[RE-wrenches] 120/240 Delta

Wed Mar 25 10:35:26 PDT 2026

Jason is spot-on with his observation.  I completely agree.  Good work!

William Miller
Miller Solar.com
805-438-5600
www.millersolar.com

On Wed, Mar 25, 2026 at 10:11 AM Jason Szumlanski via RE-wrenches <
re-wrenches at lists.re-wrenches.org> wrote:

> You're right that all three phases of a 3-phase system are 120 degrees
> apart. But the two conductors at this service aren't "A and C phase" in
> that sense -- they're the two ends of the center-tapped winding in the
> Delta transformer bank, and that's a critical distinction. The neutral is
> tapped off the midpoint of that same winding. Two ends of the same
> transformer winding are always 180 degrees out of phase.
>
> Your own measurement confirms this: two 120V phasors that are 120 degrees
> apart produce ~208V L-L. Two at 180 degrees produce 240V. You measured 240V
> L-L, so you've got standard split-phase (functionally identical to a 120/240
> split-phase residential service).
>
> The 120-degree phase angle setting on the Quattro is for when you have two
> actual phases from a 3-phase source (like a 208V L-L situation).
>
> No exotic inverter configuration needed. Any standard split-phase inverter
> should work with this service as-is.
>
> Unless I'm missing something!
>
>
> Jason Szumlanski
> Principal Solar Designer | Florida Solar Design Group
> NABCEP Certified Solar Professional (PVIP)
> Florida State Certified Solar Contractor CVC56956
> Florida Certified Electrical Contractor EC13013208
>
>
> On Wed, Mar 25, 2026 at 1:03 PM Michael Morningstar <
> mjmorningstar at gmail.com> wrote:
>
>> David, The two phases are 120V L-N and 240 L-L. The "high leg" was not
>> run to the service.
>>
>> Kienan, I do recall seeing a 120 degree phase angle option when I
>> commissioned a split phase system using Quatros. The location is in Pacific
>> Power territory which no longer does Net Metering so I guess it would be ok
>> to use a non hybrid inverter.
>>
>> Maximum KW load on that service we measured 42kw. Baseline is 38kw.
>>
>> Jason, I've always understood it that all phases of all three phase
>> configurations are 120 degrees out of phase. A=0, B=120, C=240. I'd have to
>> revisit my transformer theory books, but I'm pretty certain that's the case
>>
>>
>> On Wed, Mar 25, 2026 at 9:27 AM Jason Szumlanski <
>> jason at floridasolardesigngroup.com> wrote:
>>
>>> We do high-leg Delta commercial jobs occasionally, although I have never
>>> done a battery backup, but it should not matter if they are only using the
>>> two 120V legs to supply 120/240V split-phase service. The A and C legs of
>>> 120V to neutral should be 180 degrees out of phase with each other and work
>>> fine with any split-phase inverter. Just make sure you are not using the
>>> wild leg that is 208V to neutral, if that is even delivered to the customer
>>> from the utility.
>>>
>>> Most of the time, what we see is a 4-wire delivery to the customer with
>>> a 3-phase panelboard, and every third breaker space is empty. They are only
>>> using 120V and 240V circuits. We find this in light commercial settings in
>>> older industrial parks.
>>>
>>> The other thing to consider is whether the utility will have an issue
>>> with an unbalanced backfeed. FPL does not care around here as long as it's
>>> under 50kWac. They allow netmetering on two legs of a 240V delta high-leg
>>> service.
>>>
>>> Jason Szumlanski
>>> Principal Solar Designer | Florida Solar Design Group
>>> NABCEP Certified Solar Professional (PVIP)
>>> Florida State Certified Solar Contractor CVC56956
>>> Florida Certified Electrical Contractor EC13013208
>>>
>>>
>>> On Wed, Mar 25, 2026 at 11:54 AM Michael Morningstar via RE-wrenches <
>>> re-wrenches at lists.re-wrenches.org> wrote:
>>>
>>>> Wrenches,
>>>> I have a customer that has asked me to provide their restaurant with PV
>>>> and battery back up. Easy-peasy or so I thought. It turns out that the
>>>> service that supplies the restaurant is being fed by two phases from a
>>>> 120/240V transformer. The service entrance conductors are direct burial and
>>>> run under a large paved parking lot, so there's no changing that
>>>> arrangement. I'm at a loss as what I can do here. I don't believe that
>>>> something like a Sol-Ark 18k is going to play nice with two legs 120
>>>> degrees out of phase? The 30K definitely needs all three phases so a 240
>>>> Delta/208 WYE isn't going to help? I don't even think there are
>>>> micro-inverters or string inverters available to give them just straight
>>>> grid-tied and no BB. Are there any solutions that I'm unaware of?
>>>> Michael
>>>> ?
>>>> --
>>>>
>>>> Michael Morningstar
>>>>
>>>>
>>>> Morningstar Electric Inc
>>>>
>>>> PO Box 1494
>>>>
>>>> Mount Shasta, CA 96067
>>>>
>>>> 530-921-0560
>>>>
>>>> CSLB 1116835
>>>>
>>>> mjmorningstar at gmail.com
>>>>
>>>>
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>>
>> --
>>
>> Michael Morningstar
>>
>>
>> Morningstar Electric Inc
>>
>> PO Box 1494
>>
>> Mount Shasta, CA 96067
>>
>> 530-921-0560
>>
>> CSLB 1116835
>>
>> mjmorningstar at gmail.com
>>
>>
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