705.12 -- Source Connected at 4160V Feeder

[wyeager]

The figures below from SolarProfessional (Issue 7.3, Apr/May 2014) fairly closely illustrates a scenario I've been asked to evaluate under NEC 2014. In our case, however, there is a 200A switch with 50E main fuse on the subpanel instead of MLO. My question is whether or not the existing downstream OCPD satisfies 705.12(D)(2)(1)(b). My gut reaction is that the downstream OCPD needs to be right at the point of connection in order to protect the rest of the feeder (or that the feeder would need to be upsized as shown in figure 1). The other side of the argument is that the feeder (#2/0 Cu MV-105) can't be overloaded because the only loads on it are behind a 50E fuse (and then a 300kVA transformer).



Please let me know if I can provide further clarification and thanks in advance for your help.

William Ray Yeager, PE

 

[coop3339]

Not sure I completely understand your question but I'll give it a shot. Before your solar was added you had a 200A feeder run from a 200A OCPD to a 50A OCPD feeding a sub-panel. As long as the 200A OCPD is rated for back feed and the feeder conductors are rated at 200A you should be ok to tap the solar feed into it. The sub-panel can't draw any more than the 50A it is fused at so the feeder cant be overloaded. The main panel where the 200A feeder originates, however would have to have the correct buss rating as described in 705.12. This is more likely to be your limiting factor. If the 200A OCPD where the feeder originates is a main then you are OK. There may be other considerations because it is medium voltage, but I am not aware of any.

 

[wyeager]

Thanks for the reply Coop!

To be clear, the solar is proposed but not yet installed. I'm trying to decide what the code allows before moving forward with the design. Does the position of the downstream OCPD not matter relative to 705.12(D)(2)(1)(b)? In this case the existing 50E fuse is not close at all to the point of connection but it is certainly "on the load side of the inverter connection" and not rated greater than the ampacity of the feeder. I'm concerned that code would consider the feeder between the point of connection and the downstream OCPD to be insufficiently protected from the sum of the two sources.

Here's the actual diagram (main bus is 600A).



Thanks again,

William Ray Yeager, PE

 

[coop3339]

Hi William,

The panel with the 600A main and 3-200A breakers will work for you considering 705.12D(3)(c). This would allow for the connection of 200A @ 4160.

The portion of the feeder on the load side of the interconnection point, as long as it is rated for a minimum of 200A, is ok by 705.12(1)(b). The OCPD at the trans disco is less than the 200A feeders ampacity.

That brings us to the section of the feeder between the interconnection point and the main panel with the 200A breaker. There seem to be multiple taps made to this feeder but I cant quite make out wire sizes and amp ratings. 705.12D(2)(2) seems to indicate that any taps coming from a feeder where the inverter is connected would be required to be increased in size by the amount of the inverters full load current X 1.25. Im not sure I understand what they are trying to accomplish with this. As long as there is correctly sized over current protection at the termination of the tap, the tap conductors would never see more than the rating of that over current device. I am not sure I am interrupting this section correctly. maybe someone else can chime in. Also I don't do alot of MV design so there may be some items Im not considering.

 

[wyeager]

Thanks Coop, that's really helpful. I'm not 100% convinced that this question is settled but I'm more comfortable with it now. I hadn't thought about 705.12(D)(2)(2) affecting the other existing taps upstream. That may be the real deal killer.

 

[jaggedben]

Does the position of the downstream OCPD not matter relative to 705.12(D)(2)(1)(b)?

In my opinion it is open to interpretation. At this level and potential expensive of equipment I would ask your AHJ. I have done a couple installs like this (only at 240V!) where I have done it by putting a main breaker into the downstream MLO load center. The AHJ did not have a problem with it, but neither did we end up discussing it. I just put it in the plans and they approved it and passed our inspections.

Mike Holt has a long video discussion of the 2014 changes to 705 posted somewhere on youtube, and towards the end of the discussion Mike says 'wait a minute, I think the breaker needs to go here', meaning closer to the solar tap and source. His colleagues start to say 'that's what we were arguing about last night' and the discussion doesn't resolve.

 

[wyeager]

Thanks jaggedben, for pointing me towards that video. I'm glad to see that I'm not the only one scratching my head over this section. I'm afraid that this one might continue to be open to interpretation (which makes me sweat) until the next code revision. The distinction between feeders and taps could be much better defined (easier said than done, I know).

 

[jaggedben]

Thanks jaggedben, for pointing me towards that video. I'm glad to see that I'm not the only one scratching my head over this section. I'm afraid that this one might continue to be open to interpretation (which makes me sweat) until the next code revision. The distinction between feeders and taps could be much better defined (easier said than done, I know).

Yes, I just noticed you're in California. In that case your situation is worse because we're not on the NEC 2014 rules yet. They might not let you tap that feeder with more PV than 20% of it's ampacity. Maybe when you explain the cost difference to the client, you justify waiting 6 months for this project. My strategy was going to be to appeal to the 2014 rules if I got into a discussion with the AHJ, but I think it both cases I was also under the 120% limit.

 

[shortcircuit2]

240.21 generally requires overcurrent protection at the point of supply.

 

[Smart $]

240.21 generally requires overcurrent protection at the point of supply.

To complete the context

...except as specified in 204.21 (A) through (H).

(A) through (H) cover tap and secondary conductor situations.


Then there is also 240. 91... which covers conductor protection in supervised industrial locations. Given the OP mentions 4160V, I have to guess we are talking such a location, yes?