125% factor in 705.12

[wwhitney]

What is the purpose of the 125% factor in 705.12?

If it is a continuous use factor, why does 705.12 not have the usual exceptions for 100%-rated OCPD, such as those found in 705.28? Is this just an oversight in need of a PI?

Cheers, Wayne

 

[jaggedben]

Is there any distribution equipment (more than one disconnect, e.g. panelboard or switchboard) listed to 100% for all devices? Or does it just not matter anyway?

 

[wwhitney]

Is there any distribution equipment (more than one disconnect, e.g. panelboard or switchboard) listed to 100% for all devices? Or does it just not matter anyway?

Why would it need to be 100% rated for all devices, rather than just the devices connected to the power source output conductors? Other devices would only need to be 100% rated if they are to be carrying more than 80% of their rating continuously, but that is already covered by other sections of the NEC.

Of course, for panelboards a 100% rated breaker, as I understand it, is typically installed by itself in its own fairly large enclosure, to provide a thermal environment comparable to the free air environment under which it is required to hold indefinitely at 100% of its rated current. Such an arrangement, if contains busbars at all, would qualify under (2023) 705.12(B)(3).

But 705.12(B) covers other forms of distribution equipment. So what about switchboards and switchgear? Could they have busbars, a 100% rated OCPD, and other OCPD, all in one enclosure?

Seems like the simplest PI for 705.12(B) would be to replace all instances of "125 percent of the power source output current" with "the current specified in 705.28(B)".

Cheers, Wayne

 

[jim dungar]

Is there any distribution equipment (more than one disconnect, e.g. panelboard or switchboard) listed to 100% for all devices? Or does it just not matter anyway?

I believe the Square D I-Line panels, from Schneider Electric, can accept all 100% rated branch breakers.

 

[wwhitney]

So is there any reason that an arrangement such as depicted in the partial one line diagram below should be prohibited? As far as I'm aware, the only NEC provision it violates is the 125% factor in 705.12(B), but there is no performance reason to upsize the conductors or bus.

Cheers, Wayne

 

[ggunn]

So is there any reason that an arrangement such as depicted in the partial one line diagram below should be prohibited? As far as I'm aware, the only NEC provision it violates is the 125% factor in 705.12(B), but there is no performance reason to upsize the conductors or bus.

Cheers, Wayne

View attachment 2567009

At SPI a few years ago there was a rumor going around that the 125% rule for inverter output current was going to be eliminated from the NEC, but that did not come to pass. To state the obvious: as long as it is in the code we must design to it.

 

[Carultch]

What is the purpose of the 125% factor in 705.12?

If it is a continuous use factor, why does 705.12 not have the usual exceptions for 100%-rated OCPD, such as those found in 705.28? Is this just an oversight in need of a PI?

Cheers, Wayne

At one point, 705.12 specified the interconnection breaker rating. As of 2014, they rewrote it in a way that no longer requires you to use the breaker rating, but rather the calculation that determines it, prior to rounding to the breaker you actually use. This stops rounding errors from being a show-stopper.

One example where it would govern a design decision:
You have a 600A panel with a 600A main. This gives you 120A of headroom for interconnecting. Prior to the rewrite in 2014, this would limit you to a 110A interconnection breaker which corresponds to 88A of output current from your inverter(s). With the wording in NEC 2014, this would allow for up to 96A of output current from your inverter(s), which you'd have to land on a 125A breaker. A 125A breaker would not meet the rule prior to 2014, but now it does.

Another example:
You have a critical loads panel, with a battery backup inverter. This is a 100A subpanel that passes through the battery inverter's transfer switch, which is normally supplied from a 200A main panel with a 200A main breaker. You connect a 40A PV inverter to this critical loads panel, which follows the battery inverter during off-grid mode. Prior to 2014, you'd be limited to providing your subpanel and critical loads with no more than a 40A breaker, which extremely limiting. With the 2014 rewrite, the fact that the 100A breaker has a 100A rating, is irrelevant to the rule. You are only backfeeding 40A/(125%) worth of inverter current, even if a larger breaker is part of its path.

 

[wwhitney]

At one point, 705.12 specified the interconnection breaker rating. As of 2014, they rewrote it in a way that no longer requires you to use the breaker rating, but rather the calculation that determines it, prior to rounding to the breaker you actually use.

But the current wording is not actually the calculation that determines it, as that calculation does have an allowance for 100% rated devices. Hence my PI below (substantiation only, as the actual text change is a simple substitution as specified in the substantiation).

Cheers, Wayne

705.12(B)

. . .

Statement of Problem and Substantiation for Public Input

It is my understanding that the 125 percent factor referenced in the current version of 705.12(B) is a continuous use factor. And that busbar ratings are continuous ratings.

On that basis, 705.12(B) deserves the usual exceptions for 100% rated devices, just as every other section in the NEC containing a 125 percent continuous use factor has. Since the requisite verbiage is somewhat lengthy, and is already contained in 705.28(B), I propose replacing every instance of the phrase "125 percent of the power-source output circuit current" with the phrase "the value specified in 705.28(B)."

 

[jaggedben]

At one point, 705.12 specified the interconnection breaker rating. As of 2014, they rewrote it in a way that no longer requires you to use the breaker rating, but rather the calculation that determines it, prior to rounding to the breaker you actually use. This stops rounding errors from being a show-stopper.

One example where it would govern a design decision:
You have a 600A panel with a 600A main. This gives you 120A of headroom for interconnecting. Prior to the rewrite in 2014, this would limit you to a 110A interconnection breaker which corresponds to 88A of output current from your inverter(s). With the wording in NEC 2014, this would allow for up to 96A of output current from your inverter(s), which you'd have to land on a 125A breaker. A 125A breaker would not meet the rule prior to 2014, but now it does.

Another example:
You have a critical loads panel, with a battery backup inverter. This is a 100A subpanel that passes through the battery inverter's transfer switch, which is normally supplied from a 200A main panel with a 200A main breaker. You connect a 40A PV inverter to this critical loads panel, which follows the battery inverter during off-grid mode. Prior to 2014, you'd be limited to providing your subpanel and critical loads with no more than a 40A breaker, which extremely limiting. With the 2014 rewrite, the fact that the 100A breaker has a 100A rating, is irrelevant to the rule. You are only backfeeding 40A/(125%) worth of inverter current, even if a larger breaker is part of its path.

(There was another piece of language that worked around that last example. It went away when it was no longer necessary. But anyway...)

All that sort of precisely highlights the import of Waynes question. Why, if it isn't to account for proper operation of an overcurrent device under continuous use, are we still stuck with the 125% factor?

The 100A feed to the microgrid (or ordinary subpanel) further illustrates the issue. If we were limited to 32A on a 200/200 main because 40A was considered to be the max acceptable extra current, but a typical 40A breaker needed a 125% continuous use factor ... and if the actual breaker being backfed is 100A, such that the continuous use factor is a non-issue ... why can't I backfeed 40A?

 

[jaggedben]

But the current wording is not actually the calculation that determines it, as that calculation does have an allowance for 100% rated devices. Hence my PI below (substantiation only, as the actual text change is a simple substitution as specified in the substantiation).

Cheers, Wayne

705.12(B)

. . .

Statement of Problem and Substantiation for Public Input

It is my understanding that the 125 percent factor referenced in the current version of 705.12(B) is a continuous use factor. And that busbar ratings are continuous ratings.

On that basis, 705.12(B) deserves the usual exceptions for 100% rated devices, just as every other section in the NEC containing a 125 percent continuous use factor has. Since the requisite verbiage is somewhat lengthy, and is already contained in 705.28(B), I propose replacing every instance of the phrase "125 percent of the power-source output circuit current" with the phrase "the value specified in 705.28(B)."

Am I confused or does the following result from your proposal?

Say I start with an inverter of 32A continuous output.
If I connect it to a 200/200 panel with an ordinary 40A breaker, then I reach the max allowed (under the 120% rule).
However, if I find myself a 100% rated fused disconnect to stick in between, then I could upsize my inverter to 40A on a 50A breaker in the panel?

 

[wwhitney]

Am I confused or does the following result from your proposal?

I believe that's correct, and I also believe the arrangement you describe should be allowed.

In fact I would argue it was allowed under the 2011 NEC, which used the language "The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor". So for each supply you'd be free to choose the smallest OCPD in the circuit supplying power to the busbar. Since with multiple OCPD in series in a circuit supplying power to a busbar, you obviously don't add them all up for a single supply.

Certainly the diagram in post #5 was allowed under the 2011 NEC, but is not allowed under the 2023 NEC.

However, if I find myself a 100% rated fused disconnect to stick in between

Can you find 100% rated 40A OCPD? I was under the impression it basically doesn't exist as a mass market item.

Cheers, Wayne

 

[jaggedben]

I believe that's correct, and I also believe the arrangement you describe should be allowed.

I guess I'm not following your logic, even as I'm miffed to be cognizing clearly for the first time (after all these years) how the 120% rule became really a 116% rule. (And just confirms my feeling that number should really be relaxed, but that'd be a different PI.)

First, just because we inserted a specially listed OCPD into the circuit doesn't change any of the physics or safety of current flowing in the panelboard upstream. So it makes no sense to me that the amount of current allowed in the panelboard should change. To put it more generally, 705.12 is concerned with all the equipment between interconnected sources and the primary source, and there's no reason equipment closer to the source should affect the requirements for equipment that's farther away.

Second, the latest version of 705.12 only requires compliance for equipment that isn't otherwise listed for specific configurations of sources. So if the manufacturer of the equipment in your post #5 diagram gets it listed for that configuration then it would be allowed. Which seems more sensible and makes your PI unnecessary.

In fact I would argue it was allowed under the 2011 NEC, which used the language "The sum of the ampere ratings of overcurrent devices in circuits supplying power to a busbar or conductor". So for each supply you'd be free to choose the smallest OCPD in the circuit supplying power to the busbar. Since with multiple OCPD in series in a circuit supplying power to a busbar, you obviously don't add them all up for a single supply.

Certainly the diagram in post #5 was allowed under the 2011 NEC, but is not allowed under the 2023 NEC.

Agreed. But arguably that's another reason that going by breaker rating made no sense and why they changed all that. You're kind of pushing us back into that mistake.

Can you find 100% rated 40A OCPD? I was under the impression it basically doesn't exist as a mass market item.

No it was just an example.

 

[jaggedben]

What would ultimately make sense to me is:
- get rid of that blanket 125% factor in 705.12, but also require that no combination of OCPD and conductor be rated less than 125% of the maximum interconnected source output current that can flow through it unless it's listed for it
- have a proper debate about the number in the 120% rule

 

[wwhitney]

I guess I'm not following your logic, even as I'm miffed to be cognizing clearly for the first time (after all these years) how the 120% rule became really a 116% rule.

OK, so maybe my PI is just a way to reverse the reduction in allowances that accompanied the 2014 NEC language change in 705.12 from "ampere rating of overcurrent device" to "125 percent of the power-source output circuit current", what you are calling a change from a 120% rule to a 116% rule. (*)

Hence the question in the OP, is the 125% factor in 705.12 a continuous use factor? If it is not, and it's just a backdoor way to make the 120% rule a 116% rule, this PI is moot. But if it is, as the history suggests, then it clearly deserves an exception for 100% rated devices.

As to your example, I would hope that we can agree that 705.12 should not distinguish, as far as minimum busbar rating required, between:

1) 200A service -- busbar with loads -- 50A regular OCPD -- 40A 100% rated OCPD -- 40A of inverters
2) 200A service -- busbar with loads -- 40A 100% rated OCPD -- 40A of inverters

As far as I can see, when the 2014 NEC change was made, the phrase "125 percent of the power-source output circuit current" was chosen as a short but inaccurate version of "minimum size breaker required for the power source output circuit before needing to round up to a standard size".

Cheers, Wayne

 

[tortuga]

What is the purpose of the 125% factor in 705.12?

If it is a continuous use factor, why does 705.12 not have the usual exceptions for 100%-rated OCPD, such as those found in 705.28?

Since the upper numbered articles in ch7 only modify the earlier ones (chapter 2-4), take your idea a step further;
Article 705 is dealing with either feeder conductors, service conductors, or tap conductors then there is no need to repeat things that are already covered in 215 or 230 or 240.
Just use article 705 to classify what article the power production source conductors can fall under and send the code user there.
Delete all 125% references and do what article 210 does in 210.3 and just reference feeder rules in 215 or service rules in 230 as applicable.

705 should address the calculated 'load' for determining the ampacity of conductors like 445.13 does.
If a source cant output over 100% then no need to size for overload.

 

[jaggedben]

Since the upper numbered articles in ch7 only modify the earlier ones (chapter 2-4), take your idea a step further;
Article 705 is dealing with either feeder conductors, service conductors, or tap conductors then there is no need to repeat things that are already covered in 215 or 230 or 240.
Just use article 705 to classify what article the power production source conductors can fall under and send the code user there.
Delete all 125% references and do what article 210 does in 210.3 and just reference feeder rules in 215 or service rules in 230 as applicable.

705 should address the calculated 'load' for determining the ampacity of conductors like 445.13 does.
If a source cant output over 100% then no need to size for overload.

I think what you're saying is similar to what I'm saying in post #13.

705 has to somehow account for the possibility of multiple sources aggregating output from the same or different locations. But that's really the only special thing it has to account for when it comes to continuous use factors.

 

[wwhitney]

- get rid of that blanket 125% factor in 705.12, but also require that no combination of OCPD and conductor be rated less than 125% of the maximum interconnected source output current that can flow through it unless it's listed for it

If "listed for it" means that the OCPD is 100% rated, then isn't that exactly what my PI does?

Cheers, Wayne

 

[jaggedben]

If "listed for it" means that the OCPD is 100% rated, then isn't that exactly what my PI does?

Cheers, Wayne

I think your PI confuses the issue by seemingly having the breaker rating for a single interconnection point determine calculations up the line. It's not clear how it handles other connections in series, or how it handles the aggregation of multiple sources in various pieces of equipment.

 

[wwhitney]

First, I realize my PI should have specified 705.28(B)(1), rather than 705.28(B), so at a minimum I will change that.

But based on the feedback, I think it would be clearer and more direct to maintain the body of 705.12 as is, and add at the end the following language:

Exception to (A)(1), (A)(2)(a), (B)(1), (B)(2), and (B)(4):

Where a power source(s) output circuit is protected by an overcurrent device in accordance with 705.30(B) Exception, the rating of the overcurrent device may be used instead of 125 percent of the power source(s) output circuit current.

Comments?

Cheers, Wayne

 

[jaggedben]

I don't know how to make the PI better but I still think it's confusing and perhaps gives the wrong outcome.

Suppose a manufacturer comes out with an AC combiner where you can aggregate sources with all OCPDs used at 100% of their rating, say including a main breaker in the combiner. But then the aggregated system is connected to an existing distribution panel with no special listing. Is the "power source(s) output circuit ... protected by an overcurrent device in accordance with 705.30(B) Exception...", per your language? Seems to be. But should the 'rating of [that overcurrent device' be permitted to be 'used' to qualify the existing panelboard? I would say not.

IOW, I think you would need to make it clear that the exception applies to OCPDs that are in the same equipment that is being qualified under a particular part of 705.12, and I don't think you've done that.