High Leg Delta

[kwired]

It is useless for a 208 volt load,

It will work, just normally not a very good idea to do so, unless maybe is a very limited load like for 208 volt controls or something. It won't balance things like using all three lines to neutral on a wye system does. Assuming B phase is the high leg, on a full delta that B to N loading flows through both A and C halves of the delta. On an open delta system all that current flows through which ever side of the delta is the closed side, in addition to any other current already flowing on that side.

 

[Carultch]

It is useless for a 208 volt load,

In theory, it could work. In practice, you're facing the limitations of slash-rated breakers, if you attempt. Most single pole breakers in this voltage class, are slash-rated at 120/240V, requiring the nominal voltage-to-ground to not exceed 120V, and the voltage across poles to not exceed 240V nominal. Even if the load can be powered by 208V, the single pole breaker's limitations govern.

You'd need a straight-rated breaker that's rated for 240V, without 120V being part of the rating, to be able to do this. One way, is to use a 2 or 3 pole breaker that is straight-rated, and not use the remaining pole(s). Another way, is to use a 480V panelboard, whose breakers would at minimum, be rated for 277V nominal to ground.

There's very little practical need to do this, since most loads that can work with 208V, are also built to handle 240V. Either by configuring jumpers on a device like a motor, or with a device that automatically adjusts, like a DC power supply.

 

[electrofelon]

I think there is excessive drama about this. last year i ran a load off the 208 high leg. It was a single phase machine that needed 220 nominal. Tons of drives, i looked at the data plates and 208 was a better fit than 240+. Also its expecting line to neutral, so i didnt want to risk giving it two hots. Just used a 3 pole breaker. Its fine.

 

[jim dungar]

I think there is excessive drama about this. last year i ran a load off the 208 high leg. It was a single phase machine that needed 220 nominal. Tons of drives, i looked at the data plates and 208 was a better fit than 240+. Also its expecting line to neutral, so i didnt want to risk giving it two hots. Just used a 3 pole breaker. Its fine.

If the transformer capacity allows it there is no engineering reason you cannot use the 208V, other than OCPD voltage issues. However few transformer banks are sized using this assumption.

The problem is very few open delta banks are sized for this loading. Using the 208V connection means you are loading 1/2 of the center tapped winding as well the full winding. It is this extra loading that can cause the voltage unbalance people often mention.

 

[Jraef]

Here in PG&E country it is not going away anytime soon.

Yep. It’s listed by PG&E as “optional” but still available, but interestingly, only if you ask for “industrial” service options. If you say “commercial”, they don’t list it as an option any more. So they are definitely trying to de-emphasize it.

 

[tortuga]

If the transformer capacity allows it there is no engineering reason you cannot use the 208V, other than OCPD voltage issues. However few transformer banks are sized using this assumption.

The problem is very few open delta banks are sized for this loading. Using the 208V connection means you are loading 1/2 of the center tapped winding as well the full winding. It is this extra loading that can cause the voltage unbalance people often mention.

I think the way it plays out is say a 10kVA 208V L-N load
where T1 is the A-B phases (240) and T2 is the A-C phases (120/240)
I think the load would be VA on T1 = SQRT(3)/2 * 10kVA or 8.66kVA
and T2 would be 1/2 of 10kVA = 5kVA
13.6kva of transformer for 10kVA of real load?
The only thing worse than that is if you put a 10kVA 240V 1ph load on B-C (open set) then 10kVA on each of T1 and T2
20kVA in transformer for 10kVA of load?

 

[Birken Vogt]

Yep. It’s listed by PG&E as “optional” but still available, but interestingly, only if you ask for “industrial” service options. If you say “commercial”, they don’t list it as an option any more. So they are definitely trying to de-emphasize it.

Now that you mention it, there is a small to medium commercial building that got a single 3 phase can, presumably wye, first time I have seen that on PG&E in decades.

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Almost everything else in this area is open delta including my shop, unless it is fed underground.

Also interesting that they used open wire secondary on this job, it is a real blast from the past.

 

[wwhitney]

I think the way it plays out is say a 10kVA 208V L-N load
where T1 is the A-B phases (240) and T2 is the A-C phases (120/240)
I think the load would be VA on T1 = SQRT(3)/2 * 10kVA or 8.66kVA
and T2 would be 1/2 of 10kVA = 5kVA
13.6kva of transformer for 10kVA of real load?

The way I'd approach this question is to consider various 1A loads, each of which would use 480VA of transformer capacity as 1A in each the two 240V transformers (possibly only one half of the center-tapped A-C transformer). From least efficient to most efficient:

- A 208V load supplied B-N gives you 208VA of load.
- A 240V load supplied B-C (open winding) gives you 240VA of load.
- A 208V load supplied B-N plus a balancing 120V load supplied C-N gives you 328 VA of load.
- A 3-phase load supplied A-B-C would give you 416 VA of load.
- A 240V load supplied A-B plus another supplied B-C would give you 480VA of load.

If your transformers are supplying a mixture of loads, it gets more complicated, as the various load currents may partially cancel in the transformer coils due to phase shift. Although assuming no cancellation is conservative.

Cheers, Wayne

 

[tortuga]

The way I'd approach this question is to consider various 1A loads, each of which would use 480VA of transformer capacity as 1A in each the two 240V transformers (possibly only one half of the center-tapped A-C transformer). From least efficient to most efficient:

- A 208V load supplied B-N gives you 208VA of load.
- A 240V load supplied B-C (open winding) gives you 240VA of load.
- A 208V load supplied B-N plus a balancing 120V load supplied C-N gives you 328 VA of load.
- A 3-phase load supplied A-B-C would give you 416 VA of load.
- A 240V load supplied A-B plus another supplied B-C would give you 480VA of load.

If your transformers are supplying a mixture of loads, it gets more complicated, as the various load currents may partially cancel in the transformer coils due to phase shift. Although assuming no cancellation is conservative.

Cheers, Wayne

using you numbers this is what I get for the eight possible load types on a open delta:

Line #​	​	​	​	​	​	Open delta sizing:​
1​	Load type:​	split phase va​	1 phase VA​	2-Phase, 4 wire 90∘​	3 phase VA 120∘​	Transformer T1 load (240V) a-b​	Transformer T2 (120/240) shifted by 30∘ from T1 a-n-c​
2​	Three Phase 240V balanced​	​	​	​	480​	277​	277​
3​	Single phase 120 a-n (note 1)​	480​	​	​	​	​	480​
4​	Single phase 120 c-n (see note 1)​	480​	​	​	​	​	480​
5​	Single phase 208 b-n (note 2)​	480​	​	​	​	416​	240​
6​	240 1 phase a-b​	​	480​	​	​	480​	0​
7​	240 1 phase a-c​	​	​	​	480​	0​	480​
8​	240 1 phase b-c (note 3)​	​	480​	​	​	480​	480​
9​	2-Phase Motor​	​	​	480​	​	240​	416​
10​	Total Load ​	​	​	​	​	1893​	2853​
11​	​	​	​	​	​	​	​
12​	Load in kVA​	​	​	​	​	​	​
13​	Minimum transformer size (Scalar Sum)​	​	​	​	​	1.893​	2.853​