Service feed overcurrent protection for multi-unit EV charging

[brian.1]

My 20 unit condo building is adding EV chargers for all units and I'm looking for help managing the continuous EV charging load on my condo building's transformer and service feed wires.

There are 20 units each with a 2 phase 90A at 120/208V main breaker which gives a theoretical total of 1800A at 120/208V. IIUC that would be 1039A in 3 phase.

My condo building's service was set up long before EVs and EV charging was a consideration. As was standard at the time, the service was derated so the 3 phase service feed installed was 600A at 120/208V. The 600A is peak load. IIUC A continuous load can only be 80% of that which is 480A.

The plan is to limit each EV charging circuit current to 24A at 208V. This would give a total EV charging current of 480A at 208V which is just at the continuous rating for the service transformer and feed wires. This sounds great BUT the condos still need electrical power for other things like A/C, oven, lighting, etc. We could lower the EV charging current but how much? However, people are asking for more current not less. Besides, wouldn't it be rare for all EVs to be charging at the same time?

So 2 questions:

1. How to detect 480A continuous, not 480A peak?
2. How to disable or limit EV charging without interfering with a condo's normal power usage?

 

[tom baker]

I will approve this once the OP clarifies
1. What was occupation before retirement?
2. Who is doing the electrical work?
It’s a good question that will be of interest

(answered in a different thread...unlocked)

 

[brian.1]

I will approve this once the OP clarifies
1. What was occupation before retirement?
2. Who is doing the electrical work?
It’s a good question that will be of interest

(answered in a different thread...unlocked)

From the other thread:

• We will be using certified electricians. For insurance, it is important to have the work done by qualified people and done with permits.

 

[jaggedben]

If each unit were to draw 24A at 208V then the 3-phase line currents at the service would be approximately 277A which leaves you quite a bit of headroom without worrying about whether that full load would ever be continuous (which it almost certainly wouldn't, but you arguably still have to plan for).

You can also get units for each condo that would limit the EV charging draw if that unit were using a lot of power on other things. The jurisdiction will need to agree with the logic of what your doing in light of NEC load calculations, so limiting the draw on each unit to the existing 90A feeder rating may help.

There is a member on this forum @brycenesbitt who may be interested in helping you out. (Hope that's kosher to say here.) This sort of thing is his specialty and he's in the Bay Area.

 

[brian.1]

Ok, thank you for pointing out my 1st error: the 20 units × 24A 2 phase EV charging = 480A 2 phase would only be 480A ÷ 1.732 = 277A for 3 phase. That definitely helps but the condos could still be drawing other power.

For the individual condos, the EV charging is reduced if the current in the condo's wiring is above the 80% of 90A = 72A. Still, 20 units × 72A = 1440A 2 phase which is 1440 ÷ 1.732 = 831A 3 phase. So still seems wise to have monitoring and protection.

 

[brycenesbitt]

There is a member on this forum @brycenesbitt who may be interested in helping you out. (Hope that's kosher to say here.) This sort of thing is his specialty and he's in the Bay Area.

Yes, EV Plugbox LLC is my firm, and does specialize in just this, thanks for the shout out @jaggedben

We do all the electrical calculations, engineering, permitting, incentive matching and installation.

Now we can't call it "diversity" any more, but indeed the concept formerly known as load diversity is something a project can take advantage of. But the case is weaker for EV charging than other fields, as it's entirely credible that everyone will plug in overnight and set cars to start at midnight (due to mindbogglingly dumb rate structures with PG&E). You can't depend on luck for electrical calculations.

For EV Plugbox projects:
In Campbell, Cupertino, Gilroy, Los Altos, Los Altos Hills, Los Gatos, Milpitas, Monte Sereno, Morgan Hill, Mountain View, Saratoga, Sunnyvale and Unincorporated Santa Clara County, we have funding that can pay for much or in some cases all of an installation EVEN for HOAs. Most incentive funding excludes all reserved parking, thus don't play well with the HOA ownership model.

 

[brian.1]

Yes, EV Plugbox LLC is my firm, and does specialize in just this, thanks for the shout out @jaggedben

We do all the electrical calculations, engineering, permitting, incentive matching and installation.

Now we can't call it "diversity" any more, but indeed the concept formerly known as load diversity is something a project can take advantage of. But the case is weaker for EV charging than other fields, as it's entirely credible that everyone will plug in overnight and set cars to start at midnight (due to mindbogglingly dumb rate structures with PG&E). You can't depend on luck for electrical calculations.

Do you have a long term way to measure the building service feed's continuous current separately from the peak current?

If so, what does the system do if the continuous current is too high?

 

[brian.1]

While I am waiting for thoughts from others, let me share my idea. I am not an expert in this area so my idea may or may not be a suitable idea and I'd appreciate constructive feedback.

The overcurrent monitors I've seen do not seem sufficient. They can trip if the current is above a set level but the ones I've seen only have a few seconds on-delay for motor starting. My understanding is the 80% overcurrent limit is for loads that run longer than 3 hours. If anyone knows of an overcurrent monitor that has a long on-delay please let me know.

Otherwise, here is my idea to detect continuous overcurrent (which is below the breaker limit):

• Use an over current monitor to detect current above the 80% level.
• Use an on-delay relay timer to register that the over current has been happening continuously for a while.

The 3 current transformers step down the 3-phase current to a range suitable to the current monitor.

The current monitor (eg, a Time Mark 2742-24) is set to trip when any of the 3-phase currents is above the 80% continuous current level. However, this trips if the current is over the 80% limit for more than 20 seconds. What is needed is to detect an over 80% for a longer length of time. For this I propose that the signal from the overcurrent monitor be passed to an on-delay relay timer.

When the signal to the on-delay relay (eg, a THL-8024U-42) is on, the on-delay relay starts a timer. Any time the signal drops the timer resets. Only if the signal is continuously on for the on-delay time does the output of the on-delay trigger. This only would detect over 80% and not detect how much over the 80% the current is.

I'm not sure what the on-delay time should be. I guess it could be 3 hours but because the amount over 80% is not measured, I'm wondering if the time should be shorter. Maybe an hour? Or 1/2 an hour?

This only detects the continuous overcurrent and does not address what should happen in response. Right now I will leave that discussion for later.

I welcome constructive feedback.

 

[brycenesbitt]

Do you have a long term way to measure the building service feed's continuous current separately from the peak current?

yes, with some buts.
our local jurisdiction tag locks the compartment with the conductors.
we are working on getting a formal rule change to allow access to place ct clamps in what's called the green book.

 

[brian.1]

yes, with some buts.
our local jurisdiction tag locks the compartment with the conductors.
we are working on getting a formal rule change to allow access to place ct clamps in what's called the green book.

Thank you. That is very interesting information.