2 single phase transformer as 1 three phase Open delta transformer

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PowerdT

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I need help here with ocpd requirement for open delta corner grounded transformer

If you have (2) single phase transformer as one 3 phase open delta transformer 480/240,3P. Are we required to put disconnect on primary and secondary or Is it fine only on the primary side if it’s sized per 450.3B?
 
Etanuye said:
I need help here with ocpd requirement for open delta corner grounded transformer

If you have (2) single phase transformer as one 3 phase open delta transformer 480/240,3P. Are we required to put disconnect on primary and secondary or Is it fine only on the primary side if it’s sized per 450.3B?
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Remember that the rules in Article 450 only apply to the transformer itself. The secondary conductors must be protected in accordance with one of the rules in 240.21(C).
 
Because your transformation is a single voltage to a single voltage the rules in article 240 may allow for primary side only protection. Don't forget article 408 would require a main device if you are using a secondary side panelboard.
Honestly I think it has been more than 25 years since I have seen a new installation without a secondary main.
 
jim dungar said:
Because your transformation is a single voltage to a single voltage the rules in article 240 may allow for primary side only protection. Don't forget article 408 would require a main device if you are using a secondary side panelboard.
Honestly I think it has been more than 25 years since I have seen a new installation without a secondary main.
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The current installation does not have secondary protection and it’s feeding 3P motor straight from the secondary transformer
 
jim dungar said:
Because your transformation is a single voltage to a single voltage the rules in article 240 may allow for primary side only protection. Don't forget article 408 would require a main device if you are using a secondary side panelboard.
Honestly I think it has been more than 25 years since I have seen a new installation without a secondary main.
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The transformer is 2kva that’s why panel is not used
 
If memory serves on an open delta like that. Two 2 kva single phase transformers
Max KVA Load= sq root 3 x 2
1.732 x 2 = 3.464
Or 57.7% of the three 2 kva transformers
6 x .577= 3.462

Around 8 amps@ 240 three phase load.
 
240.21(c)(1)

“multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.”

this is where I have my doubt that I need secondary protection
 
Etanuye said:
240.21(c)(1)

“multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.”

this is where I have my doubt that I need secondary protection
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Didn't you say your transformer was an open delta connection with three output conductors?
 
Etanuye said:
Open delta connection , 4 wire output
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That eliminates it from qualifying for primary only protection. If it were corner grounded secondary you possibly can have primary only protection.

You also said it directly feeds a motor load, did not mention if there is motor overload protection, primary only protection possibly could be sufficient for short circuit/ground fault protection but probably not for motor overload protection.
 
synchro said:
That sounds more like a high leg open delta instead of a corner grounded one. What are the 4 wires on the output?
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So the transformer secondary output are L1, L2, L3, L1’. L2 is grounded

that way L1,L2,L3 feeds 240V, 3P motor and L1’, L2/N feeds single phase 120V load
 
T
kwired said:
That eliminates it from qualifying for primary only protection. If it were corner grounded secondary you possibly can have primary only protection.

You also said it directly feeds a motor load, did not mention if there is motor overload protection, primary only protection possibly could be sufficient for short circuit/ground fault protection but probably not for motor overload protection.
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The motors are for door control, I can’t be sure if they have motor overload protection
 
Tulsa Electrician said:
If memory serves on an open delta like that. Two 2 kva single phase transformers
Max KVA Load= sq root 3 x 2
1.732 x 2 = 3.464
Or 57.7% of the three 2 kva transformers
6 x .577= 3.462

Around 8 amps@ 240 three phase load.
Click to expand...

Etanuye said:
So the transformer secondary output are L1, L2, L3, L1’. L2 is grounded

that way L1,L2,L3 feeds 240V, 3P motor and L1’, L2/N feeds single phase 120V load
Click to expand...
The N should be midpoint of one the transformers, and that should be what is grounded. It will be 120 volts to each side of that transformer and 208 volts to the far side of the other transformer, which is commonly connected to the B bus in panelboards and therefore commonly referred to as B phase. Though A B C is nothing more than relationship between them and you can pick any one as a particular reference.
 
kwired said:
The N should be midpoint of one the transformers, and that should be what is grounded. It will be 120 volts to each side of that transformer and 208 volts to the far side of the other transformer, which is commonly connected to the B bus in panelboards and therefore commonly referred to as B phase. Though A B C is nothing more than relationship between them and you can pick any one as a particular reference.
Click to expand...
This is how they wired it
 
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