2 single phase transformer as 1 three phase Open delta transformer

[kwired]

Wrong as is in non-standard (agree), or wrong as in an NEC violation (disagree)?

Cheers, Wayne

Is connected properly for three phase four wire with high leg, but I didn't notice the ground symbol was on the wrong lead first time around.

Is still three phase four wire with high leg, but does not comply with 250.26 if grounding the lead that is shown grounded.

If only using as three phase three wire (not utilizing either center tap at all besides putting each half of each secondary coil in series with one another) then it becomes corner grounded, in which case you can ground any one of the three leads being utilized as system conductors.

 

[wwhitney]

Also if you read through 250.26 that midpoint of that phase (when extended for use as a system conductor anyway) is the conductor of such a system that NEC requires to be grounded.

Disagree on 250.26. From 2017:

250.26 Conductor to Be Grounded — Alternating-Current Systems. For ac premises wiring systems, the conductor to be grounded shall be as specified in the following:
(1) Single-phase, 2-wire — one conductor
(2) Single-phase, 3-wire — the neutral conductor
(3) Multiphase systems having one wire common to all phases — the neutral conductor
(4) Multiphase systems where one phase is grounded — one phase conductor
(5) Multiphase systems in which one phase is used as in (2) — the neutral conductor

(4) and (5) are not written to be mutually exclusive, so you can install a center-tapped delta to comply with either (4) or (5). I.e. it doesn't say:

. . .
(4) Multiphase systems in which one phase is used as in (2) — the neutral conductor
(5) All other multiphase systems--one phase conductor

(although even that wouldn't be 100% clear, as what exactly does "used as in (2)" mean? If you have a center tap, but don't supply loads from the center-tap to both of the ends of the coil (separately as 2-wire loads), does that mean you aren't using it as in (2)?)

Cheers, Wayne

 

[PowerdT]

Are these two transformers sold as a pair to use as an open delta? Or just two individual single phase transformers?

How about posting pictures of the two labels?

Cheers, Wayne

They are sold as a pair.

I will try to post labeling

 

[jim dungar]

The wiring and labeled are From the transformer manufacturer.

I agree L1’ neutral makes sense but for some reason they went to ground L2 which is unusual. And they labeled as L2/N

The terminal designations, from the manufacturer, don't really mean anything. The NEC requires you need to identify conductors per your system. For example, the industry standard for transformers is to identify the low voltage terminals using X plus a number, like X1, X2, X3, and X4 but once they are installed a standard practice is to use L1, N, and L2.

 

[kwired]

They are sold as a pair.

I will try to post labeling

Maybe so, but I'd bet exact same units are also sold individually as single phase transformers. When used as pair for this you need to connect properly to get three phase out of them, and can do so with pretty much any similar design pair of single phase transformers.

 

[kwired]

Disagree on 250.26. From 2017:

250.26 Conductor to Be Grounded — Alternating-Current Systems. For ac premises wiring systems, the conductor to be grounded shall be as specified in the following:
(1) Single-phase, 2-wire — one conductor
(2) Single-phase, 3-wire — the neutral conductor
(3) Multiphase systems having one wire common to all phases — the neutral conductor
(4) Multiphase systems where one phase is grounded — one phase conductor
(5) Multiphase systems in which one phase is used as in (2) — the neutral conductor

(4) and (5) are not written to be mutually exclusive, so you can install a center-tapped delta to comply with either (4) or (5). I.e. it doesn't say:

. . .
(4) Multiphase systems in which one phase is used as in (2) — the neutral conductor
(5) All other multiphase systems--one phase conductor

(although even that wouldn't be 100% clear, as what exactly does "used as in (2)" mean? If you have a center tap, but don't supply loads from the center-tap to both of the ends of the coil (separately as 2-wire loads), does that mean you aren't using it as in (2)?)

Cheers, Wayne

Understand. you can't ground more than one point of any system, that results in high current flow between those two points.

If you need (in this case) 120 volt loads then you must ground the mid point of the one phase where you are taking that "single phase three wire" mentioned in (2), therefore (5) applies to you. If you don't need 120 volts, you have option to not utilize either center tap and then (4) applies to you, you still can arrange it as (5) if you want to, but you can't do both, regardless of code doing both just isn't going to work out so well.

 

[wwhitney]

If you need (in this case) 120 volt loads then you must ground the mid point of the one phase where you are taking that "single phase three wire" mentioned in (2), therefore (5) applies to you.

If you need 120V, then you need to use the center tap and one end of the coil. But you still (functionally) have a choice of which of those two conductors to ground. If you ground the center tap, then you obviously aren't under 250.26(4), so you are under 250.26(5). This has the advantage that you could supply 120V from the center tap to either end of the coil.

But if you ground the end of the center-tapped coil, then the system meets the description in 250.26(4), so I don't see the problem. You are restricted to supplying 120V from that grounded conductor to the center tap.

Cheers, Wayne

 

[Tulsa Electrician]

Are these part of a piece of equipment.
NEC may not apply. 450.1 exception #2

 

[kwired]

If you need 120V, then you need to use the center tap and one end of the coil. But you still (functionally) have a choice of which of those two conductors to ground. If you ground the center tap, then you obviously aren't under 250.26(4), so you are under 250.26(5). This has the advantage that you could supply 120V from the center tap to either end of the coil.

But if you ground the end of the center-tapped coil, then the system meets the description in 250.26(4), so I don't see the problem. You are restricted to supplying 120V from that grounded conductor to the center tap.

Cheers, Wayne

You would need overcurrent devices in all the ungrounded conductors.

Not saying this is not possible but leaves you with something rather unconventional and likely very confusing to some.

will have two ungrounded conductors that are 240 to ground.

will have one ungrounded conductor(the center tap of the one coil) that is 120 volts to the grounded conductor, 120 to one ungrounded conductor and 208 to the remaining ungrounded conductor.

 

[kwired]

If you need 120V, then you need to use the center tap and one end of the coil. But you still (functionally) have a choice of which of those two conductors to ground. If you ground the center tap, then you obviously aren't under 250.26(4), so you are under 250.26(5). This has the advantage that you could supply 120V from the center tap to either end of the coil.

But if you ground the end of the center-tapped coil, then the system meets the description in 250.26(4), so I don't see the problem. You are restricted to supplying 120V from that grounded conductor to the center tap.

Cheers, Wayne

Also keep in mind 250.26 is titled Conductor to be grounded. If using one side of the delta to get single phase three wire voltages, then (5) says the neutral conductor is the one to be grounded.

"(4) Multiphase systems where one phase is grounded — one phase conductor" is kind of redundant. If one phase is grounded, laws of physics say you shouldn't be grounding any other point of said system.