Tap rules

[ggunn]

We are contemplating connecting a three phase PV system with maximum output current of 153A to the load side of an 800A disco. This is governed by 2017 NEC 705.12(B)(2), which in turn evokes 240.21(B)(2), as our tap conductors are between 10 and 25 feet in length.

My problem is with the way 705.12 interacts with 240.21. 705.12(B)(2) says "...any taps shall be sized based on the sum of 125% of the power source(s) output circuit current and the rating of the overcurrent device protecting the feeder conductors as calculated in 240.21(B)." Parsing that sentence in different ways yields different results.

Does all this mean that our tap conductors must have ampacity equal to 1/3 of 800A plus (1.25)(153A)? That doesn't make a whole lot of sense to me, since if the tap were a load the conductors would only need to be rated for 800A/3, and any current injected into the feeder through the tap would serve to reduce, not increase, the current in the tap conductors.

 

[synchro]

If it were another tap on this feeder, then adding 125% of the inverter output current to the feeder's OCPD rating in the tap calculation certainly makes sense because it adds to the total current available for loads. Unfortunately, the wording is "any taps" so I don't think you can make any exception for the one connected to the inverter.

 

[jaggedben]

...

Does all this mean that our tap conductors must have ampacity equal to 1/3 of 800A plus (1.25)(153A)?...

I believe this is indeed what they intended. I tried to get this section revised to make this more clear and grammatically sensible, but to no avail.

I wouldn't expect it to make much sense from a physics standpoint, but for what it's worth... The tap rules are awfully arbitrary to begin with. My understanding is that putatively the intent is to make sure the conductor provides little enough resistance to trip an oversized upstream breaker. If there's another source on the feeder, then the conductor would need to be bigger to ensure that enough current is still drawn from the utility. I think this is the same as when you say...

...any current injected into the feeder through the tap would serve to reduce, not increase, the current in the tap conductors.

Since we are trying to ensure a breaker trips, reduced current is a bad thing.

In most cases, as we both know, the inverter's affect on fault current is in fact probably negligible enough to be ignored. But I suppose in extreme cases that might not be.

 

[ggunn]

Since we are trying to ensure a breaker trips, reduced current is a bad thing.

In most cases, as we both know, the inverter's affect on fault current is in fact probably negligible enough to be ignored. But I suppose in extreme cases that might not be.

Actually, I was thinking of operating current, not fault current. IMO, a faulted condition that doesn't shut down the inverter(s) is a very unlikely occurrence.

 

[pv_n00b]

It's easy to misread 705.12(B)(2) as applying to the PV interconnection tap conductor, but it applies to feeder taps downstream of the PV interconnection tap and describes how the tap rules have to take into account the feeder OCPD and the contribution of the PV current.

NEC 240.21 is written for adding load taps and does a poor job when applied to generator taps such as PV interconnections. It's focused on protecting conductors from overload by putting OCPD on the load end of the conductor and accepting that the conductor has little or no protection from faults. It does fault protection by limiting the size and length of the tap conductor and protecting it from physical damage to limit the chance of a fault. None of this is as applicable to a current limited generator. But we have to play the NEC way on this and this is what we have to deal with.

Now if I can find a way to keep people from tapping a tap when they tap the unprotected secondary of an existing transformer I would be happy.

 

[jaggedben]

It's easy to misread 705.12(B)(2) as applying to the PV interconnection tap conductor, but it applies to feeder taps downstream of the PV interconnection tap and describes how the tap rules have to take into account the feeder OCPD and the contribution of the PV current.
...
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I disagree. I think it applies to any taps on a feeder that is fed by more than one source.

The requirements in 240.21(B) have essentially nothing to do with the operating current of the circuit or which direction it flows during normal operation. If the purpose of requiring 10percent or 33percent ampacity on the tap conductor is to increase the chance of the oversized upstream OCPD tripping during a fault, it applies to source taps and load taps equally. The language makes no distinction and in fact says 'any taps.'

 

[pv_n00b]

I disagree. I think it applies to any taps on a feeder that is fed by more than one source.

The requirements in 240.21(B) have essentially nothing to do with the operating current of the circuit or which direction it flows during normal operation. If the purpose of requiring 10percent or 33percent ampacity on the tap conductor is to increase the chance of the oversized upstream OCPD tripping during a fault, it applies to source taps and load taps equally. The language makes no distinction and in fact says 'any taps.'

NEC 240.21 is written assuming a single source of supply to a larger conductor where that conductor is protected by an OCPD, and then a number of smaller load conductors that will have reduced protection are tapped off that larger conductor. It requires OCPD at the load end of those smaller conductors to prevent the smaller conductor from being overloaded by the operating current or a fault after the load end OCPD. Not all of the sections of 240.21 size the conductors so that they will be protected by the OCPD on the larger conductor.

NEC 240.21 is not written to size two conductors from two sources coming together, one a larger conductor with an OCPD at the source and one a smaller conductor also with an OCPD at the source. We can make it work after a fashon, but we are putting a square peg in a round hole. The reason 705.12(B)(2)(2) is needed is becasue 240.21 does not deal with multiple sources. If it did then 705.12(B)(2)(2) would not be necessary.

 

[ggunn]

NEC 240.21 is written assuming a single source of supply to a larger conductor where that conductor is protected by an OCPD, and then a number of smaller load conductors that will have reduced protection are tapped off that larger conductor. It requires OCPD at the load end of those smaller conductors to prevent the smaller conductor from being overloaded by the operating current or a fault after the load end OCPD. Not all of the sections of 240.21 size the conductors so that they will be protected by the OCPD on the larger conductor.

NEC 240.21 is not written to size two conductors from two sources coming together, one a larger conductor with an OCPD at the source and one a smaller conductor also with an OCPD at the source. We can make it work after a fashon, but we are putting a square peg in a round hole. The reason 705.12(B)(2)(2) is needed is becasue 240.21 does not deal with multiple sources. If it did then 705.12(B)(2)(2) would not be necessary.

When you are dealing with PV, some sections of the NEC require you to stand on your head while reading it.