3-Phase Delta 4W Service upgrade concerns

[william_87]

I am working on a commercial building, and planning a service upgrade from a 400amp delta system with phase-b grounded (high-leg) to an 800 amp system. While doing this upgrade I will also be doing some panel additions and change arounds within the building.. This may be a bit naive, but I this is the first time working with a high-leg system and just wanted to double check a couple concerns before proceeding

1) does this system work like a standard ungrounded 3-phase system when using 3-pole breaker for equiptment

2) Is there anything else I should be aware of besides avoiding phase-b when doing standard 120v connections

3) Can the wild leg be used when doing a 2-pole breaker for 240v single phase applications.

Again I'm new to the delta grounded systems so I apologize if these are dumb questions

 

[don_resqcapt19]

First we need to understand exactly what system you have. A corner grounded system does not have a high leg. I just has one of the phases connected to earth and is a 3 wire system. A delta high leg system is a 4 wire system.

 

[Smart $]

As Don noted, a grounded [B phase] delta is only a 3 wire system.

If you have 4 wires, you likely have a 240/120V 3Ø 4W [delta high leg] system. The grounded neutral conductor is a center tap to the A-C winding. The "B" high leg to the grounded neutral should measure approximately 208V. Note any breaker connected to "B" phase must be straight rated for 240V, i.e. it cannot be a slash-rated 120/240 breaker. Circuit wise, you can use the "B" leg for:

• 240V 3Ø 4W
• 240V 3Ø 3W
• 240V 1Ø 2W

...but nothing 208V or 120V.

 

[electrofelon]

1. Yes
2. Keep in mind that because of having to avoid the b phase for certain loads, you nay need more breaker spaces than you think. Also balancing the whole system out can be a little more tricky. As mentioned, make sure you use fully rated breakers: some breakers lines are all fully rated, some are not, some will have you use a three pole for two pole loads.
3. Yes, but see #2.

 

[Iron_Ben]

I don't like 120/240 wild leg services. We had thousands of them at the POCO where I worked, but give me 208Y/120 or 480Y/277 any day. That said, those thousands of services have been around since the 50's and 60's, so obviously they can do the job.

Note that we had a published limit of 400 amps for such a service. Your provider might have a similar restriction. We allowed 600 and 800 amp services on a case by case basis though. If the load is mostly single phase your phase currents will not be anywhere near balanced. I remember when we amped a troublesome service at a small to medium sized grocery store. The readings were roughly 450, 460, 100. Something like that.