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

[Carultch]

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.

There is a difference between SLASH RATED and STRAIGHT RATED breakers. Most single pole breakers are slash rated, meaning they are rated for 120/240V.

A breaker connected to the neutral and the high leg will experience 208Volts. If it is slash rated, it is not rated to exceed 120 Volts to ground. Even if it is rated for 240V to the neighboring phase.

It is rare that the high leg is usable. If it is, you would need a straight rated breaker. Rated at 240 volts no matter what.

All 3-pole breakers are straight rated, but not all 2-pole or 1-pole breakers are. Straight rated 2-pole breakers would be a lot more common than straight rated 1-pole.

 

[fandi]

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?

I think it would be 16.
150kW/7.2kW = 20.833.
due to the fact that the evcs are continuous loads:
80% x 20.833 = 16.666 so the max is 16 stations.

 

[Smart $]

I think it would be 16.
150kW/7.2kW = 20.833.
due to the fact that the evcs are continuous loads:
80% x 20.833 = 16.666 so the max is 16 stations.

125% continuous load factoring is 1) to meet UL breaker rating requirements, and 2) padding to reduce conductor insulation degradation.

On the other hand, transformers are rated for their ability to sustain a continuous load at specified kVA while their maximum protection ratings are permitted at 125% of their rated current with upsizing permitted to next higher standard size. So a 150kVA transformer can handle 150kVA continuous load (180.42A PRI, 360.84A SEC rated currents) while protected by up to a 250A primary ocpd or 500A secondary ocpd.

 

[fandi]

125% continuous load factoring is 1) to meet UL breaker rating requirements, and 2) padding to reduce conductor insulation degradation.

On the other hand, transformers are rated for their ability to sustain a continuous load at specified kVA while their maximum protection ratings are permitted at 125% of their rated current with upsizing permitted to next higher standard size. So a 150kVA transformer can handle 150kVA continuous load (180.42A PRI, 360.84A SEC rated currents) while protected by up to a 250A primary ocpd or 500A secondary ocpd.

Your transformer protection calculation is right but how do you come up with 18 stations though?

 

[Smart $]

Your transformer protection calculation is right but how do you come up with 18 stations though?

Technically you could go to 19 as calculated line currents would be 338.1A, 338.1A, 311.8A, but if you go to 20, the calculated line currents would be 363.7A, 338.1A, 338.1A... exceeding transformer rated current by almost 3A on one leg.

I should note there is no NEC provision which explicitly prohibits this. The only one that could would be an interpretation of 110.3(A) to this effect.

 

[fandi]

Technically you could go to 19 as calculated line currents would be 338.1A, 338.1A, 311.8A, but if you go to 20, the calculated line currents would be 363.7A, 338.1A, 338.1A... exceeding transformer rated current by almost 3A on one leg.

I should note there is no NEC provision which explicitly prohibits this. The only one that could would be an interpretation of 110.3(A) to this effect.

Sorry for my ignorance but can you elaborate how do you come up with 'calculated line currents would be 338.1A, 338.1A, 311.8A'?

Here's my calc for 19 stations:
input power = 7.2kVA at 240V.
7.2 x 1.25 = 9 kVA (continuous loads)
9 x 19 = 171 kVA > 150kVA (of the transformer).

 

[Smart $]

Sorry for my ignorance but can you elaborate how do you come up with 'calculated line currents would be 338.1A, 338.1A, 311.8A'?

Here's my calc for 19 stations:
input power = 7.2kVA at 240V.
7.2 x 1.25 = 9 kVA (continuous loads)
9 x 19 = 171 kVA > 150kVA (of the transformer).

Your calc's assume a balanced load. It is not possible to balance 19 or 20 units. To calculate that way, you should fictitiously pad your load to the next greater multiple of 3 (21 units).

You are also factoring your loads by 125% for continuous. While Code requires you to factor the load by 125% for conductor ampacity and OCPD rating determination, it is not required for comparison with transformer rating. The actual continuous load is still just 100%... and transformer ratings are based on continuous load. So if you factor your loads at 125%, then you should also factor your transformer rating at 125%... or just use non-factored load compared to non-factor transformer rating.

As to calculating line current for unbalanced loading without padding as mentioned above, you have to calculate each line current separately. The conventional method is to add up half the load values connected to that leg then divide by line-to-wye-neutral voltage. A delta system does not have a line-to-wye-neutral voltage, so you use a virtual voltage of 240V/sqrt(3) [138.564V]. Doing it this way will result in values close to what I posted. The values I posted are results of doing vector calculations.
http://forums.mikeholt.com/showthread.php?t=167171&p=1655882#post1655882

 

[fandi]

Your calc's assume a balanced load. It is not possible to balance 19 or 20 units. To calculate that way, you should fictitiously pad your load to the next greater multiple of 3 (21 units).

You are also factoring your loads by 125% for continuous. While Code requires you to factor the load by 125% for conductor ampacity and OCPD rating determination, it is not required for comparison with transformer rating. The actual continuous load is still just 100%... and transformer ratings are based on continuous load. So if you factor your loads at 125%, then you should also factor your transformer rating at 125%... or just use non-factored load compared to non-factor transformer rating.

As to calculating line current for unbalanced loading without padding as mentioned above, you have to calculate each line current separately. The conventional method is to add up half the load values connected to that leg then divide by line-to-wye-neutral voltage. A delta system does not have a line-to-wye-neutral voltage, so you use a virtual voltage of 240V/sqrt(3) [138.564V]. Doing it this way will result in values close to what I posted. The values I posted are results of doing vector calculations.
http://forums.mikeholt.com/showthread.php?t=167171&p=1655882#post1655882

Thanks a lot Smart. You're so smart.