NEC 240.21(b)(1) Load side taps "rating" confusion

[wwhitney]

The language in (2) for the 25' tap rule is much better than that in the 10' tap rule.

Sorry, I don't see the difference between 240.21(B)(1)(1)(b)'s text "the overcurrent protective device at the termination of the tap conductors" and 240.21(B)(2)(2) text "the tap conductors terminate in a single circuit breaker or a single set of fuses". I would think any configuration would that meets one description meets the other, and vice versa.

Cheers, Wayne

 

[don_resqcapt19]

Sorry, I don't see the difference between 240.21(B)(1)(1)(b)'s text "the overcurrent protective device at the termination of the tap conductors" and 240.21(B)(2)(2) text "the tap conductors terminate in a single circuit breaker or a single set of fuses". I would think any configuration would that meets one description meets the other, and vice versa.

Cheers, Wayne

Enough difference for someone to say that (1) requires conductors to a fusible disconnect to be sized for the rating of the disconnect and not for the rating of the fuses.

The "rating of the equipment containing an overcurrent device(s)" vs "terminate in a single circuit breaker or a single set of fuses" The first applying to the rating of the fusible disconnect or the rating of an MLO panelboard, and the second applying to the rating of the OCPD itself.

Not going to waste my time digging to see why the language is different, but there must be a reason.

 

[wwhitney]

Enough difference for someone to say that (1) requires conductors to a fusible disconnect to be sized for the rating of the disconnect and not for the rating of the fuses.

Still not seeing it. Let's say that I don't want to use the "rating of the equipment containing an overcurrent device(s) supplied by the tap conductors" clause in 240.21(B)(1)(1)(b). Then the rest of 240.21(B)(1)(1)(b) becomes:

(A) "[The ampacity of the tap conductors is] not less than the rating of the overcurrent protective device at the termination of the tap conductors"

Whereas 240.21(B)(2)(2) says

(B) "The tap conductors terminate in a single circuit breaker or a single set of fuses that limit the load to the ampacity of the tap conductors."

Those are just two different wordings of the identical idea. I don't see how one can read (B) to allow 70A ampacity conductors to land on a 100A fused disconnect switch with 70A fuses installed, while not reading (A) to allow the same thing.

In other words, if we agree that conductors landing in a fused disconnect "terminate in a single set of fuses", which we must if we say that clause (B) above is compatible with the use of a fused disconnect, then clearly the fuses are an "overcurrent protective device at the termination of the tap conductors," and so satisfy clause (A).

Cheers, Wayne

 

[don_resqcapt19]

Still not seeing it. Let's say that I don't want to use the "rating of the equipment containing an overcurrent device(s) supplied by the tap conductors" clause in 240.21(B)(1)(1)(b). Then the rest of 240.21(B)(1)(1)(b) becomes:

(A) "[The ampacity of the tap conductors is] not less than the rating of the overcurrent protective device at the termination of the tap conductors"

Whereas 240.21(B)(2)(2) says

(B) "The tap conductors terminate in a single circuit breaker or a single set of fuses that limit the load to the ampacity of the tap conductors."

Those are just two different wordings of the identical idea. I don't see how one can read (B) to allow 70A ampacity conductors to land on a 100A fused disconnect switch with 70A fuses installed, while not reading (A) to allow the same thing.

In other words, if we agree that conductors landing in a fused disconnect "terminate in a single set of fuses", which we must if we say that clause (B) above is compatible with the use of a fused disconnect, then clearly the fuses are an "overcurrent protective device at the termination of the tap conductors," and so satisfy clause (A).

Cheers, Wayne

I am saying the "or" does work that way. It does not give the installer a choice. That choice is made by the type of equipment being installed.

This should have said the "or" does not work that way.

 

[wwhitney]

I am saying the "or" does work that way. It does not give the installer a choice. That choice is made by the type of equipment being installed.

Presumably you mean "does not work that way."

I don't agree, "or" does work that way. If an affirmative code rule ("do A or B or C") has an "or" list, you can comply with the rule by complying with any one of the items that is applicable to your situation. The other items could be deleted from the list, and your compliance with the rule would be unchanged.

Cheers, Wayne

 

[don_resqcapt19]

Presumably you mean "does not work that way."

I don't agree, "or" does work that way. If an affirmative code rule ("do A or B or C") has an "or" list, you can comply with the rule by complying with any one of the items that is applicable to your situation. The other items could be deleted from the list, and your compliance with the rule would be unchanged.

Cheers, Wayne

(1975 NEC)240-21 Exception No. 2 b. The ampacity of the tap conductors is: (1) not less than the computed loads on the circuits supplied by the tap conductors, and (2) not less than the ampere rating of the switchboard, panelboard, or control device supplied by the tap conductors unless they are terminated in an overcurrent protective device not exceeding the ampacity of the tap conductors.

That language makes it very clear that once you install an OCPD in the equipment, the tap conductor can be sized to that OCPD. I still don't think that the current rule is completely clear, however the intent is to permit the tap conductor to be sized to the OCPD in the equipment as I said in post 30.

I guess the current language would still permit the tap conductor to terminate in equipment that does not have an OCPD, and in that case the tap conductor would have have an ampacity equal or greater than the current rating of the equipment that the tap is terminated on.

 

[jaggedben]

I have a new question on this thread:
When we install a solar system on an existing service, we usually make the interconnection as either a feeder tap or a backfeed breaker. When it's a feeder tap, we size the solar output conductors per 690.8 at 125% of the maximum output current, then we check the size of the tap per 240.21 on field connections (since we are installing on an existing service) and insure the tap size is also at or above the percentage of the rating of the feeder being tapped. Then referring to 705.11 for supply-side connections, this is OK. But then section 705.12(B)(1)a for load-side taps requires my tap to be sized as the sum of the 125% solar rating and the percentage assigned by the length of the tap. Question: Why the sum, and why is this the only place that requires it? Why not just the larger of the two? (This jurisdiction is using the 2020 NEC.)

The intent is that the calculation in 240.21(B) - e.g. 1/10 for a 10ft tap or 1/3 for a 25ft tap - is performed on the sum of the feeder OCPD and 125% of the interconnected source current. e.g. 100A feeder breaker, 32A solar output, and a 25ft tap would require (100+(32*1.25))/3= 46.67A ampacity for your tap conductors. In many cases your minimum requirement for the output will still govern.

At one point I submitted a PI to try to get them to make the language clearer, and it improved a little, but I agree it's still badly worded.

The theory behind the requirement, which I believe is largely erroneous, is that interconnected sources could contribute a roughly proportional amount of fault current as the grid. In reality, in most cases, the interconnected source is an inverter that can't contribute nearly that much fault current.

 

[ryangittens]

@wwhitney I think I know your answer to this, but would you agree that if a tap is less than 10' long the 25' tap rule can still be applied? There is not prohibitive verbiage in the code stating that if it's under 10' that it must comply with the 10' tap rule. After all that, these guys are still sticking to their logic so I am just not referncing the 10' at all once we complay with all the 25' tap rules.

 

[wwhitney]

@wwhitney I think I know your answer to this, but would you agree that if a tap is less than 10' long the 25' tap rule can still be applied?

Of course.

Cheers, Wayne

 

[ggunn]

@wwhitney I think I know your answer to this, but would you agree that if a tap is less than 10' long the 25' tap rule can still be applied?

In general, being more conservative than what the NEC requires is always allowed, although there is the occasional inspector that does not understand this. One case we ran into was a guy who insisted on failing us on our inspections if the minimum PV interconnection breaker was 35A and we used a 40A breaker with appropriately sized conductors. His "reasoning" was that the inverter "will never trip that 40A breaker". Well no, of course not. Duh.

 

[electrofelon]

One case we ran into was a guy who insisted on failing us on our inspections if the minimum PV interconnection breaker was 35A and we used a 40A breaker with appropriately sized conductors. His "reasoning" was that the inverter "will never trip that 40A breaker". Well no, of course not. Duh.

Well his reasoning was off, but I would agree with him that you cannot just put as large a breaker as you want on a piece of equipment even if the conductors are sized to match. There are sections in the NEC that dictate max OCPD size, and then of course there are manufacturer instructions which often provide a max size. Even though an inverter doesnt "use" current, I could see a max size being part of the testing/rating/listing process and a certain size OCPD could protect it from worse or more catastrophic damage if there was an internal fault. Sort of like how something like an ATS has a variety of different OCPD options and the characteristics of those OCPD's such as clearing times can effect the allowed max size of that OCPD.

 

[jaggedben]

I presume in ggunn's case there was no max ocpd specified by the inverter manufacturer or else it was 40A or higher. While you're correct that manufacturers can impose a max, the code does not, and it was not the inspector's reasoning.

 

[ggunn]

I presume in ggunn's case there was no max ocpd specified by the inverter manufacturer or else it was 40A or higher. While you're correct that manufacturers can impose a max, the code does not, and it was not the inspector's reasoning.

Correct. We were building a lot of smaller load side connected resi PV systems using microinverters, and we wanted to standardize the interconnections to 40A breakers so that the installers wouldn't have to carry around a lot of different breaker and conductor sizes. This guy also wanted us to downsize the individual breakers in the AC combiners where fewer than the maximum number of microinverters was on a branch. He was convinced that the OCPD was there to protect the conductors from the inverters.

He was eventually transferred out of the solar department of the POCO to a position in the metering department, where he still found ways to make things difficult for solar contractors by making up his own rules.

 

[electrofelon]

I presume in ggunn's case there was no max ocpd specified by the inverter manufacturer or else it was 40A or higher. While you're correct that manufacturers can impose a max, the code does not, and it was not the inspector's reasoning.

I agree there is no code addressing Max ocpd size for an inverter. I would however be very surprised if there was not a Max stated in the instructions and or as part of the product standard. I mean is there really such thing as a string inverter that does not State Max breaker size and you could put it on a 4000 amp breaker? I guess of course the terminals will have a Max conductor size and maybe that could determine the largest breaker size.

 

[ggunn]

I agree there is no code addressing Max ocpd size for an inverter. I would however be very surprised if there was not a Max stated in the instructions and or as part of the product standard.

It was not a factor in these cases. The OCPD in question was between the AC combiner panel and the service and not directly connected to the (micro)inverters. I guess it could have been a 4000A breaker without any issues, but of course it was not.

 

[Tulsa Electrician]

I feel the difference is the conductor sizing.
Key word for me is " equipment containing an over current device(s)" 2014 new language.

If under 1/3 for the tap conductor, 10' rule is default. Than the rating of equipment must meet the rating or the tap conductor and not exceed 10'.

Comment: In this case even if the tap conductor only needs 20 amp #12 THHN and you use a 30 fused switch. The conductors feeding this switch need to rated for 30 amp. Further more the the conductors rating would be at the terminals max temperature rating for conductor sizing.
60c,75c etc.

If 10' or under and meets 1/3 tap conductor it defaults into the 25' rule.
Than it's the over current device rating The equipment has to meet the over current device as a minimum.

To say you put in 65 amp fuses it has to be in a 100 switch or they don't fit. Or a 60 amp breaker in an 100 amp circuit breaker enclosure.

If was 100 amp OCPD.
Then the switch is 100 amp for the 100 amp fuses to fit.
If you put 100 breaker in an 60 amp circuit breaker enclosure you have a violation of the equipment based on it's rating.

Therefore I would say it based on the tap conductor size and then distance.
Smaller than 1/3 tap conductor ,10' rule is default.
Tap conductor 1/3 or greater. Default into the 25' rule even if 10' or under.

Now for 240.21 (B) (1) b, where it says "or".

I take this as circuit breaker in an enclosure. Here we two types one with a buss and one with out. The tap conductor terminates on the the terminals of the breaker with in the equipment. I see this as a back feed breaker on a panel board or a bolted in main breaker. Either way the current has to run thru the breaker first before the load.
If it was a circuit breaker enclosure with a buss you hit the Breaker not the lugs or the conductors have to meet the rating of the panels board with in the enclosure.

Then we have a circuit breaker enclosure with out a buss. Then the tap conductor (s) has to land on the breaker. You would just need to make sure it's not marked line or load. If so marked the tap conductor(s) has to land on the line marked terminals.

We must remember the tap conductors not only apply to ungrounded conductors. It also applies to grounded conductor (neutral). This conductor does not over current protection. It's responsible for carrying current in normal conditions and in fault conditions.
With that in mind it only makes since why the 10' rule is written the way it is. This also make since why the EGC never has to be larger than the tap conductor. With that in mind and the 1/10 rule you can see a correlation between the 1/10 rule and table 250.122.

Run some numbers based on CM.

Example: 200 amp feeder using 3/0 cu and applied the 10' tap rule.
CM for 3/0 = 167,800
10% of 167,800= 16,780
The tap would be #6 with a cm of 26,240.
Table 250.122 for 200 amp is a #6 cu and at 65 amps would require the minimum EGC to be a #8 cu. However the NEC says the EGC is sized off of the feeder OCPD. Which requires a #6. The EGC need not be larger than the tap conductor which is a #6.

Then the grounded tap conductor if one was used would also be a #6. You would not be able to down size it based on other NEC rules until after the OCPD protecting the tap conductor.

Based on that I can why it's written the way it is.

Using the same example using the 1/3 25' rule the minimums and using the OCPD and not equipment conductor sizing is all good.

3/0 at 1/3 the CM would be
167800/3= 55,943.33 or a #2
#2 THHN cu at 75c tap = 115 amps. That's 15 amps over 100. So if the 65 amp fuses were changed to 100 for a 100 amp fuses switch the tap would still be protected from over current. The load is a separate issue.

Just the way I see it and why.
It helps comply with the other rules in the NEC.

Edit, 60 fuses to 65 amp.

 

[electrofelon]

I guess it could have been a 4000A breaker without any issues, but of course it was not.

Sometime I'm going to do something like that, just make people scratch their heads....

 

[pv_n00b]

The UL 1741 standard requires that the manufacturer of the inverter specify the maximum OCPD rating to be used on the AC side of the inverter. Some of the testing is done based on this OCPD size and it becomes part of the inverter listing. If the OCPD is installed separately from the inverter the maximum size must be specified in the installation instructions.

 

[ggunn]

The UL 1741 standard requires that the manufacturer of the inverter specify the maximum OCPD rating to be used on the AC side of the inverter. Some of the testing is done based on this OCPD size and it becomes part of the inverter listing. If the OCPD is installed separately from the inverter the maximum size must be specified in the installation instructions.

Once again, this has no bearing on the case I cited. The OCPD in question was between the service and the AC combiner and not directly connected to any inverter. It was not in violation of either the NEC or the manufacturer's specifications.

 

[jaggedben]

And just to repeat, neither UL 1741 nor the NEC limits the OCPD to 125% of inverter output, as the inspector seemed to think. String inverters sometimes have higher max OCPD allowances. But it's particularly true of micros, where a single micro can be on the same OCPD as a full circuit. Also applies to Enphase's ESS; for example the 5P can be on an 80A breaker whether there is one (output 16A) or eight (output limited to 64A by PCS).