code violation for feeding 240/208V EV chargers from the 'high leg' of delta xformer?

[fandi]

Hello All,
I have an existing 150kVA delta - delta 480V-240/208/120V. The secondary is center grounded between phase A and C (Van=Vcn=120V). Phase B is usually called 'high leg' (Vbn=208V).


The EVCS (Electric Vehicle Charging Station) is Leviton ever-green Level 2 charger using SAE J1772 standard with this Elec input info:
- Input power: 7.2kW.
- Input voltage: 208/240VAC.
- Input current: 30A.
- Input power connections: Line 1, Line 2, Earth. (I see Neutral mentioned in their cutsheet for Level 1 but not for this Level 2 which is the product will be used in this case)
- Recommended breaker: 40A double pole breaker on a dedicated circuit.


What I'm getting at is, from the 150kVA transformer secondary via a 400A 3 phase, 4 wire panel, I can use 40A two pole breaker between phases AB, BC and CA (240V) to feed the EVCS. And I don't have to worry anything about the fact that it's a delta transformer with 'high leg' phase B.
I need to contact the manufacturer to see how efficient charging station is but for now let's assume 1kVA = 1kW.
This is the maximum charging stations I can hook up to the 150kVA transformer: 150/7.2 = 20.8333 so the max is 20 stations.




Please let me know if my calculation is wrong. Thanks.

 

[infinity]

I let someone else handle the calculations but a 2 pole CB on AB, BC or AC is OK.

 

[Smart $]

Actually the max you can hook up is 24... 8 per line pair. You can go up to 125% because the EVCS' are not continuous loads. Whether to drop to 7 per line pair is dependent on likelihood of all 24 drawing max current at the same time. 7 per line pair drawing max concurrently shouldn't be a problem.

 

[gar]

150607-1217 EDT

fandi:

Around here wild leg deltas are usually one large center tapped transformer for single phase loads with a single smaller transformer to supply the wild leg. Lighting load is generally much greater than air conditioning load.

Why did you even mention 208? You do not want to use the 208 from a wild leg system.

Provide more detail on the three phase transfomers.

If you had a single 100 kVA transformer plus a 50 kVA to the wild leg, then you could put up to 100 kVA on the first one and up to 50 kVA on the other. This could be modified by continuous or non-continuous.

If there were three 50 kVA transformers connected delta, then each 50 kVA could be loded to 50 kVA.

.

 

[fandi]

Actually the max you can hook up is 24... 8 per line pair. You can go up to 125% because the EVCS' are not continuous loads. Whether to drop to 7 per line pair is dependent on likelihood of all 24 drawing max current at the same time. 7 per line pair drawing max concurrently shouldn't be a problem.

I believe it takes more than 3 hours for a level 2 charging station to fully charge an EV so I think it's continuous load.

 

[fandi]

150607-1217 EDT

fandi:

Around here wild leg deltas are usually one large center tapped transformer for single phase loads with a single smaller transformer to supply the wild leg. Lighting load is generally much greater than air conditioning load.

Why did you even mention 208? You do not want to use the 208 from a wild leg system.

Provide more detail on the three phase transfomers.

If you had a single 100 kVA transformer plus a 50 kVA to the wild leg, then you could put up to 100 kVA on the first one and up to 50 kVA on the other. This could be modified by continuous or non-continuous.

If there were three 50 kVA transformers connected delta, then each 50 kVA could be loded to 50 kVA.

.

The reason I mentioned 208V because that 150kVA transformer is capable of producing 240/208/120V.
Can you elaborate the reason why I don't want to use 208V from a wild leg system? I thought it's there to feed some 208V single phase loads?
Anyway, 208V will not be used because the EVCS loads will be fed from 40A double pole breakers with the voltage is 240V.

 

[ActionDave]

I believe it takes more than 3 hours for a level 2 charging station to fully charge an EV so I think it's continuous load.

A continuous load has to run at its max for three hours. I don't know of any chargers that do this.

 

[GoldDigger]

Can you elaborate the reason why I don't want to use 208V from a wild leg system? I thought it's there to feed some 208V single phase loads?

It is there only to power three phase loads, such as motors or rectifiers.
The problem with using a L to N load off the high leg is that the transformer (usually only one pot for an open delta) is not sized for much extra load and the circuit is completed through the windings of the 120-N-120 pot, which could overload that pot and/or increase voltage drop.
If you have a true three phase delta transformer with one winding center tapped, you still need to know if all three windings have the same design load.

With the right design and planning it can be done, but there is little reason for it.

 

[fandi]

It is there only to power three phase loads, such as motors or rectifiers.
The problem with using a L to N load off the high leg is that the transformer (usually only one pot for an open delta) is not sized for much extra load and the circuit is completed through the windings of the 120-N-120 pot, which could overload that pot and/or increase voltage drop.
If you have a true three phase delta transformer with one winding center tapped, you still need to know if all three windings have the same design load.

With the right design and planning it can be done, but there is little reason for it.

Thanks for your input. The delta transformer with 'high' leg is existing so there's not much I can do.
So is there any problems if I hook up all (20) 208/240V EVCSs with the transformer's AB, BC, CA 240V phases (to utilize all the transformer capacity)?

 

[GoldDigger]

Thanks for your input. The delta transformer with 'high' leg is existing so there's not much I can do.
So is there any problems if I hook up all (20) 208/240V EVCSs with the transformer's AB, BC, CA 240V phases (to utilize all the transformer capacity)?

That would be the best way to balance the added load. You should probably do a quick load analysis of the existing loads on the center tapped winding (A-N-C) to see whether you would be better off putting more of the new loads on the other two phases. (AB and BC)
But like I said, first confirm that all three windings of the transformer are the same wire size. If not, then look up the load capacity of each winding.

 

[fandi]

That would be the best way to balance the added load. You should probably do a quick load analysis of the existing loads on the center tapped winding (A-N-C) to see whether you would be better off putting more of the new loads on the other two phases. (AB and BC)
But like I said, first confirm that all three windings of the transformer are the same wire size. If not, then look up the load capacity of each winding.

Thanks a lot. Here's my last question: if it's a true three phase delta transformer with all three windings have the same wire size, what happen if I can't balance the loads on AB, BC and CA but each winding is still not overloaded?

 

[Smart $]

I believe it takes more than 3 hours for a level 2 charging station to fully charge an EV so I think it's continuous load.

As noted by ActionDave, each EVCS would have to draw the 7.2kW continuously for more than 3 hours. The nature of battery charging demands the charging current level drop off as the battery charges. I'm not familiar with EVCS charging output voltage and current rates, but I have to assume a completely diminished battery cannot draw max EVCS current after 3 hours of restoration charging. Please advise if it would...

Thanks for your input. The delta transformer with 'high' leg is existing so there's not much I can do.
So is there any problems if I hook up all (20) 208/240V EVCSs with the transformer's AB, BC, CA 240V phases (to utilize all the transformer capacity)?

If you think exceeding 20 chargers is because it would slightly exceed the transformer's rating, then for that very same reason, you cannot install #19 and #20 because that would put your current on the common leg of those two at 363.7A, and the rated current is 360.8A.

Thanks a lot. Here's my last question: if it's a true three phase delta transformer with all three windings have the same wire size, what happen if I can't balance the loads on AB, BC and CA but each winding is still not overloaded?

See reply above.

 

[hmspe]

To use the "high leg" to neutral for a 208 volt load would require an appropriately rated circuit breaker. Most single pole breakers designed for 240 volt class panels are not rated for a 208 volt load.

 

[kwired]

To use the "high leg" to neutral for a 208 volt load would require an appropriately rated circuit breaker. Most single pole breakers designed for 240 volt class panels are not rated for a 208 volt load.

And most double pole circuit breakers are rated 120/240 and can not be used on the high leg either - needs to be a breaker with a straight 240 volt rating. Can't speak for other brands but Square D QO is not even stocked by my supplier in the straight 240 volt - and are not cheap either if you want to get some of them compared to the standard 2 pole breakers.

 

[kwired]

The reason I mentioned 208V because that 150kVA transformer is capable of producing 240/208/120V.
Can you elaborate the reason why I don't want to use 208V from a wild leg system? I thought it's there to feed some 208V single phase loads?
Anyway, 208V will not be used because the EVCS loads will be fed from 40A double pole breakers with the voltage is 240V.

High leg is not intentionally there to supply 208 volt loads, it is just something that is inherent to that configuration. Make a triangle with all three sided 240 units long (these units represent voltage when we draw this out). Now make a mark at the midpoint of one side of the traingle to represent where the grounded conductor would get connected. That point is 208 measuring units away from the far corner of the traingle. There is a constant ratio between those two points, try to make that distance 120 or 240 and you then mess with the distance between the corners. If we want 120 from each phase of an electric system to a common point - the 208/120 wye system is the way to do it with accepted standard components - but then you are giving up a little voltage on the phase to phase voltage. So the decision of which system to choose depends on what the majority of the load will be - or in some cases where utility companies are supplying with an "open delta" it can be to save cost on equipment as with the way a lot of them build their systems they would need all three primary phases plus the neutral for a full delta bank but can build an open delta with only two phase conductors and the neutral - saves cost for remote areas where they would have to spend extra on line costs to get all 4 lines there. Or sometimes there is a lot of 120/240 single phase load and very limited three phase load so they run the open delta with a larger 120/240 transformer and a smaller high leg transformer again mostly to reduce cost, both install as well as operational costs.

 

[gar]

150607-2117 EDT

fandi:

The 208 V voltage from a wild leg 240 V delta is happenstance of the secondary circuit. It is not something provided to give you a 208 voltage.

If you put a resistive load from the high leg to the secondary center tap (neutral for the grounded single phase loads), then there is 208 across the resistor and the current thru the resistor is in phase with the 208 voltage. But the 208 voltage is not in phase with either phase supplying the wild leg.

If there are two secondaries connected to the wild leg, then each supplies half of this current, but the secondary current ( phase current ) of each secondsary has to be greater than the current it supplies to the resistor because of the phase relationships between the phase voltage and the wild leg to neutral voltage. I quickly looked on the Internet for some vector drawings to illustrate the relationship, but did not find any. Possibly someone will create some for you.

There seems to be a lot of negative comment on two transformer high leg deltas on the Internet. My son's shop and his building (40,000 sq-ft) runs from one of these with no problems (I believe 150 kVA and 50 kVA). And as I mentioned before many businesses and some homes (two at least in my neighborhood) are supplied by two transformer high leg deltas.

.

 

[Smart $]

150607-2117 EDT

fandi:

The 208 V voltage from a wild leg 240 V delta is happenstance of the secondary circuit. It is not something provided to give you a 208 voltage.

If you put a resistive load from the high leg to the secondary center tap (neutral for the grounded single phase loads), then there is 208 across the resistor and the current thru the resistor is in phase with the 208 voltage. But the 208 voltage is not in phase with either phase supplying the wild leg.

If there are two secondaries connected to the wild leg, then each supplies half of this current, but the secondary current ( phase current ) of each secondsary has to be greater than the current it supplies to the resistor because of the phase relationships between the phase voltage and the wild leg to neutral voltage. I quickly looked on the Internet for some vector drawings to illustrate the relationship, but did not find any. Possibly someone will create some for you.

There seems to be a lot of negative comment on two transformer high leg deltas on the Internet. My son's shop and his building (40,000 sq-ft) runs from one of these with no problems (I believe 150 kVA and 50 kVA). And as I mentioned before many businesses and some homes (two at least in my neighborhood) are supplied by two transformer high leg deltas.

.

I don't think a vector diagram is necessary.

Look at it this way... say a 10A 208V line-to-neutral load. All 10A has to pass through one and a half winding (open delta), or 5A has to pass through one full winding and one half winding each (full delta), amounting either way to 10A?(240V+120V)=3600VA of transformer power rating consumed for 2080VA worth of load.

 

[Rampage_Rick]

Those EVSEs may not like being connected to the high leg, as many of them interface with the ground for proper operation of the internal GFCI. If they're only designed for 120V to ground they may not tolerate 208V to ground...

From Clipper Creek's installation manual:
Caution
With the delta connection, one leg must be center-tapped, and only the two phases on either side of the center tap can be used. The two phases must both measure 120V to neutral. The third line (L3) of the delta is 208V, with respect to neutral, and is sometimes referred to as a ?stinger?. Do not use this third line! Consult the transformer manufacturer?s literature to be sure the single leg can supply the required power.

 

[wwhitney]

As noted by ActionDave, each EVCS would have to draw the 7.2kW continuously for more than 3 hours. The nature of battery charging demands the charging current level drop off as the battery charges. I'm not familiar with EVCS charging output voltage and current rates, but I have to assume a completely diminished battery cannot draw max EVCS current after 3 hours of restoration charging. Please advise if it would...

Per NEC 625.14 (2011), the EVSE is considered a continuous load. As a practical matter, if you plug in a fully discharged Tesla, it will draw the maximum current for over 3 hours.

Cheers, Wayne

 

[Smart $]

Per NEC 625.14 (2011), the EVSE is considered a continuous load. As a practical matter, if you plug in a fully discharged Tesla, it will draw the maximum current for over 3 hours.

Cheers, Wayne

Thanks for pointing that out. I keep forgetting there is a section specifically for EVCS.

FWIW, 625.14 was removed in the 2014 edition, but 625.41 contains the "continuous load" language.

So as a result, the maximum he can connect is 18 with 6 per phase, right?