Multiple transformers to step down v (1st ungrounded)

[Jpflex]

I wrote a post earlier regarding a 4800 p to 480 secondary delta ungrounded system transformer being pad mounted with only case or transformer cabinet being grounded but not secondary windings (ungrounded system by owners choice)

If feeding a second transformer to a building from this first (ungrounded) transformer to obtain a 2 phase 120/240 sub-panel how would this be done if second transformer is grounded at its secondary system windings and bonded to second transformer case as well as derived neutral (y system?) for line to neutral loads and to obtain a ground fault circuit path?

Is this feasible to have the first transformer ungrounded but second transformer system grounded at its system secondary windings, neutral, main bonding jumper etc?

First transformer powers only 3 phase motors and control circuits at a mine

Thanks

 

[jim dungar]

I wrote a post earlier regarding a 4800 p to 480 secondary delta ungrounded system transformer being pad mounted with only case or transformer cabinet being grounded but not secondary windings (ungrounded system by owners choice)

If feeding a second transformer to a building from this first (ungrounded) transformer to obtain a 2 phase 120/240 sub-panel how would this be done if second transformer is grounded at its secondary system windings and bonded to second transformer case as well as derived neutral (y system?) for line to neutral loads and to obtain a ground fault circuit path?

Is this feasible to have the first transformer ungrounded but second transformer system grounded at its system secondary windings, neutral, main bonding jumper etc?

First transformer powers only 3 phase motors and control circuits at a mine

Thanks

120/240V 3 wire is not 2-phase. Some people call this split-phase, but I don't see the need to.

Every time you install a transformer you create a new system and your 'grounding' starts over. The NEC calls this a Separately Derived System. The only time this is not true is when some conductor, other than the grounding (green), is common on both sides of the transformer.

 

[winnie]

A transformer can provide 'galvanic isolation' between its coils. Power is transferred between coils by the changing magnetic field, but there isn't a current flow path from primary to secondary.

So a 480V:120/240V transformer can simply be connected to a 480V ungrounded system on its primary side, and create a 120/240V grounded system on the secondary side. The secondary side grounding won't somehow create a primary side connection to ground.

-Jon

 

[Jpflex]

120/240V 3 wire is not 2-phase. Some people call this split-phase, but I don't see the need to.

Every time you install a transformer you create a new system and your 'grounding' starts over. The NEC calls this a Separately Derived System. The only time this is not true is when some conductor, other than the grounding (green), is common on both sides of the transformer.

What I was saying was obtaining 120/240 from only 2 legs of the 3 phase transformer to primary of second step down transformer to obtain 120/240 volts at secondary and derived neutral, not exactly a split phase but perhaps usable voltage?

 

[winnie]

What I was saying was obtaining 120/240 from only 2 legs of the 3 phase transformer to primary of second step down transformer to obtain 120/240 volts at secondary and derived neutral, not exactly a split phase but perhaps usable voltage?

No problem at all.

All the transformer cares about is that the two primary terminals have 480V between them. Doesn't matter if that 480V comes from two terminals of a 480/277V wye system, or from a 480/240V high leg delta system, or from an ungrounded delta or from a single phase 480V source. All that matters is the 480V connected to the primary.

The secondary will just be a single phase 240V with a center tap to get 120/240V.

-Jon

 

[jim dungar]

What I was saying was obtaining 120/240 from only 2 legs of the 3 phase transformer to primary of second step down transformer to obtain 120/240 volts at secondary and derived neutral, not exactly a split phase but perhaps usable voltage?

2 legs is still only 1-phase.
The primary of a step down transformer is isolated from the secondary side, unless you have a rare autotransformer.
Your grounding connection and grounded conductor on the secondary side of your 480:120/240 transformer are not concerned with those connections on the primary.

 

[Jpflex]

2 legs is still only 1-phase.
The primary of a step down transformer is isolated from the secondary side, unless you have a rare autotransformer.
Your grounding connection and grounded conductor on the secondary side of your 480:120/240 transformer are not concerned with those connections on the primary.

How do I determine ampere limit of second transformer when coming from a 3 phase separately derived source. I heard it was 5% of something or depends on kva of transformer upstream?

Is there a section in NEC to go over this? The section on transformers in my 2020 NEC book is only 3-4 pages but my book is badly worn and missing some parts of the page in this section

 

[jim dungar]

How do I determine ampere limit of second transformer when coming from a 3 phase separately derived source. I heard it was 5% of something or depends on kva of transformer upstream?

Is there a section in NEC to go over this? The section on transformers in my 2020 NEC book is only 3-4 pages but my book is badly worn and missing some parts of the page in this section

This is not an NEC issue.

The 5% limit on single phase loading is when you have a 240/120v 3-phase 4-wire transformer secondary wound on a common core. It does not apply to a standard 480:120/240V step down transformer.

The single phase transformer, connected to 2 legs of the 480V supply will cause some unbalance, similar to how 480V also causes an unbalance. The amount of 480V currentis simply the amount of 240V load current divided by 2 (the ratio between the primary and secondary voltages) because this is single phase.

 

[winnie]

I agree with @jim dungar , the 5% limit that you are thinking of us a transformer balance limit not applicable in this case.

Your 480:120/240V transformer is simply a single phase load added in with whatever other loads are on the 480V source.

If you are designing a system with a 4800:480 three phase transformer feeding a 480:120/240V single phase transformer as its only load, why? The maximum single phase load which you can place on an ordinary 3 phase transformer is 58% of the transformer kVA, which leaves lots of unusable capacity.

Jon

 

[jim dungar]

I agree with @jim dungar , the 5% limit that you are thinking of us a transformer balance limit not applicable in this case.

Your 480:120/240V transformer is simply a single phase load added in with whatever other loads are on the 480V source.

If you are designing a system with a 4800:480 three phase transformer feeding a 480:120/240V single phase transformer as its only load, why? The maximum single phase load which you can place on an ordinary 3 phase transformer is 58% of the transformer kVA, which leaves lots of unusable capacity.

Jon

The 58% load is a restriction on an open delta configuration, as opposed to a closed delta. This has to do with the total kVA being being calculated differently than simply summing the kVA of the winding sets.

Generally, a three phase transformer with three equal sets of windings, connected in a closed delta or wye, can handle a maximum single phase unbalance of 1/3 the transformer total kVA, or the value of one of the equal winding sets. Because an open delta configuration is only 2 sets of windings, which are often different sizes, the maximum single phase unbalance depends on which set of windings are being used.

 

[winnie]

I think that the 58% number is a coincidence here. I am familiar with it in the open delta vs closed delta case.

But now we are talking about a single phase load placed on a three phase transformer. I don't see why the load limit would be only 1/3 of the transformer kVA, because 2 or 3 windings will be participating in the output (2 in wye, 3 in delta), though they won't all be used optimally.

Consider a 75kVA wye secondary. It is rated for 90A. If I draw the full 90A through a load connected from A to B, with nothing connected to C, then I am delivering 43 kVA to that load., or about 58%.

-Jon

 

[jim dungar]

I think that the 58% number is a coincidence here. I am familiar with it in the open delta vs closed delta case.

But now we are talking about a single phase load placed on a three phase transformer. I don't see why the load limit would be only 1/3 of the transformer kVA, because 2 or 3 windings will be participating in the output (2 in wye, 3 in delta), though they won't all be used optimally.

Consider a 75kVA wye secondary. It is rated for 90A. If I draw the full 90A through a load connected from A to B, with nothing connected to C, then I am delivering 43 kVA to that load., or about 58%.

-Jon

Yes, my example was if only one of the windings was involved.

 

[Jpflex]

Yes, my example was if only one of the windings was involved.

Yes, my example was if only one of the windings was involved.

I think that the 58% number is a coincidence here. I am familiar with it in the open delta vs closed delta case.

But now we are talking about a single phase load placed on a three phase transformer. I don't see why the load limit would be only 1/3 of the transformer kVA, because 2 or 3 windings will be participating in the output (2 in wye, 3 in delta), though they won't all be used optimally.

Consider a 75kVA wye secondary. It is rated for 90A. If I draw the full 90A through a load connected from A to B, with nothing connected to C, then I am delivering 43 kVA to that load., or about 58%.

-Jon

Can you explain your calculation

As you did not compute 1.732 or sqrt of 3 for 3 phase system when using 2 phase legs to power a load to obtain 43kva

480v x 90i = 43,200 KVA. Why was 1.732 omitted?

If this is 58% of total kva output are you not over 1/3 of total or 2/3 total kva?

The total kva of transformer has always confused me whether total kva is sum of kva for all 3 phases windings or something else?

 

[Jpflex]

The 58% load is a restriction on an open delta configuration, as opposed to a closed delta. This has to do with the total kVA being being calculated differently than simply summing the kVA of the winding sets.

Generally, a three phase transformer with three equal sets of windings, connected in a closed delta or wye, can handle a maximum single phase unbalance of 1/3 the transformer total kVA, or the value of one of the equal winding sets. Because an open delta configuration is only 2 sets of windings, which are often different sizes, the maximum single phase unbalance depends on which set of windings are being used.

Is this 1/3 total kva one phase winding end to neutral or phase leg to leg

 

[winnie]

The 1.732 factor was implied when I said that a 75kVA 480V transformer has a 90A current rating.

75000VA / 480V / 1.732 = 90A

In this case the 1.732 factor is used because this is a 3 phase calculation. What is the amp rating of a three phase transformer?

Now given a transformer with a 480V L-L voltage rating and a 90A current rating, what is the maximum single phase load that can be supplied?

480V * 90A = 43kVA

In this case we are doing a single phase calculation because we are only connecting a single phase load.

The transformer is limited to 90A per output leg. If you use a 3 phase transformer to supply a single phase load, then you throw away the capacity of that third output leg.

Usually when a 3 phase transformer is supplying single phase loads, you will have multiple loads distributed across all 3 phases. So the transformer sees 3 phase loading.

But you are asking about the maximum single phase transformer connected as a load to a 3 phase transformer.

Jon

 

[Jpflex]

The 1.732 factor was implied when I said that a 75kVA 480V transformer has a 90A current rating.

75000VA / 480V / 1.732 = 90A

In this case the 1.732 factor is used because this is a 3 phase calculation. What is the amp rating of a three phase transformer?

Now given a transformer with a 480V L-L voltage rating and a 90A current rating, what is the maximum single phase load that can be supplied?

480V * 90A = 43kVA

In this case we are doing a single phase calculation because we are only connecting a single phase load.

The transformer is limited to 90A per output leg. If you use a 3 phase transformer to supply a single phase load, then you throw away the capacity of that third output leg.

Usually when a 3 phase transformer is supplying single phase loads, you will have multiple loads distributed across all 3 phases. So the transformer sees 3 phase loading.

But you are asking about the maximum single phase transformer connected as a load to a 3 phase transformer.

Jon

If transformer is limited to 90i per output leg then total transformer kva would be 90i x 3 or 270 amperes

270 amperes x 480 E x 1.732 = 224,467 VA which doesn’t add up to transformer kva mentioned

Also when you say a single phase load is between two phase legs it’s considered single phase?

 

[Tulsa Electrician]

Does this help
Edit added one, round up to 58

 

[winnie]

If transformer is limited to 90i per output leg then total transformer kva would be 90i x 3 or 270 amperes

No. 90A per leg is 90A per leg. The convention for any circuit is to name the current per phase.

A 90A breaker trips at 90A per pole, no matter how many poles.

270 amperes x 480 E x 1.732 = 224,467 VA which doesn’t add up to transformer kva mentioned

The 3 phase formula is 90A * 480V * 1.732

Also when you say a single phase load is between two phase legs it’s considered single phase?

Yup. A two wire circuit is always single phase, even if supplied by a 3 phase supply.

Jon

 

[Joethemechanic]

One of the big advantages of three phase is you are only using 50% more copper, but getting the ability to transmit 73% more power