2020 NEC 705.12(B)(3)(2) Questions for 120% rule

[fandi]

Hello All,
An engineer connects (4) 240V Energy Storage System (ESS) to an 800A 208/120V switchboard via (4) 37.5kVA Delta(240V)/Wye(208V) transformers using 120% rule method. Per his calculations, 52A (ESS output) on the 240V side of the transformers which is (52*240)/208=60A on the 208V side of the transformers.
T1 connects to phase A and B.
T2 connects to phase A and C.
T3 connects to phase B and C.
T4 connects to phase A and B.

He then uses vector addition (parallelogram rule) to calculate the line currents:
I1=I2=158.75A, I3=103.92A
Per 2020 NEC 705.12(B)(3)(2), max backfeed is 800A busx120%-800A main breaker=160A.
My question about the wordings of the code section 705.12(B)(3)(2) is:
Is 160A max backfeed per phase (A, B or C)? Because if it is then I think he's fine: on 208V side at the 800A switchboard, max phase current is 158.75A < 160A.
Please see the diagram sketch for the connection configuration.
Thank you.

 

[Elect117]

The connections are a little confusing. They are written as phase to phase but noted as the phase to neutral values. Are these boost transformers or single phase transformers wired 3ph? Two of your transformer connections are A B.

But to answer the question, where you have marked 225A 2P, that should be a maximum of the 705.12(B) value. Which ever one applies to the installation. If you are using (B)(3)(2), 120% of busbar > Main + Other sources breaker

So typically, if you assume the main is 100% of the busbar, then only 20% of the bus bar's rating can be the breaker size. so 160A. Since 160A is not a standard size, 150A would be the maximum.

 

[wwhitney]

My question about the wordings of the code section 705.12(B)(3)(2) is:
Is 160A max backfeed per phase (A, B or C)?

Certainly it is per bus in the panelboard, so yes.

Your diagram appears to show (4) separate single phase 208 : 240V transformers, with four separate 225A double pole primary side supplies. But presumably this would be done with a single 225A 3 pole breaker supplying a single 208Y120V : 240/120V Delta transformer sized at 37.5 kVA.

Note that if the maximum continuous inverter output current is 52A, then the configuration shown doesn't comply with the 120% rule, as the 125% factor on the continuous inverter output current has been omitted. If the 52A figure already includes a 125% factor, so the real inverter output current is 41.6A, then the schematic complies with 705.12(B)(3)(2).

Cheers, Wayne

 

[wwhitney]

So typically, if you assume the main is 100% of the busbar, then only 20% of the bus bar's rating can be the breaker size. so 160A. Since 160A is not a standard size, 150A would be the maximum.

2020 NEC 705.12(B)(3)(2) refers to 125% of the inverter output current, not the breaker size. So the breaker size isn't a problem, 225A is fine as long as 125% of the inverter output current is no more than 160A.

Cheers, Wayne

 

[Elect117]

I read it as both with the "and" statement. Both of the "and" statements need to be less than 120% of the busbar rating.

"the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar."

Am I misunderstanding it?

 

[jaggedben]

On the question of using vector math to calculate the line currents, I believe the industry consensus has always been that it is allowed. I can recall discussions about this back at least a decade, particularly around when the 2014 NEC allowed 125% of output current to be used instead of overcurrent device ratings. With those changes, and the code now referring only to busbars and conductors and currents, there is nothing in the code that suggests it should be done different under this rule. (705.12(B)(3)(3) would be different.)

As Wayne said, the install would only be compliant if the 52A ESS output already includes a 125% factor. The max allowable actual line current in this situation is 128A.

In addition, 705.12(B)(3)(2) requires all the connections to be at the opposite end of the busbar from the main overcurrent device, and I wonder how that is being achieved here.

Finally, given how little sense the diagram makes (225A breakers make no sense, description is of delta wye transformers but drawing is of single phase connections), I have to wonder if we're getting a correct description of the design.

 

[jaggedben]

I read it as both with the "and" statement. Both of the "and" statements need to be less than 120% of the busbar rating.

"the sum of 125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar shall not exceed 120 percent of the ampacity of the busbar."

Am I misunderstanding it?

Yes you are misunderstanding. It is "the sum of [125 percent of the power-source(s) output circuit current and the rating of the overcurrent device protecting the busbar] shall not exceed 120 percent of the ampacity of the busbar."

 

[wwhitney]

Am I misunderstanding it?

Yes, the "the rating of the overcurrent device protecting the busbar" is referring to the main breaker, or whatever breaker is satisfying 408.36. So in the OP, that's the 800A main breaker for the 800A bus.

So with an 800A breaker on a 800A bus, 125% of the inverter output current shall not exceed 20% of 800A = 160A. Downsize the main breaker to 600A, and now you can have 125% of the inverter output current as high as 360A on an 800A bus.

Cheers, Wayne

 

[fandi]

Certainly it is per bus in the panelboard, so yes.

Your diagram appears to show (4) separate single phase 208 : 240V transformers, with four separate 225A double pole primary side supplies. But presumably this would be done with a single 225A 3 pole breaker supplying a single 208Y120V : 240/120V Delta transformer sized at 37.5 kVA.

Note that if the maximum continuous inverter output current is 52A, then the configuration shown doesn't comply with the 120% rule, as the 125% factor on the continuous inverter output current has been omitted. If the 52A figure already includes a 125% factor, so the real inverter output current is 41.6A, then the schematic complies with 705.12(B)(3)(2).

Cheers, Wayne

Thanks for your input. It's not a single 225A 3 pole breaker or a single 37.5kvA transformer. It's (4) transformers, (4) 225A double pole breakers.
Regarding the ESS max continuous output current, plans state 'Tesla PCS controlled setting is set to 52A at the conductor at line side of the Tesla Backup Gateway'. It's not clear.
This is how the system sets up:
ESS---Gateway---37.5kvA transformer---800A panel

 

[fandi]

On the question of using vector math to calculate the line currents, I believe the industry consensus has always been that it is allowed. I can recall discussions about this back at least a decade, particularly around when the 2014 NEC allowed 125% of output current to be used instead of overcurrent device ratings. With those changes, and the code now referring only to busbars and conductors and currents, there is nothing in the code that suggests it should be done different under this rule. (705.12(B)(3)(3) would be different.)

As Wayne said, the install would only be compliant if the 52A ESS output already includes a 125% factor. The max allowable actual line current in this situation is 128A.

In addition, 705.12(B)(3)(2) requires all the connections to be at the opposite end of the busbar from the main overcurrent device, and I wonder how that is being achieved here.

Finally, given how little sense the diagram makes (225A breakers make no sense, description is of delta wye transformers but drawing is of single phase connections), I have to wonder if we're getting a correct description of the design.

Thanks. I didn't show the entire diagram but the (4) 225A double pole breakers indeed on the opposite side of the main three pole 800A breaker.

 

[Elect117]

So when they say power source(s) they are not including the utility in it until the main breaker.

I also though that the solar OC protection is also protecting the bus bar so it is included in that phrase.

 

[wwhitney]

Thanks for your input. It's not a single 225A 3 pole breaker or a single 37.5kvA transformer. It's (4) transformers, (4) 225A double pole breakers.

OK, that's just a crazy design, I bet someone on the design side has made the drawing based on a misunderstanding. You could use (4) separate transformers, but you'd use single phase transformers sized on the basis of 240V * 52A (or less, see below) = 12.5 kVA. And the primary side OCPD for each transformer would then be sized on the basis of the the 60A equivalent at 208V.

Having said that, there's no code violation for oversizing the transformers, primary OCPD and primary conductors by a factor of 3.

Regarding the ESS max continuous output current, plans state 'Tesla PCS controlled setting is set to 52A at the conductor at line side of the Tesla Backup Gateway'. It's not clear.

See the last sentence of 705.12 first paragraph: "Where a power control system (PCS) is installed in accordance with 705.13, the setting of the PCS controller shall be considered the power-source output circuit current in 705.12(A) through (E)."

So each PCS needs to be set at 41A, not 52A.

Cheers, Wayne

 

[jaggedben]

Thanks for your input. It's not a single 225A 3 pole breaker or a single 37.5kvA transformer. It's (4) transformers, (4) 225A double pole breakers.
Regarding the ESS max continuous output current, plans state 'Tesla PCS controlled setting is set to 52A at the conductor at line side of the Tesla Backup Gateway'. It's not clear.
This is how the system sets up:
ESS---Gateway---37.5kvA transformer---800A panel

The PCS controlled current setting would need to be 1/1.25 or 80% of that. i.e. about 41.6A

Are the transformers actually single phase like we said?
Are the 225A breakers sized for transformer inrush, or for loads?
There will be four backup micro-grids? Just clarifying. This strikes me as an odd system, but I suppose there could be good reasons for it.

 

[fandi]

OK, that's just a crazy design, I bet someone on the design side has made the drawing based on a misunderstanding. You could use (4) separate transformers, but you'd use single phase transformers sized on the basis of 240V * 52A (or less, see below) = 12.5 kVA. And the primary side OCPD for each transformer would then be sized on the basis of the the 60A equivalent at 208V.

Having said that, there's no code violation for oversizing the transformers, primary OCPD and primary conductors by a factor of 3.


See the last sentence of 705.12 first paragraph: "Where a power control system (PCS) is installed in accordance with 705.13, the setting of the PCS controller shall be considered the power-source output circuit current in 705.12(A) through (E)."

So each PCS needs to be set at 41A, not 52A.

Cheers, Wayne

You're right.

 

[jaggedben]

The PCS controlled current setting would need to be 1/1.25 or 80% of that. i.e. about 41.6A

Alternatively, the controlled current settings could be set different on each system, e.g. lower on one or both of the A-B connected systems.

 

[jaggedben]

So when they say power source(s) they are not including the utility in it until the main breaker.

Correct. In context, 'power source' in article 705 refers to non-utility sources. They could spell it out better but see the scope in 705.1 as well as the definition of Power Production Equipment in Article 100.

Article 705 used to refer specifically to 'utility interactive inverters' instead of 'power production equipment' or 'power sources'. That changed in I think 2017 when I guess they wanted to make the rules the same for non-inverter sources as well.

I also though that the solar OC protection is also protecting the bus bar so it is included in that phrase.

They sort of do that, but not really, and that's not what the language refers to. I agree the wording isn't as clear as it could be. It makes more sense if you've seen how the code has changed over time.