Deriving A 3-Phase, 3-Wire Service

[Jbmyers09]

Hi All,

I used to work at a company that would do dual-services to a critical facility with two utility switchgear (A & B) and generator paralleling switchgear (A & B) to provide standby power. To avoid the cost of a modified differential ground fault system (MDGF), they'd only provide a 3-wire system but the utility would only provide a wye-connected transformer secondary. Based on my understanding of the NEC, section 250.24(C) requires you to bring the neutral into your service disconnect. This is probably a stupid question but do you just bond the neutral to the ground bus in the switchgear and keep it from being distributed throughout your system and therefore it's technically a 3-wire system? How else would you do it? Based on my understanding / experience, delta-connected secondaries are not really used anymore since they'd be corner-grounded and you'd have a wild-leg phase which can be problematic if you aren't meticulously labeling equipment.

Any feedback would be greatly appreciated. Thank you.

 

[zbang]

To avoid the cost...

IMNSHO, any company who wants that much redundancy/etc shouldn't be trying to skip out of the necessary elements.

 

[jim dungar]

Hi All,

I used to work at a company that would do dual-services to a critical facility with two utility switchgear (A & B) and generator paralleling switchgear (A & B) to provide standby power. To avoid the cost of a modified differential ground fault system (MDGF), they'd only provide a 3-wire system but the utility would only provide a wye-connected transformer secondary. Based on my understanding of the NEC, section 250.24(C) requires you to bring the neutral into your service disconnect. This is probably a stupid question but do you just bond the neutral to the ground bus in the switchgear and keep it from being distributed throughout your system and therefore it's technically a 3-wire system? How else would you do it? Based on my understanding / experience, delta-connected secondaries are not really used anymore since they'd be corner-grounded and you'd have a wild-leg phase which can be problematic if you aren't meticulously labeling equipment.

Any feedback would be greatly appreciated. Thank you.

You have a solidly grounded Wye system and service, the NEC does not care that you are only using 3 wires for your loads.
This is why High Resistance Grounding is popular for these types of installations.

 

[David Castor]

If you have no line-to-neutral loads, then you can get a small neutral bus in the service entrance compartment in order to make the main bonding jumper. You don't have to run a neutral bus everywhere. As far as the utility is concerned it's a four-wire service, but they don't actually care. We used to try to avoid 480/277 V systems due to costs associated with running the neutral everywhere. Not many 277 V loads other than lighting, and with LED lights, not much reason to use 277 V for lighting for the most part. We installed small 480-480/277 V transformers if 277 V was wanted. This isolated the lighting systems from the Ground Fault protection on the main system, eliminating a lot of coordination issues.

 

[augie47]

Unsure if entertaining this option is reasonable....or compliant.
In the 60s a great number of the industrial facilities in this are had ungrounded systems fed by pole transformers,. Ad the power companies switched to pads which all had Y secondaries, in many cases they (POCOs) simply left the neutral terminal unbonded in the pad and continued to supply the customer with an ungrounded system.

So much equipment now is not compatible with ungrounded or corner grounded system I never see those services. Solid grounded and High-Resistance Grounded is all I see. That seems to be the most recommended by engineers in this area.

 

[David Castor]

Ungrounded 480 V delta systems were universally used in industrial facilities until the early 1960s. But high rates of motor winding failures and overvoltage issues caused a fairly quick transition to solidly grounded wye systems. While there are still ungrounded 480 V delta systems in service, for anything new, solidly-grounded or high-resistance grounded should really be used.