Wire sizing for 100% Rated Breakers

[Matt Donovan]

Here's the situation:

I have a continuous load that draws 2800A. Normally, I would size the circuit for 1.25 x 2800A = 3500A. But if I use a 100% rated breaker sized at 3000A, does the wire still need to be sized for 1.25x2800A = 3500A? Or will a 3000A feeder still work? (8) sets of 500mcm CU are existing, and I'd like to not upgrade those.

 

[Matt Donovan]

apply this to a PV interconnection

apply this to a PV interconnection

I asked that question to keep it simple. The REAL situation is a little more nuanced. I have a 3000A service with (8) sets of 500mcm, and I want to interconnect a PV system line-side to that service. The PV system output is 2800A, and I'd like to avoid upsizing those service feeders if possible. Article 705.12(A) says "the sum of the ratings of the overcurrent devices connected to power production sources shall not exceed the rating of the service."

Is the answer any different in this scenario? Can the (8) sets of 500mcm still handle the 2800A of solar with a 3000A 100% rated breaker?

 

[ron]

230.90(A) says service conductors can't be smaller than the protection.

Now that you updated the info, I'll have to think about it more, but you could always do a line side tap with the PV.

Although intuitively, they are likely concerned that the building be be drawing no load and all generation is backfed to the grid, and the service conductors would be smaller than the combined protection, which is a problem.

 

[Smart $]

230.90(A) says service conductors can't be smaller than the protection.
...

Perhaps whoever 'engineered' the service thought that 100% rated service disconnect breaker allowed them to use the 90°C ampacity of the conductors. That is incorrect. We still have to use the 75°C ampacity but are required to use 90°C-rated wire. The other difference is we don't have to pad the continuous load portion by 25%.

Another question is whether a 100%-rated breaker is even permitted to take advantage of the 100% rating on its line side in a service application. There is no exception for service conductors as there is in the feeder and branch circuit articles.

@Matt Donovan

By chance, is that 3000A breaker adjustable and trimmed to not more than 2880A?

500mcm is only rated 360A@75°C
8 x 360A =2880A

Are you certain the load on the service is 2800A continuous?

 

[infinity]

P

By chance, is that 3000A breaker adjustable and trimmed to not more than 2880A?

500mcm is only rated 360A@75°C
8 x 360A =2880A

Aren't 500 kcmil @ 75° C conductors rated for 380 amps? (380*8=3040 amps)

 

[Smart $]

Aren't 500 kcmil @ 75° C conductors rated for 380 amps? (380*8=3040 amps)

Oops! :blink: Yes, they are.

 

[dkidd]

Aren't 500 kcmil @ 75° C conductors rated for 380 amps? (380*8=3040 amps)

That's what I get.

 

[Smart $]

That's what I get.

Still the questions of whether the 100% rating applies to service conductors (I think not), and if the load is truly 2800A continuous...?

 

[Matt Donovan]

lets ignore the service...

lets ignore the service...

In the case of feeders, I think I've found an answer in 215.12(A)(1) expection 1. Paraphrasing: where 100% rated breakers are used, the feeder may be sized for 100% of non-continuous load + 100% of continuous load.

For services, I'm not sure if the rules are different. I can't find a comparable exception.

@Smart$, this 3000A breaker does not yet exist, but for arguments sake, lets say its dialed up to 3000A. I'm trying to decide if I can spec it as 100% rated and get away with the existing conductors.

 

[Smart $]

...
@Smart$, this 3000A breaker does not yet exist, but for arguments sake, lets say its dialed up to 3000A. I'm trying to decide if I can spec it as 100% rated and get away with the existing conductors.

As I see it 2400A of PV is the most you can connect to a 3000A service.

Exactly what breaker are we talking about: the PV system breaker or the service breaker, or both? Are you tapping into an existing service? Or just using the service conductors of a demo'd service? I just feel like we are not getting the complete picture... :blink:

 

[Matt Donovan]

more detail

more detail

The existing service is metered at the transformer, it has 8-sets underground landing in a two-section switchboard. The left section is a termination and pull section, the right section has up to 6 service disconnects sized 400A to 1200A each (there is no 3000A main existing). I would tap into that left section, and install a new section to the far left with a 3000A breaker for solar. Thats the 100% rated breaker we're talking about. The existing service conductors are the ones I am concerned about.

Does that clear it up?

 

[Smart $]

...
Does that clear it up?

Yes. Thank you.

Hmmm.... this is a tough one. You'd be good on the PV side of the 3000A 100% breaker. Don't forget the new section for it must also be listed for the use of 100%-rated breaker(s). The question is whether the 100% rating can be applied to the service side... and I simply cannot say yes with no reservations. I'm leaning towards no because there is no exception in Article 230 for 100%-rated breakers like there is in Articles 210 and 215. Yet it seems stupid if you can run 8 sets of 500mcm on the PV side and not be able to on the service side. If I were you I'd be contacting my AHJ on this one and present the argument I just stated (which assumes the service conductors are 90°C-rated copper conductors).

 

[ramsy]

..I'd like to not upgrade those.

Prove original service conductors were installed with <= 3 CCC's per conduit, and perhaps a few ºC below ambient conditions (underground Duct Bank), and 230.42(A)(2) may avoid service-conductor upgrades.

 

[Smart $]

Prove original service conductors were installed with <= 3 CCC's per conduit, and perhaps a few ºC below ambient conditions (underground Duct Bank), and 230.42(A)(2) may avoid service-conductor upgrades.

Aha! I was going to say that doesn't get around 230.42(A)(1) but I just did a reread and noticed the general statement says (1) or (2)... or (3) if 2014... and there it is....

(3) The sum of the noncontinuous load plus the continuous
load if the service-entrance conductors terminate in an
overcurrent device where both the overcurrent device
and its assembly are listed for operation at 100 percent
of their rating

I was thinking that section said all numbered conditions must be met like 210.19 and 215.2... but it does not.

Under 2017, that was moved to 230.41(A)(1) Exception No. 2. Probably done just to make it consistent with 210.19 and 215.2.

 

[ramsy]

Sorry, should clarify 2017 NFPA-70 230.42(A)(2)

 

[Unbridled]

Here's the situation:

I have a continuous load that draws 2800A. Normally, I would size the circuit for 1.25 x 2800A = 3500A. But if I use a 100% rated breaker sized at 3000A, does the wire still need to be sized for 1.25x2800A = 3500A? Or will a 3000A feeder still work? (8) sets of 500mcm CU are existing, and I'd like to not upgrade those.

240.4(C) Overcurrent Devices Rated over 800 Amperes. Where the overcurrent device is rated over 800 amperes,
the ampacity of the conductors it protects shall be equal to
or greater than the rating of the overcurrent device defined
in 240.6.

 

[Unbridled]

NEC 2017 215.2(A)(1)(a) allows feeder conductors to be sized using the 90 degree column, as long as the terminals at each end are rated at 90 degrees. If terminated in a panelboard, the C/B must be rated at 100% (not 80%). It also permits feeder sizing at 100% of the continuous load (not 125%) and 100% of the non-continuous load.

 

[Smart $]

NEC 2017 215.2(A)(1)(a) allows feeder conductors to be sized using the 90 degree column, as long as the terminals at each end are rated at 90 degrees. If terminated in a panelboard, the C/B must be rated at 100% (not 80%). It also permits feeder sizing at 100% of the continuous load (not 125%) and 100% of the non-continuous load.

100%-rated breakers are still 75*C equipment. To be utilized at the 100% rating, 90*C-rated conductors are required, but you still have to use 75*C for termination temperature limits.

 

[Unbridled]

100%-rated breakers are still 75*C equipment. To be utilized at the 100% rating, 90*C-rated conductors are required, but you still have to use 75*C for termination temperature limits.

You are correct Smart. My interpretation is that you can install 90 degree lugs on a C/B, and pull 90 degree conductors, however i,f the entire assembly is not rated at 90 degree, then you must work with 75 Degree ampacity of the conductors.
One option the poster has is to install two enclosures with termination blocks. One close to his switch board and also one near his sub panels. He can Install Feeder conductors from the switchboard to the enclosure with terminal blocks and work with 75 degree ampacities ( short Distance). Then he can continue his feeder from the load side of the 90 degree rated terminal block in this enclosure to the 2nd enclosure with 90 degree conductors and Terminations, using 90 degree ampacities. then continue up to his sub-panel with 75 degree ampacities.

Why? He can save an enormous amount of money using a 90 degree rated conductors for his feeders between enclosures, rather than 75 degree.

Just a thought.:thumbsup::thumbsup:

 

[Smart $]

...
One option the poster has is to install two enclosures with termination blocks. One close to his switch board and also one near his sub panels. He can Install Feeder conductors from the switchboard to the enclosure with terminal blocks and work with 75 degree ampacities ( short Distance). Then he can continue his feeder from the load side of the 90 degree rated terminal block in this enclosure to the 2nd enclosure with 90 degree conductors and Terminations, using 90 degree ampacities. then continue up to his sub-panel with 75 degree ampacities.

Why? He can save an enormous amount of money using a 90 degree rated conductors for his feeders between enclosures, rather than 75 degree.

Just a thought.:thumbsup::thumbsup:

While true, I do not see how this relates to the situation presented.