50k 120/240 3phase 4wire advice

Interesting and the first time for a commercial install that we have come across this configuration, 120/240 3phase 4wire with a 208 High leg.
So... any advice on interconnecting? First thought is a sub panel for 240 phase to phase with 2/3 of the system and a separate subpanel for the 208 high leg, all run as separate taps..... would this avoid an unbalanced system? we can vary up to 6k per phase, but shoot for a harmonius situation.
Thanks in advance.



  • boBboB Solar Expert Posts: 972 ✭✭✭✭
    Re: 50k 120/240 3phase 4wire advice

    I had a fairly recent need to learn about these myself...

    It's basically just a 3 phase delta configuration with one of those
    deltas having a center tap. So, on that one arm, you have two 120VAC legs referenced to the arm's center tap.

    It is kind of weird, I agree, but is evidently quite common.
    Reason being that you get all the voltages, pretty much.

    There are two ways that people ground these too... One at a time of course. I think it's either that arm center tap, OR one end of that center tapped arm.

    Here is my favorite picture of this (I think)


    Here is a forum thread with another decent image link of that configuration:


  • System2System2 Posts: 6,290 admin
    Re: 50k 120/240 3phase 4wire advice

    TY for the links much appreciated, I am still searching for a code compliant and practical, balanced way to deal. It is a building that is 100 years old in Queens, and the service fits with my readings on the time period. Troubling over the ability of attaching a sub with just the 208 feed neutral and ground. Inverters are available that can do this, Are breakers?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: 50k 120/240 3phase 4wire advice

    You should check with the utility and see what they say about loading one leg of their 3-phase system. Some will allow it, but only to a certain extent. Others they won't allow it at all.
  • System2System2 Posts: 6,290 admin
    Re: 50k 120/240 3phase 4wire advice

    Wont load one leg, will have 2/3 of the system one the 240, and one third on the 208..... been searching all over online and really can not find a similar install at 50k. Just got an idea.... endicott NY ETM Solar works....Sending an email :) Went to a commercial class there. 2 yars ago. worth a try.
  • VicVic Solar Expert Posts: 2,984 ✭✭✭✭
    Re: 50k 120/240 3phase 4wire advice

    My 2c ...

    Think that you have center-tapped delta, which is what is in my workshop. In that case a 3-pole breaker feeds a single subpanel. Each single or split phase (120 or 240 VAC) breaker occupies one or two positions in the sub. There are ONLY two positions where these breakers ma be placed. Any 208 loads can occupy a single spot in the remaining positions. And, of course any three-phase loads can occupy a correct spot picking all three of the busses in the sub.

    This subpanel with both single/three phase loads might no longer be code-compliant, as there is no natural way to keep a person from installing a breaker in an incorrect location, and have incorrect outputs.

    Having a seperate three-phase/208 VAC (single position, high leg to neut) is perhaps safer.

    I personally like this center-tapped Delta, as one can derrive single/split phase and three pahse loads from a single service without an additional transformer. In my neighborhood, non-eaqual leg/leg balancing is permitted, as the utility configured the servise to supply the needs of mixed industrial/commerrcial users.

    The title of the original post notes 50 K. What does this refer to ? Not KV I hope.

    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
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