Delta-Delta Primary only Transformer Protection

[LibertyEngineering]

Good Day Folks!
Looking at a design installation for a local Township which the drawings were prepared by another engineer who is being unresponsive to the electrical inspectors questions.
Installed currently (but not energized yet) is a 240V 1-phase Delta Primary to 480V 1-phase Delta secondary 225KVA transformer with #600MCM secondary conductors feeding an 800A MCB distribution panel. Would someone please explain by way of writing out the calculations NEC 240.21(C) (1). I am trying to determine if the primary only protection is acceptable or not since it is a Delta-Delta transformer. My secondary conductors are over 25' in length. Not quite understanding the whole primary to secondary ratio portion.

Thank you for your input

 

[iceworm]

.... a 240V 1-phase Delta Primary to 480V 1-phase Delta secondary

... since it is a Delta-Delta transformer.

Was the "1 phase" just a mis-type? And the primary and secondary are 3 phase?

 

[LibertyEngineering]

No it is single phase.

 

[LibertyEngineering]

It is a sports field located in the middle of a residential area where 3 phase is not available.

 

[electrofelon]

Was the "1 phase" just a mis-type? And the primary and secondary are 3 phase?

Right, that makes no sense. Although in local jargon here Sometimes a single phase transformer is referred to as a "Delta" meaning the voltage and/or bushing configuration is intended for an ungrounded or "Delta" distribution system (i.e. primary voltage: 4800 Delta) It's pretty sloppy but a lot of people say it. Hopefully op will clarify.

 

[electrofelon]

No it is single phase.

Then please clarify what "Delta" means in this sense.

 

[Carultch]

Good Day Folks!
Looking at a design installation for a local Township which the drawings were prepared by another engineer who is being unresponsive to the electrical inspectors questions.
Installed currently (but not energized yet) is a 240V 1-phase Delta Primary to 480V 1-phase Delta secondary 225KVA transformer with #600MCM secondary conductors feeding an 800A MCB distribution panel. Would someone please explain by way of writing out the calculations NEC 240.21(C) (1). I am trying to determine if the primary only protection is acceptable or not since it is a Delta-Delta transformer. My secondary conductors are over 25' in length. Not quite understanding the whole primary to secondary ratio portion.

Thank you for your input

What that section of the NEC is saying, is that in certain topologies of transformers, the primary OCPD can effectively protect the secondary conductors too, as if they are a direct extension of the primary conductors (that operate at a different mix of voltage and current). So for a 480V (primary) to 240V transformer that qualifies, a 300A OCPD on the primary, will act as a 600A OCPD protecting the secondary. That's what that whole "primary to secondary voltage ratio" statement is all about.

Think about transformers like a gear system. Gear systems trade torque for speed, transformers trade voltage for current. A transformer that halves the voltage, will double the current. Suppose you had a 1:2 overdrive gear system, and you needed to stop the output shaft from reaching 6000RPM. You could either put a "mechanical fuse" on the output shaft that breaks at 6000 RPM, or you could put a "mechanical fuse" on the input shaft at 3000 RPM. Same concept with transformers that qualify for the primary OCPD protecting the secondary.

What makes a transformer qualify for this, is when fault currents have no other option than to line up directly across each pair of windings. So it is single phase 2-wire to 2-wire transformers, or delta-to-delta that are 3-wire on both sides. Introduce a WYE system, high leg delta, or center tap, and it no longer qualifies.

 

[kwired]

Good Day Folks!
Looking at a design installation for a local Township which the drawings were prepared by another engineer who is being unresponsive to the electrical inspectors questions.
Installed currently (but not energized yet) is a 240V 1-phase Delta Primary to 480V 1-phase Delta secondary 225KVA transformer with #600MCM secondary conductors feeding an 800A MCB distribution panel. Would someone please explain by way of writing out the calculations NEC 240.21(C) (1). I am trying to determine if the primary only protection is acceptable or not since it is a Delta-Delta transformer. My secondary conductors are over 25' in length. Not quite understanding the whole primary to secondary ratio portion.

Thank you for your input

Since you later said this is single phase - there is no "delta" to talk about. If you do indeed have a delta to delta transformer and only input single phase to it, you only are getting 1/3 the kVA rating out as you would only energize one side of the delta.

Primary to secondary ratio isn't that complicated, especially for single voltage on both sides.

If you have 240 two wire in and 480 volt two wire out, the secondary current is half the primary current, regardless what kVA is. A primary overcurrent protection device can protect secondary in this situation because we know what primary current will be for any given secondary current, it will be 2 times the secondary current. So if your secondary current were say 100 amps then secondary current will be 200 amps and 200 amp primary protection will be equivalent to having 100 amp protection on the secondary.

If you had split secondary (120/240 most common) and line to neutral loads you can't protect that secondary with the primary. In that case it is possible to put more load on one half of the secondary than it is rated for but still be less than total kVA rating of the unit, primary device still sees less than full rating and doesn't trip in this situation though a portion of secondary is overloaded.

 

[Besoeker3]

Installed currently (but not energized yet) is a 240V 1-phase Delta Primary to 480V 1-phase Delta secondary

Good day to you too.
Care to draw a diagram of a 1-phase Delta??

 

[iceworm]

... 240V 1-phase ... Primary to 480V 1-phase ... secondary 225KVA transformer with #600MCM secondary conductors feeding an 800A MCB distribution ...

I need a bit of context

The transformer is:

2 wire 240V input (no neutral) this part sounds okay

This part is odd
2 wire 480V output (no neutral?)
I don't think I have ever seen a 1 phase, 480V, 800A panelboard. Doesn't mean there aren't available - likely not common.
Is either 480V wire bonded to ground? This doesn't matter much, mostly curiousity)​

 

[LibertyEngineering]

Since you later said this is single phase - there is no "delta" to talk about. If you do indeed have a delta to delta transformer and only input single phase to it, you only are getting 1/3 the kVA rating out as you would only energize one side of the delta.

Primary to secondary ratio isn't that complicated, especially for single voltage on both sides.

If you have 240 two wire in and 480 volt two wire out, the secondary current is half the primary current, regardless what kVA is. A primary overcurrent protection device can protect secondary in this situation because we know what primary current will be for any given secondary current, it will be 2 times the secondary current. So if your secondary current were say 100 amps then secondary current will be 200 amps and 200 amp primary protection will be equivalent to having 100 amp protection on the secondary.

If you had split secondary (120/240 most common) and line to neutral loads you can't protect that secondary with the primary. In that case it is possible to put more load on one half of the secondary than it is rated for but still be less than total kVA rating of the unit, primary device still sees less than full rating and doesn't trip in this situation though a portion of secondary is overloaded.

Boy this is embarrassing - you are all correct. I had a little tunnel vision as I typically live in the world of 3-phase power systems so went straight to that portion of the code section, no Delta involved here. This is an odd set up but I understand why it was done. The township had a large amount of sports lighters donated which were all 480V single phase. Thank you for your input.

 

[iceworm]

... The township had a large amount of sports lighters donated which were all 480V single phase. .

So the 480V output is center tap grounded?
end grounded?
un-grounded?

 

[PaulMmn]

Since you later said this is single phase - there is no "delta" to talk about. If you do indeed have a delta to delta transformer and only input single phase to it, you only are getting 1/3 the kVA rating out as you would only energize one side of the delta.

Primary to secondary ratio isn't that complicated, especially for single voltage on both sides.

If you have 240 two wire in and 480 volt two wire out, the secondary current is half the primary current, regardless what kVA is. A primary overcurrent protection device can protect secondary in this situation because we know what primary current will be for any given secondary current, it will be 2 times the secondary current. So if your secondary current were say 100 amps then secondary current will be 200 amps and 200 amp primary protection will be equivalent to having 100 amp protection on the secondary.

If you had split secondary (120/240 most common) and line to neutral loads you can't protect that secondary with the primary. In that case it is possible to put more load on one half of the secondary than it is rated for but still be less than total kVA rating of the unit, primary device still sees less than full rating and doesn't trip in this situation though a portion of secondary is overloaded.

Is this a mis-statement??

 

[LibertyEngineering]

I need a bit of context

The transformer is:

2 wire 240V input (no neutral) this part sounds okay

This part is odd
2 wire 480V output (no neutral?)
I don't think I have ever seen a 1 phase, 480V, 800A panelboard. Doesn't mean there aren't available - likely not common.
Is either 480V wire bonded to ground? This doesn't matter much, mostly curiousity)​

Not Sure did not gewt to see the inside of the panel The engineer who did this originally messed up when specifying the transformer. They forgot to add the /277V portion to the call out and ended up with a change order revision to add another transformer which was 277V single phase only on the secondary side just a big mess if you ask me. Pics attached of both xfmers labels

 

[kwired]

Is this a mis-statement??

yes, read what I mean and not what I write

Should have been "primary current will be 200 amps"

 

[LibertyEngineering]

yes, read what I mean and not what I write

Should have been "primary current will be 200 amps"

Yes- I picked up on that but thanks for clarifying

 

[kwired]

Not Sure did not gewt to see the inside of the panel The engineer who did this originally messed up when specifying the transformer. They forgot to add the /277V portion to the call out and ended up with a change order revision to add another transformer which was 277V single phase only on the secondary side just a big mess if you ask me. Pics attached of both xfmers labels

If your luminaires are rated 480 volts, there is no need for 277 to be present.

Single phase transformer with 277 either side is sort of rare other than pole mounted transformers like POCO's use. Would be special order most the time and likely higher cost than stocked 120/240 x 240/480 units. If your secondary is 240/480 you can center ground it or end ground it. If you end ground it it is a two wire system and primary protection can protect the secondary. If you center ground it then it is a multiwire secondary and you must also have secondary protection.

If you have multi-volt luminaires you likely have a 240 volt tap you could use if you wanted, but then there isn't much need for a transformer if you do that either, only good reason to center ground the secondary would be to have lower voltage to ground - which may make a difference in what kind of breakers you could use if using fuses you are mostly stuck with 600 volt class either way.

 

[LibertyEngineering]

If your luminaires are rated 480 volts, there is no need for 277 to be present.

Single phase transformer with 277 either side is sort of rare other than pole mounted transformers like POCO's use. Would be special order most the time and likely higher cost than stocked 120/240 x 240/480 units. If your secondary is 240/480 you can center ground it or end ground it. If you end ground it it is a two wire system and primary protection can protect the secondary. If you center ground it then it is a multiwire secondary and you must also have secondary protection.

If you have multi-volt luminaires you likely have a 240 volt tap you could use if you wanted, but then there isn't much need for a transformer if you do that either, only good reason to center ground the secondary would be to have lower voltage to ground - which may make a difference in what kind of breakers you could use if using fuses you are mostly stuck with 600 volt class either way.

Fixtures are 480 only already installed. Than they found some were 277V only so a second transfomer was purchased as 277V only and is already on site, all they needed to do was order the first one with 277V but the original drawings did not call for it. We were brought into the project secondary to the original engineer to try and staighten things out.

 

[david luchini]

Fixtures are 480 only already installed. Than they found some were 277V only so a second transfomer was purchased as 277V only and is already on site, all they needed to do was order the first one with 277V but the original drawings did not call for it.

Then how would they have supplied the 480V fixtures?

 

[augie47]

What size is your primary protection ?
You have a 2:1 ration on your transformer so the 2:1 ratio will apply to your conductor protection.
FGrom you description it sounds as if your secondary conductors might be located outside in which case 240.21(C)(4) could be applied and the ratio would no matter as your secondary conductors are protected by your 800 amp breaker,.