Transformer size vs secondary output

[cppoly]

If you have a 225 kVA transformer, maximum output on the secondary is 625 amps @ 208/120V.

How would you size the secondary OCPD? 650 amp fuses or 800A fuses if you wanted to assume a continuous load at 125% (781 amps).

The part that I'm questioning / confused about is, if someone down the road sees an 800 amp fused disconnect - is it not wrong for them to assume they can load that up to 800 amps of non continuous load ? But they'll only get 625 amps max out of it.

 

[charlie b]

We don't design or build with the expectation that some future person might misinterpret our installation. That person is responsible for assuring that what they change is code-compliant.

 

[ron]

Although Table 450.3(A) Maximum Rating or Setting of Overcurrent Protection for Transformers Over 1000 Volts (as a Percentage of Transformer-Rated Current), shows the maximum, Footnote 1 allows you to round up.
If someone wants to load it to 800A without looking at the feeder, that is their mistake.

 

[cppoly]

A follow up. Looking at Table 450.3(B) with primary protection at 125% and secondary protection not required, this would mean the OCPD upstream would never trip unless the primary load exceed 125% of the transformer's FLA?? Transferring that onto the secondary, does that mean a the transformer would never trip with 125% of it's secondary FLA? So a 225 kVA transformer could run 625A x 125% = 781 without damage or just without tripping?

 

[jim dungar]

So a 225 kVA transformer could run 625A x 125% = 781 without damage or just without tripping?

Probably.

The NEC is only concerned with conductor sizing and conductor protection. Transformer, and motor, loading is a design issue to be taken up with the manufacturer.

 

[don_resqcapt19]

Note that where you size the secondary overcurrent protection to 800 amps the conductors on the load side of that OCPD must have an ampacity of 800 amps. Also note that between the secondary terminals and that OCPD, you need conductors with an actual ampacity of 800 amps because 240.21(C) says "Section 240.4(B) shall not be permitted for transformer secondary conductors.".

 

[ron]

Note that where you size the secondary overcurrent protection to 800 amps the conductors on the load side of that OCPD must have an ampacity of 800 amps. Also note that between the secondary terminals and that OCPD, you need conductors with an actual ampacity of 800 amps because 240.21(C) says "Section 240.4(B) shall not be permitted for transformer secondary conductors.".

Also recall that 240.21(C)(1) says that a delta wye transformer isn't protected by primary only protection.

 

[Joethemechanic]

I think just the nature of your typical 208/120 WYE service is that even if they are the type to max it out without checking, they probably aren't really. The loads are mostly not continuous.

And really like said above, it's their bad

 

[jim dungar]

Also recall that 240.21(C)(1) says that a delta wye transformer isn't protected by primary only protection.

Actually, it says the secondary conductors of a delta-wye transformer aren't protected.
450.3 allows the transformer, itself, to be protected by a primary device sized at not more 125%.

 

[kwired]

The transformer is still rated at 625 secondary amps, it can be protected at 125% and next standard size up which allows 800 amp overcurrent device.

Conductors will still need to be 800 amp conductors if you are providing 800 amp protection and has also been mentioned you can not protect the secondary with the primary protection on this particular system.

 

[Carultch]

A follow up. Looking at Table 450.3(B) with primary protection at 125% and secondary protection not required, this would mean the OCPD upstream would never trip unless the primary load exceed 125% of the transformer's FLA?? Transferring that onto the secondary, does that mean a the transformer would never trip with 125% of it's secondary FLA? So a 225 kVA transformer could run 625A x 125% = 781 without damage or just without tripping?

It's only qualifying topologies, where the primary OCPD can be considered to protect the secondary conductors. To qualify, it requires overcurrents to line-up across each winding pair, and not get redistributed. Anything involving a centertap, a wye system on one side, or both, does not qualify. If you have a centertap, but don't connect anything to it inside the transformer, it counts as if it wasn't there in the first place.

As an example of why, consider a 480V to 120/240V 1-phase transformer, with a 50A breaker on the primary. This would imply a 100A secondary circuit if it were 480V to 240V straight instead. With the centertap, consider drawing 150A on Leg 1, and 10A on Leg 2. This will show up as 40A on the primary, which the primary breaker ignores. The overload on Leg 1 is in its blindspot, which is why a breaker on the 120/240V side is required, to meet 240.21(C).

In your situation, the fact that it could run at the full 225 KVA doesn't govern what you need to connect for a secondary circuit. You are only required to size your secondary circuit, for the calculated load that will be installed at the time you finish your job, with the continuous load safety factor (if applicable). If you only use 170 kVA continuous (hypothetical number) of the 225kVA, that's 472A yielding a 600A OCPD, and it would be OK to use a 600A OCPD and circuit. The fact that the transformer has more capacity, doesn't require it to be higher.

800A would give you the full capacity whether continuous or non-continuous, while the 625A figure you mentioned would likely round up to 700A in practice, if you wanted to use the 225A as non-continuous. It's best to stick to standard fuse ratings where you can, to facilitate replacements in the future.

 

[LarryFine]

It's only qualifying topologies, where the primary OCPD can be considered to protect the secondary conductors.

Does that mean that only delta-delta and wye-wye transformers qualify?

 

[Carultch]

Does that mean that only delta-delta and wye-wye transformers qualify?

Delta-delta 3-wire on both sides, and single phase 2-wire on both sides.

 

[wwhitney]

Delta-delta 3-wire on both sides, and single phase 2-wire on both sides.

Those are the topologies that the NEC recognizes for allowing primary side OCPD to protect secondary conductors, but it seems to me that the physics works for wye-wye as well.

Also, for other topologies, it seems to me you can assign a worst case discrepancy factor, and if you oversized your secondary conductors accordingly, they would still be protected by the primary side OCPD. For single phase transformers with 3 wire secondaries, the factor would be 2; for delta-wye transformers, it would be sqrt(3).

Cheers, Wayne

 

[jim dungar]

but it seems to me that the physics works for wye-wye as well.

But Yg-Yg transformers arrangements are hard to install and still conform to the NEC. This is primarily because these installs usually depend on multigrounded neutrals which the NEC frowns on.

 

[wwhitney]

But Yg-Yg transformers arrangements are hard to install and still conform to the NEC. This is primarily because these installs usually depend on multigrounded neutrals which the NEC frowns on.

If you have, say, a 480Y/277V service, and need a 208Y/120V system for some loads, I understand it is common to choose a 480V delta : 208Y/120V transformer.

What is the difficulty of instead using a 480Y/277V : 208Y/120V wye-wye transformer, or set of three 277V : 120V 2-wire 2-wire transformers? You could install it as an SDS by connecting a GEC to X0, as well as an SBJ from X0 to the EGC. Or you could install it as a non-SDS by connecting H0 to X0 and omitting any GEC or SBJ.

Thanks,
Wayne

 

[jim dungar]

What is the difficulty of instead using a 480Y/277V : 208Y/120V wye-wye transformer, or set of three 277V : 120V 2-wire 2-wire transformers?

If you do not connect the source side Y point at the transformer you no longer have a Yg-yG transformer you have a Y-Yg (ungrounded wye primary to grounded wye secondary).
If you install a Yg-Yg (grounded wye to grounded wye), with a neutral conductor, you are rebonding the source side wye neutral point to ground which is not allowed by the NEC.

I know back in the 80's the Sorgel division of Square D once sold a 480Y277-208Y/120 autotransformer , but it was not an SDS..

 

[wwhitney]

If you install a Yg-Yg (grounded wye to grounded wye), with a neutral conductor, you are rebonding the source side wye neutral point to ground which is not allowed by the NEC.

Only if the source side wye neutral point is bonded to the secondary side neutral point at the transformer, at which point it is not an SDS and you would not install a GEC or SBJ, so you would not have such a rebonding.

Does a wye-wye transformer typically come with such a neutral-neutral bond?

Cheers, Wayne

 

[LarryFine]

Delta-delta 3-wire on both sides, and single phase 2-wire on both sides.

So not wye-wye? Why?

 

[jim dungar]

Does a wye-wye transformer typically come with such a neutral-neutral bond?

It depends on your transformer. Padmount transformers are usually Yg-Yg with an internal jumper between neutrals

Have you done a current analysis of an ungrounded wye primary to a grounded wye secondary when one secondary phase is faulted? How different are they from a similarly faulted D-Yg transformer?

I am not trying to justify the NEC wording. I am trying to discourage wye-wye transformers in general usage for LV applications.