Another 120% busbar question

[Jeff S]

I think it does make sense that a disconnect have an equal or greater rating than the conductors connected to it.

Jaggedben,
It would seem that the logic of that falls apart when you have upsized the wires due to long runs and voltage drop. The voltage drop within the disconnect will be negligible if sized for the actual current X 1.25.

Jeff

 

[jaggedben]

First of all, I think this is definitely an area where the code is genuinely ambiguous. The code doesn't even define 'conductor' clearly enough to say that the conductive parts of a disconnect aren't conductors.

With that said, to me the key to your question is that you mentioned an unfused disconnect. If it were a fused disconnect, then the application of the 120% rule would be separately calculated for the conductors on either side of that disconnect, depending on the other overcurrent devices at the other ends of those conductors. But for an unfused disconnect, the same 120% calculation would apply to the conductors on both sides of the disconnect. It simply seems illogical that it wouldn't also apply to the disconnect itself.

If you are oversizing conductors for voltage drop and not the 120% rule that is really a different situation.

 

[Jeff S]

Thanks for your input on this. Apparently there are plenty of gray areas in the code to sustain forums for a long time. As a co-author of city planning and zoning codes for many years, I can vouch for the fact that it is very difficult to think of every situation when writing codes. Obviously that's why the NEC needs continuing revisions.

Jeff

 

[Marvin_Hamon]

Marvin,
Are you saying then that in my example, if the wire fits, you would use a 30A disconnect?

If a 30A disco is sized for the inverter current and it is listed for the conductor size you are good. There is nothing that I know of that requires the ampacity of the disco to match the ampacity of the conductors terminated at it. If there were a requirement like this it would cause all kinds of problems when dealing with voltage drop issues, which for the most part the NEC does not consider.

Some people feel that the minimum size for the first disconnect after a supply side interconnection has to be 60A due to the 60A minimum in 230. This may also effect your choice of disco rating.

 

[Marvin_Hamon]

First of all, I think this is definitely an area where the code is genuinely ambiguous. The code doesn't even define 'conductor' clearly enough to say that the conductive parts of a disconnect aren't conductors.

With that said, to me the key to your question is that you mentioned an unfused disconnect. If it were a fused disconnect, then the application of the 120% rule would be separately calculated for the conductors on either side of that disconnect, depending on the other overcurrent devices at the other ends of those conductors. But for an unfused disconnect, the same 120% calculation would apply to the conductors on both sides of the disconnect. It simply seems illogical that it wouldn't also apply to the disconnect itself.

If you are oversizing conductors for voltage drop and not the 120% rule that is really a different situation.

I see it as different because while you can tap a conductor someone would be hard pressed to put an internal tap in a disconnect. Therefore disconnects don't fall under the 120% rule.

 

[jaggedben]

I see it as different because while you can tap a conductor someone would be hard pressed to put an internal tap in a disconnect. Therefore disconnects don't fall under the 120% rule.

I don't really follow this line of reasoning for unfused disconnects. If there is a reason that it is dangerous for a conductor that is backfed to have an ampacity less than 5/6ths of the OCPDs that protect it at either end, then I take it that such a danger exists at all points between those OCPD's. An unfused disconnect is just another point in between those OCPDs.

Now if it's a fused disconnect, then the disconnect is at the point of the OCPD, not in between two OCPDs, so there's no reason that the rating of the disconnect has to exceed the rating of the fuses.

 

[Marvin_Hamon]

I don't really follow this line of reasoning for unfused disconnects. If there is a reason that it is dangerous for a conductor that is backfed to have an ampacity less than 5/6ths of the OCPDs that protect it at either end, then I take it that such a danger exists at all points between those OCPD's. An unfused disconnect is just another point in between those OCPDs.

Now if it's a fused disconnect, then the disconnect is at the point of the OCPD, not in between two OCPDs, so there's no reason that the rating of the disconnect has to exceed the rating of the fuses.

I don't know what else to say other than to go back to the reason the 120% rule is applied to a conductor, it's to protect a conductor if it is tapped. It is not to protect an untapped conductor since the current through the conductor will never be greater than the larger of the two OCPDs on either end. You can't tap a disconnect so it does not apply.

To gain a full understanding you have to go and read all the NEC CMP comments on 690.64(B) for the last 5 or so code cycles. They explain their reasoning in their comments and it provides the background and reasoning that does not make it into the final code.