480/240V 3-phase 4-wire center side ground delta

[beanland]

I find no mention in the 2008 NEC of 240/480V 3-phase 4-wire center-side grounded delta services. 100 "Voltage Nominal" mentions 120/240V, 480Y/277V amnd 600V. 100 "Voltage to Ground" mentions 208Y/120, 480Y/277, and 480V 3-phase 3-wire. The Handbook also talks about 120/240V 3-phase 4-wire delta. 220.5 mentions 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347 and 600V. I find no mention of 480/240V. Is 480/240V 3-phase 4-wire center-side ground permitted? Or, since it is not mentioned, it is not forbidden?

 

[Smart $]

I find no mention in the 2008 NEC of 240/480V 3-phase 4-wire center-side grounded delta services. 100 "Voltage Nominal" mentions 120/240V, 480Y/277V amnd 600V. 100 "Voltage to Ground" mentions 208Y/120, 480Y/277, and 480V 3-phase 3-wire. The Handbook also talks about 120/240V 3-phase 4-wire delta. 220.5 mentions 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347 and 600V. I find no mention of 480/240V. Is 480/240V 3-phase 4-wire center-side ground permitted? Or, since it is not mentioned, it is not forbidden?

TTBOMK, there is no such standard system.

 

[LarryFine]

I find no mention in the 2008 NEC of 240/480V 3-phase 4-wire center-side grounded delta services.

That's because we have no 240v line-to-neutral loads.

The high-leg Delta (especially the open Delta) started out as a modification to existing 1ph services.

 

[beanland]

240/480V 3ph 4-wire delta

240/480V 3ph 4-wire delta

The 240/480V delta bank is actually common in the rural west. It is a common way to provide 480V irrigation pump service. You do not want an ungrounded 480V service because of transient voltage spikes passing through the transformers and nailing the equipment. Corner ground 480V is generally frowned upon. So, to assure a good reliable fault return path, the center-side is grounded. It is just like a 120/240V 3ph 4w service just at twice the voltage. Still, it seems that whoever wrote the NEC was unaware of its existance.

 

[elohr46]

The 240/480V delta bank is actually common in the rural west. It is a common way to provide 480V irrigation pump service. You do not want an ungrounded 480V service because of transient voltage spikes passing through the transformers and nailing the equipment. Corner ground 480V is generally frowned upon. So, to assure a good reliable fault return path, the center-side is grounded. It is just like a 120/240V 3ph 4w service just at twice the voltage. Still, it seems that whoever wrote the NEC was unaware of its existance.

I just googled "center side delta" and nothing came up. Would it be possible for you to post a drawing of that configuration? I've seen corner grd. delta configurations used for oil pump systems, irrigation pump systems, etc. but was unaware of what you mentioned. Thanks.

 

[beanland]

Sketch

Sketch

Typical transformer bank. Three 7200V-240x480V transformers. Primary star point is left floating.

 

[Smart $]

I just googled "center side delta" and nothing came up. Would it be possible for you to post a drawing of that configuration? I've seen corner grd. delta configurations used for oil pump systems, irrigation pump systems, etc. but was unaware of what you mentioned. Thanks.

Search on 240/120V 3? Delta... it is wired the same but only half the voltage under discussion.

 

[LarryFine]

Still, it seems that whoever wrote the NEC was unaware of its existance.

Same here, until just now.

 

[charlie]

. . . Still, it seems that whoever wrote the NEC was unaware of its existence.

You are the "whoever" that wrote the NEC. Generally, the code making panels just accept or reject the proposals that are made.

 

[hurk27]

Sounds like a good time to put in a proposal for it:grin:

Unless it is such a rarity that it's not needed?

 

[LazyS]

Just ran into it last week in West Texas on a irrigation pump. Replacing a meter base that was hit by lighting and one of my guys called saying he had no idea what he was looking at. It took me a while to realize what it was but according to the POCO its halfway common.

 

[charlie]

By the way, if the code doesn't prohibit it, it is permitted.

Also, for overhead transformer banks that provide a delta secondary and three transformers are used, the center point of the wye primary is normally floated.