Alternate power source interconnection - NEC 240 / 705 compliant?

[ChargedUp]

Please see attached sketch and give opinion on whether or not you feel the existing 2000A feeder is adequately protected as shown, and the installation is compliant with NEC 240 and 705.12(B)(1)b.

If required, and if the existing 2000A feeder is considered a tap conductor, could outside taps of unlimited length apply if it is found that the existing 2000A feeder is below the electrical room concrete slab, being that it is over 80 feet long?

 

[jaggedben]

My experience on this consists of installing a few similar installations but at much lower amperages, plus discussions on this forum.

This is a grey area because 705 is unclear about the required location of the OCPD going to the load. Some people might say that 240 requires it to be closer to the tap location than you've shown, or else the highlighted section to the load should be considered a tap. But the code doesn't explicitly address this, it's reading between the lines. If it's a tap, and it's an outside tap, then it seems to be compliant. If it's an inside tap, you can only go 25ft max.

It should be noted that the downstream 2000A OCPD at the load provides overload protection, so we're only really concerned about short circuit protection. In the kind of residential installations I've done (for example) the available utility fault current might be assumed to be 10,000A and the inverter fault current maybe 40A, so it's somewhat unreasonable to require considering inverter fault current and require such a feeder to be considered a tap. But when you have a 2000A alternative source, maybe one should be less cavalier about this aspect.

The language in 705 concerning taps has been awful (ungrammatical and illogical), But I'm pretty sure the intention it is that in this case you're required you to use 4000a instead of 2000A when doing the precentage calculations required in 240.21(B).

 

[ChargedUp]

Thanks Jaggedben, your email above is essentially exactly our opinion as well, which unfortunately is still gray, particularly if that feeder is not underground. The consideration of underground feeder in this case is coming from the service conductors NEC articles which lists the requirements to consider an unprotected feeder as underground or "outside" - in order to apply outside taps of unlimited length in this case.

We would not be able to increase this existing feeder from 2000A to 4000A.

 

[wwhitney]

There is an apparent conflict between the 705.12(B)(1)(b) allowance and the rules in 240.21. Since Chapter 7 is allowed to modify Chapters 2-4, and since the scope of 705.12(B)(1) is more specific and limited than the scope of 240.21, I would say that 705.12(B)(1) controls. Note also that if 240.21 applies, then 705.12(B)(1)(b) is superfluous and could be deleted.

So while it's not 100% clear, on balance it seems to me that 705.12(B)(1)(b) trumps 240.21. Are there any factors that lean the other way?

Cheers, Wayne

 

[jaggedben]

I didn't realize until now that between 2017 and 2020 they changed the language from 'overcurrent device on the load side of the power source connection' to 'overcurrent device at the load side of the power source connection'. Sheesh. Why? (Is 'at' actually clear enough to mean anything different at all? What does it mean to be 'at [a] side' of something? If they really mean 'at' the same place of the connection, how could one possibly comply? Was their intention to effectively prohibit this method of interconnection?)

The change slightly affects my opinion that 705.12(B)(1)(b) does not actually modify or conflict with 240.21. If the OCPD is merely 'on the load side' then 705 is agnostic about the location, so you defer back to 240. If it's 'at' the load side... I submit that just makes everything more vague, confusing, and unclear.

 

[ChargedUp]

There is an apparent conflict between the 705.12(B)(1)(b) allowance and the rules in 240.21. Since Chapter 7 is allowed to modify Chapters 2-4, and since the scope of 705.12(B)(1) is more specific and limited than the scope of 240.21, I would say that 705.12(B)(1) controls. Note also that if 240.21 applies, then 705.12(B)(1)(b) is superfluous and could be deleted.

So while it's not 100% clear, on balance it seems to me that 705.12(B)(1)(b) trumps 240.21. Are there any factors that lean the other way?

Cheers, Wayne

Personally, I would agree with you Wayne and don't really have any other factors that lean the other way.

 

[wwhitney]

Any suggestions on some clear simple language to add to 705.12(B)(1)(b) to make it clear that it is in fact amending 240.21? Now is the time to make a PI, which I'm happy to do.

Cheers, Wayne

 

[jaggedben]

Clear and simple? I don't know about that, but let's be clear on what any PI should or should not do.

I would be in favor of either:
a) Making it clear that the placement of the load side overcurrent device can be anywhere on the feeder before load(s), or
b) making it clear that 240.21(B) does apply to the placement of the overcurrent device, and that the section of the feeder in between the power source connection and the overcurrent device is a tap (if the sum of both source OCDs exceeds the ampacity).

What any change should not do is further codify the notion that the load side overcurrent device cannot be any meaningful distance from the connection. That would make installations more difficult and expensive while holding 705 installs to a discriminatory higher standard than other installations that comply with 240.21(B).