Question on 210.20(a)

[Provision&installation]

I'm looking to install a new sub panel for multiple EV charging stations and I'd like to maximize the amount of stations the customer can install. The chargers require (2) 40a 208v single phase circuits which makes staying under 80% capacity and balancing the load for maximum capacity a bit difficult.

Ideally, if we can get away with running the panel at 100% I'd like to do it.

Please let me know if I'm understanding the exception below, specifically the underlined portion. The way I'm reading it, if we install a 400 amp panelboard rated for 100% load with main breaker rated at 100%,i still need all of my branch circuit OCPD's to also be rated at 100% even if they are only loaded to <80%? I want to load the panel feeders and main breaker at 100%, not the individual branch circuits, but it looks like a catch-22 based upon my understanding of the code section below. I haven't asked my supplier if 40A breakers rated for 100% use are available, but after a brief googling it doesn't look like they are.

"
As explained in Section 210-20(a) of the 1999 National Electrical Code
(NEC):
�Where a branch circuit supplies continuous loads or any combination of
continuous and noncontinuous loads, the rating of the overcurrent device
shall not be less than the noncontinuous load plus 125 percent of the
continuous load.


�Exception: Where the assembly, including the overcurrent devices
protecting the branch circuit(s), is listed for operation at 100 percent of its
rating, the ampere rating of the overcurrent device shall be permitted to be
not less than the sum of the continuous load plus the noncontinuous load.�

 

[jcleavit]

210.20 only applies to the branch circuit. Look at 215.2(A)(1)(a) and the exception. This specifies only the assembly and the OCPD protecting the feeders be rated for 100%. So you might need to verify that the assembly your feeders originate from is rated at 100% but i think you can use 210.20(A) as it is written and not dip into the exception.

 

[Provision&installation]

210.20 only applies to the branch circuit. Look at 215.2(A)(1)(a) and the exception. This specifies only the assembly and the OCPD protecting the feeders be rated for 100%. So you might need to verify that the assembly your feeders originate from is rated at 100% but i think you can use 210.20(A) as it is written and not dip into the exception.

Doh, I completely overlooked the fact that 210.20 refers to branch circuits and not feeder.

My feeders will originate from a 112.5KVA transformer, which brings in an entirely new set of questions for me and leads me to believe I am going cart before the horse with this thread. The secondary rating of the transformer should be 112,500 / 360.356 = ~ 312 amps. 125% of that is what gets me to 400 amps for my panelboard. However, if i load the panelboard to 100% it seems i will be overloading the transformer by over 25%.

I think rather than trying to come up with different scenarios for how many chargers the customer can run i just need to get a straight answer on how many they will be using and go from there.

 

[Smart $]

... The chargers require (2) 40a 208v single phase circuits which makes staying under 80% capacity and balancing the load for maximum capacity a bit difficult. ...

Who is saying the chargers require (2) 40A 208V 1Ø circuits?

The NEC requires the OCPD for both feeders and branch circuits to be sized for continuous duty and rated 125% of the maximum load [625.40—2014]. No exception is provided for 100%-rated equipment.

That said, if the (2) 40A circuits meet that criteria, you are technically not loading your transformer more than 32A per circuit.

 

[Provision&installation]

Who is saying the chargers require (2) 40A 208V 1Ø circuits?

The NEC requires the OCPD for both feeders and branch circuits to be sized for continuous duty and rated 125% of the maximum load [625.40—2014]. No exception is provided for 100%-rated equipment.

That said, if the (2) 40A circuits meet that criteria, you are technically not loading your transformer more than 32A per circuit.

The manufacturer's specification sheet calls for (2) 40a single phase 208 circuits per unit and states that the actual load is 30 amps per circuit.

My load calc was using 3600VA across (2) phases for each unit.

This only allows me to run 38,400VA per phase for a maximum load of 320 amps (320 x 1.25 = 400) This allows me to run a total of (7) charging station with 2 phases loaded to 36,000VA and one loaded at 28,800. I cannot add another without exceeding 320 amps any any given phase.

 

[Smart $]

The manufacturer's specification sheet calls for (2) 40a single phase 208 circuits per unit and states that the actual load is 30 amps per circuit.

My load calc was using 3600VA across (2) phases for each unit.

This only allows me to run 38,400VA per phase for a maximum load of 320 amps (320 x 1.25 = 400) This allows me to run a total of (7) charging station with 2 phases loaded to 36,000VA and one loaded at 28,800. I cannot add another without exceeding 320 amps any any given phase.

Ummm... doesn't 30A per circuit equate to 60A per unit? That's 12,480VA per unit. 112,500VA transformer rating will handle 9 units (which can be balanced).

9 units × 60A/unit × 208V = 112,320VA
112,320VA ÷ 208 ÷ sqrt(3) = 312A

 

[Chamuit]

Ummm... doesn't 30A per circuit equate to 60A per unit? That's 12,480VA per unit. 112,500VA transformer rating will handle 9 units (which can be balanced).

9 units × 60A/unit × 208V = 112,320VA
112,320VA ÷ 208 ÷ sqrt(3) = 312A

+1

Those are the numbers I came up with.

 

[Provision&installation]

Ummm... doesn't 30A per circuit equate to 60A per unit? That's 12,480VA per unit. 112,500VA transformer rating will handle 9 units (which can be balanced).

9 units × 60A/unit × 208V = 112,320VA
112,320VA ÷ 208 ÷ sqrt(3) = 312A

Maybe I'm looking at it all wrong. I have a panel schedule load calc spreadsheet that has inputs for amperage. it multiplies amps x 120 to fill the VA column. At the bottom of the sheet it takes demand KVA/.360 and spits out amps. I am inputting 30a per phase over 4 phases for each unit.

For my 75KVA/225A load calc i have 4 charging units with a connected load of (A)21,600, (B)21,600, (C)14,400. These are all continuous loads which are multiplied by 1.25 to which comes to 72,000. It then takes 72kva/.360 and spits out 200 demand amps.

sorry for changing it up to 75kva, i've been going back and forth between the two trying to figure out which is the better value and that's the spreadsheet i had in front of me.

 

[Provision&installation]

 

[Smart $]

Maybe I'm looking at it all wrong. I have a panel schedule load calc spreadsheet that has inputs for amperage. it multiplies amps x 120 to fill the VA column. At the bottom of the sheet it takes demand KVA/.360 and spits out amps. I am inputting 30a per phase over 4 phases for each unit. ...

Your spreadsheet is not set up to properly calculate line-to-line loads...

 

[Provision&installation]

Sorry about the poor image quality, I seem to be struggling to upload the image properly.

 

[Provision&installation]

Your spreadsheet is not set up to properly calculate line-to-line loads...

View attachment 17452

Attached linky no worky.

 

[Smart $]

Sorry about the poor image quality, I seem to be struggling to upload the image properly.

Forum programming has major tendency to downsize images...

Attached linky no worky.

...which is why my post temporarily did not display. I had uploaded image and it got downsized to the point it was illegible... so I uploaded to another server and linked it in the post.

 

[wwhitney]

Maybe I'm looking at it all wrong. I have a panel schedule load calc spreadsheet that has inputs for amperage. it multiplies amps x 120 to fill the VA column. At the bottom of the sheet it takes demand KVA/.360 and spits out amps. I am inputting 30a per phase over 4 phases for each unit.

That right there is the issue: 4 * 30A * 120V = 14,400 VA, but the calculation should be 2 * 30A * 208V = 12,480 VA. You are off by a factor of 2 / sqrt(3).

Cheers, Wayne