch so that there is never more than one bond actually connected at a time.
hillbilly1
Senior Member
- Location
- North Georgia mountains
- Occupation
- Owner/electrical contractor
I agree with iwire, that is the correct way to do it, though I had a state inspector require that the bond be on the load side of a service rated groundfault protected transfer switch (four pole also). Could not get him to understand why it would defeat the ground fault protection!
erickench
Senior Member
- Location
- Brooklyn, NY
The ATS should have two ground fault current paths. One for each source. If you bond the neutral to the ATS enclosure you will have objectionable current on either of these EGCs.
- Location
- Massachusetts
This may help, with a switched neutral bonding is required in two places but only one is actually connected at any given time.
erickench
Senior Member
- Location
- Brooklyn, NY
Lets assume that the neutrals are tied together. What would happen if you had fault current flowing through the neutral from one source to the other? This is what would happen if you only had one main bonding jumper for both systems. Maybe it's better to have two main bonding jumpers?
Last edited:
erickench
Senior Member
- Location
- Brooklyn, NY
Okay it suddenly dawned on me what this inspector wants. If there are no main bonding jumpers at both services and only one main bonding jumper at the ATS, then the fault current will not loop through one service and then on to the other. Also, there would be no parallel paths for the fault current to flow through. All the fault current would flow through the neutral to whichever service is supplying power.
Last edited:
hillbilly1
Senior Member
- Location
- North Georgia mountains
- Occupation
- Owner/electrical contractor
That is what the fourth pole is for, to prevent the current loop, that is why the service is bonded at the line side, with the generator bonded on it's line side. As the drawing that iwire posted shows. If you bond only once on the load side of the transfer switch, any ground faults will return to the neutral and pass through the ground fault CT, and not be seen as a fault. If bonded correctly on the line side, the fault will go around the CT, and will be sensed by the ground fault circuitry.
- Location
- Massachusetts
Here are some drawings from a past member named Ed MacLaren .
I saved many of his drawings as they helped me a great deal with many electrical discussions.
I saved many of his drawings as they helped me a great deal with many electrical discussions.
erickench
Senior Member
- Location
- Brooklyn, NY
I'm a little confused. A circuit breaker would only sense the overcurrent flowing through the ungrounded conductors. The only way a protection system would sense the neutral current is if you were using a phase overcurrent relay that senses the imbalance the same way that a GFCI would sense it. Is that what we have here?
- Status