Disconnect needed?

[electro7]

Do I need a disconnect where the red circle is? I'm being told according to 225.31 that I need a disconnect there. My argument is that the fused disconnect upstream from the xfrmr disconnects those conductors. But I am questioning myself since the xfrmr is a separately derived system.

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[PWDickerson]

See 690.9(D) including the exception. It states:

"Power Transformers. Overcurrent protection for a transformer with a source(s) on each side shall be provided in accordance with 450.3 by considering first one side of the transformer, then the other side of the transformer, as the primary.

Exception: A power transformer with a current rating on the side connected toward the interactive inverter output, not less than the rated continuous output current of the inverter, shall be permitted without overcurrent protection from the inverter."

 

[electro7]

See 690.9(D) including the exception. It states:

"Power Transformers. Overcurrent protection for a transformer with a source(s) on each side shall be provided in accordance with 450.3 by considering first one side of the transformer, then the other side of the transformer, as the primary.

Exception: A power transformer with a current rating on the side connected toward the interactive inverter output, not less than the rated continuous output current of the inverter, shall be permitted without overcurrent protection from the inverter."

In my example, we have a solar panel board on the solar side of the transformer (I didnt show that in the drawing), with overcurrent protection. So that would satisfy the exception, correct?

The argument is that the disconnect is needed where I circled in red because the conductors cross over a building. Wouldn't the fused disconnect on the primary side of the transformer suffice for disconnecting those conductors going across the building and to the solar?

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[electro7]

Updated drawing showing the panel board on the solar side of the building.

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[Carultch]

The argument is that the disconnect is needed where I circled in red because the conductors cross over a building. Wouldn't the fused disconnect on the primary side of the transformer suffice for disconnecting those conductors going across the building and to the solar?

Disconnecting, yes. But overcurrent is another matter entirely. And the topology of the transformer, can redistribute a secondary overcurrent on an individual phase so they are in the blindspot of the primary OCPD.

240.21(C) only allows a primary OCPD to act as protection for secondary conductors, for qualifying topologies that keep currents lined up between corresponding phase conductors. If you had a qualifying topology on a 208V:480V transformer, this would mean a 400A primary fuse at 208V, would effectively act on the 480V side as a 173A fuse for the secondary. Qualifying toplogies for this, are delta:delta 3-wire, and single phase 2-wire to 2-wire. A wye system and any system with a centertap will not qualify, and requires secondary overcurrent to meet 240.21(C).

The way I read these rules, it is telling you that you don't need to protect the transformer from overload caused by the inverter, because the inverter is current limited by design. However, one thing this rule doesn't clarify, is how it is meant to interact with 240.21(C). The transformer doesn't require protection from the inverter, as long as it has enough KVA (unlikely that it wouldn't). But the secondary conductors still require protection, which this rule doesn't tell me otherwise. Assuming you have a wye system on at least one side, I see as requiring a secondary side fused disconnect, or circuit breaker, to meet 240.21(C) in one form or another.

 

[electro7]

Disconnecting, yes. But overcurrent is another matter entirely. And the topology of the transformer, can redistribute a secondary overcurrent on an individual phase so they are in the blindspot of the primary OCPD.

240.21(C) only allows a primary OCPD to act as protection for secondary conductors, for qualifying topologies that keep currents lined up between corresponding phase conductors. If you had a qualifying topology on a 208V:480V transformer, this would mean a 400A primary fuse at 208V, would effectively act on the 480V side as a 173A fuse for the secondary. Qualifying toplogies for this, are delta:delta 3-wire, and single phase 2-wire to 2-wire. A wye system and any system with a centertap will not qualify, and requires secondary overcurrent to meet 240.21(C).

The way I read these rules, it is telling you that you don't need to protect the transformer from overload caused by the inverter, because the inverter is current limited by design. However, one thing this rule doesn't clarify, is how it is meant to interact with 240.21(C). The transformer doesn't require protection from the inverter, as long as it has enough KVA (unlikely that it wouldn't). But the secondary conductors still require protection, which this rule doesn't tell me otherwise. Assuming you have a wye system on at least one side, I see as requiring a secondary side fused disconnect, or circuit breaker, to meet 240.21(C) in one form or another.

So on the 480V PV side of the transformer (secondary side) at the groundmount array which is on the other side of a building from the MSB, Utility fused disconnect and transformer there will be a main breaker in the panelboard. On the primary side, there is the utility fused disconnect. The engineer is saying I need a disconnect located on the MSB side of the building between the transformer 480V secondary side and the solar panel board which is on the other side of the building since we are routing the conductors over the building rooftop. My argument is that the utility disconnect disconnects those conductors and that 480V disconnect is not needed. I think I am correct.

But I can see how since the transformer is a separately derived system, those conductors aren't actually disconnected. But they kinda are by the utility disconnect. From a safety standpoint I don't see any reason for the extra 480V secondary disconnect on the MSB side of the building since when you turn off the utility disconnect there is no potential on these conductors on the 480V side.


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[wwhitney]

The OP references 225.31, which says:

225.31 Disconnecting Means. Means shall be provided for disconnecting all ungrounded conductors that supply or pass through the building or structure.

So is the transformer outdoors away from the building, with the secondary conductors passing near the building on the way to the ground mount array?

If so, 225.31 may require a disconnect at the point the secondary conductors hit the building, to be able to disconnect all sources of power to the building. If the conductors pass through the building, even with no junctions or splices within the building, then 225.31 clearly requires that. However, when the conductors are purely on the outside surface of the building, it is then a judgement call if that counts as "supplying" or "passing through" the building.

If, however, the transformer is within the building, then its source of supply will already have a means to disconnect power from the building (per 225.31 or Article 230), so its secondary conductors certainly don't need a 225.31 disconnect.

Cheers, Wayne

 

[electro7]

The OP references 225.31, which says:



So is the transformer outdoors away from the building, with the secondary conductors passing near the building on the way to the ground mount array?

If so, 225.31 may require a disconnect at the point the secondary conductors hit the building, to be able to disconnect all sources of power to the building. If the conductors pass through the building, even with no junctions or splices within the building, then 225.31 clearly requires that. However, when the conductors are purely on the outside surface of the building, it is then a judgement call if that counts as "supplying" or "passing through" the building.

If, however, the transformer is within the building, then its source of supply will already have a means to disconnect power from the building (per 225.31 or Article 230), so its secondary conductors certainly don't need a 225.31 disconnect.

Cheers, Wayne

Hi Wayne,

Thanks. Yes, the transformer is exterior near MSB, and the conduit is being routed on the roof, crossing the building to the array groundmount.

Are you thinking that since the utility disconnect is on the primary side of the transformer and since the transformer is a separately derived system, by turning the utility disconnect off you are actually not disconnecting the secondary conductors?

I think that's what the engineer is thinking.

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[ggunn]

Hi Wayne,

Thanks. Yes, the transformer is exterior near MSB, and the conduit is being routed on the roof, crossing the building to the array groundmount.

Are you thinking that since the utility disconnect is on the primary side of the transformer and since the transformer is a separately derived system, by turning the utility disconnect off you are actually not disconnecting the secondary conductors?

I think that's what the engineer is thinking.

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But the inverter(s) will shut down when the disco is opened or the power goes out, so no conductors will remain energized.

 

[wwhitney]

Are you thinking that since the utility disconnect is on the primary side of the transformer and since the transformer is a separately derived system, by turning the utility disconnect off you are actually not disconnecting the secondary conductors?

As regards 225.31, I believe the intent is to be able to remove all sources of power "at" the building when doing maintenance, renovations, etc. So I think there are a few questions to be answered here:

1) If you kill the other building disconnect(s) (for service(s) or feeder(s) supplying the building), will these secondary conductors still be energized? If yes, then Article 225 definitely applies. Even if no, if the transformer is far enough away from the building, Article 225 still applies. How far is unclear; I'd say definitely yes at 50', definitely no at 1'.

2) If Article 225 applies, do the secondary conductors count as "supplying" or "passing through" the building, so that 225.31 applies? As they are only attached to the exterior of the building without penetrating the building envelope, you could argue no. On the other hand, some might consider being attached the building to be a potential hazard comparable to "passing through".

3) If 225.31 applies, is the primary side disconnect sufficiently close to the building to count as the 225.31 disconnect? Again, how far away it may be is unclear.

Note that the presence of the transformer is more or less incidental to the above; it's just a question of where the conductors get their supply, and how far away they travel from the building, and how far away from the building your disconnect it.

Cheers, Wayne

 

[wwhitney]

As regards 225.31

To summarize, in the drawing of post #4, we need the location of the items shown relative to the building. Possible extremes are:

- The MSB and Fused Disconnect are mounted on the side of the building, and the transformer is a couple feet away from the building. You still need to decide if the primary side fused disconnect is a 225.31 disconnect by deciding question #2 in the previous post, because of signage requirements, IIRC. But there's certainly no need for a secondary disconnect for 225.31, I would think; I don't see how the presence of the transformer changes anything.

- The MSB, Fused Disconnect and transformer are all 50 feet away from the building. Then the feeder from the MSB to the building should itself have a 225.31 disconnect on the building. And depending on the decision on question #2 above, the secondary conductors may need a disconnect on the building.

Cheers, Wayne

 

[jaggedben]

It's been unclear to me from the start of this thread if the transformer is at the same building whose roof the conduit is crossing over.

If the transformer and conduit are not at the same building, I agree that 225.31 requires a disconnect. If they are at the same building, I don't.

 

[electro7]

Okay, thanks for the feedback. In this case, the MSB, utility disconnect, and transformer are maybe 5-10 feet max away from the building. Neither the MSB nor any of the other equipment are attached to the building. The building is the same building the MSB serves and therefore the same building the PV is for.

Thanks again, guys!

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[wwhitney]

Okay, thanks for the feedback. In this case, the MSB, utility disconnect, and transformer are maybe 5-10 feet max away from the building. Neither the MSB nor any of the other equipment are attached to the building. The building is the same building the MSB serves and therefore the same building the PV is for.

Assuming the MSB is existing, and the fused disconnect and transformer are new, and are all the same distance from the building, then one thing you can do is take a clue from how the existing feeder from the MSB to the building was installed. If that feeder was considered subject to Article 225, then it would have its own 225.31 disconnect on the building itself (even if that's only 10' from the MSB). Whereas if that MSB was already considered to be "at" the building, then there would be no such disconnect.

Cheers, Wayne

 

[electro7]

Assuming the MSB is existing, and the fused disconnect and transformer are new, and are all the same distance from the building, then one thing you can do is take a clue from how the existing feeder from the MSB to the building was installed. If that feeder was considered subject to Article 225, then it would have its own 225.31 disconnect on the building itself (even if that's only 10' from the MSB). Whereas if that MSB was already considered to be "at" the building, then there would be no such disconnect.

Cheers, Wayne

Gotcha. The MSB is existing and only has feeder breakers going to sub panels inside the building- no external disconnect switches. The utility disconnect and transformer are new for the PV.

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[coop3339]

I don't see any reason for the disconnect. Wouldn't the outside tap rule apply in this case? As long as the conductors are sized for the load or in this case the output of the inverter?

 

[Elect117]

450.14 - Transformer disconnecting means should be within in line of sight or lockable. In this case your power source is the solar. So there should be a single disconnecting means that is lockable either near the inverters or near the transformer depending on where everything is at.

P.S. Transformers are considered separately derived because they are not electrically connected. They are stepped up or down by electromagnetic fields and windings. The ground in a transformer is separately derived and wired similar to that of a service entrance for that reason. You can not rely on the current in the primary to protect the secondary side. There are some rules for taps but it starts to get nuanced when we talk about single phase 240.21(C)(1).

 

[jaggedben]

... In this case your power source is the solar. ...

For grid-tie only systems, I disagree with this sentence, and will go as far as to say that under the 2020 or 2023 NEC it is incorrect. See the 2020 revision to 705.30(C), which defines the greater fault current source (the grid) as the primary, and provides for secondary protection to not be required. Regardless of code cycle, the transformer would be de-energized by a disconnect on the grid side, not by one on the solar side.

In this case a disconnecting means (but not necessarily OCPD) is probably required at the inverter for other reasons. (See 690.15). However in the case that a primary disconnect, transformer, and single inverter are all grouped within sight of each other, it is debatable if there needs to be a disconnect between the transformer and inverter.