Hey there...I am currently working out refeeding a line of 208volt battery chargers (on 30amp two pole breakers) with a system that is standard three phase. The chargers are presently fed from a three phase system that contains a high leg. All circuit breakers feeding the chargers are attached to the high leg and a standard leg providing 208volts. If I make this switchover to standard three phase power will my load increase on the new panel? Its an old system and it seems that all chargers were purposely attached to the high leg. I'm not sure why and therefore I'm a bit suspicious about feeding from standard voltage.

 

The high leg system you described is a way of providing for 120 and 208 volt loads from a 240 volt delta transformer equipped with a "lighting tap" or a "neutral" on one of the secondary windings.

Your three phase system should therefore be 240 volts and the chargers would have to be single phase and should be connected to the high leg (normally, by convention, the "B" phase) and the neutral. It worries me that you say they are connected to the "high leg" and a "standard leg" to get 208 volts. You need to verify what is connected to what. The misunderstanding might be semantic in nature.

Powering these 208 volt units from a 240 volt source could do all sorts of things to them and to the batteries connected to them. Check the units to see if the transformer in the charger has "taps" that would allow you to change the primary voltage requirement of the charger.

As to the load on the system... power is power is power... whether it is at 208 or 240 volts, the demand will be the same.

Best Regards...

Rick Kelly

Regional Electrical/Elevator Inspector and Senior Plans Review Officer,
Qikiqtaaluk Region (Baffin)

Safety Services C&GS Protection Services Government of Nunavut

Phone (867) 975-5419
Fax (867) 975-5453

 

Responding to Anonymous' Jun 3, 10:31 am query:

The information you provided is not clear regarding your load distribution. Under the old hi-leg system one can only presume that the ratio of single-phase load kVA to transformer's kVA capacity, was not too large!

That said, I agree with Richard Kelly's Jun 5, 10:10 pm, admonition! Make sure the new single-phase loads do not exceed the capacity of the new? (3-ph, 4w, 208y/120) transformer windings. A reminder, each winding's capacity is 1/3 of the total rating. Be especially cautious if
distribution is delta-connected, i.e, 3-ph, 3w, 240V!

A single-line diagram would be helpful!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

After discussing this with some American colleges (I am in Canada), they have told me that it is against the NEC to use the high leg / neutral pair to power 208 volt loads. This practice is not against the Canadian code. Further investigation on your part might be wise.

Best Regards...

Rick Kelly

Regional Electrical Inspector and
Senior Plans Review Officer,
Qikiqtaaluk Region (Baffin)

Safety Services
C&GS Protection Services
Government of Nunavut

Phone (867) 975-5419
Fax (867) 975-5453

 

Responding to Richard Kelly's Jun 13, 12:16 am, comment... such practice is permitted by NEC!

Details are provided in Article 250, section 250.20(B)(3) and Article 250.26. Perhaps the local authority having jurisdiction has not allowed it!

Regards,
Phil Corso, PE {Boca Raton, FL, USA}
[[email protected]] ([email protected])

 

Hi Phil...

Thanks for the comment on this issue. I must admit that I was surprised when the American officials I was discussing this issue with (on another industry forum) said it was a practice, the high leg neutral pair feeding a 208 volt load, that was not condoned by the NEC. It used to be a common practice here in Canada in older neighborhoods. Over time they have slowly been replaced with 208/120 4w Y systems.

Brings me to a pet peeve, there are, at least IMHO, to many AHJ making there own "rules" as they go. The codes, as written, are meant to be prescriptive in nature and not subjective. The rewriting of the various codes is best left to the bodies responsible for them. Not the individual AHJ in the field.

Best Regards...

Rick Kelly

Regional Electrical Inspector and
Senior Plans Review Officer,
Qikiqtaaluk Region (Baffin)

Safety Services
C&GS Protection Services
Government of Nunavut