Transformer OCPD sizing question and confirmation

[sokbok]

Howdy forum, its been awhile since I had to size OCPD for a transformer.
The transformer is feeding a chiller which cools a laser cutter.
The machine is technically not continuous, but I think for the sake of safety, I will size it if it is.
The laser will never run for more then 3 hours, but the chiller could stay on in between jobs so CONTINUOUS.

I have a 208 wye > 480V Delta 75kva transformer. The chiller runs on 480V, 3 phase. The service is 208V.

The full load amps @ 208V are 127. So 127 x 1.25 = 158.75 amps continuous on the 208V side.
The full load amps @ 480V are 55. So 55 x 1.25 = 68.79 amps continuous on the 480V side.

I believe I am correct that I size the primary OCPD to 160Amps. I size the primary wire accordingly.
I was questioning myself that I size it based on the transformer max ampacity, but this is a single dedicated load on that XFMR, only for the chiller.

Location is using NEC 2020.
What I am confused about is section 450.3 of the code.
Its my understanding that based on 240.4(F) since this is neither a 2 wire transformer or a delta>delta transformer, I need OCPD protection on both the
primary and secondary......but then I look at table 450.3(B) and it seems to imply I dont need secondary OCPD if I size the
primary OCPD within 125% or the continuous amps....which is 158.75.......so do I need secondary OCPD?

thanks so much for any help.

 

[LarryFine]

I have a 208 wye > 480V Delta 75kva transformer. The chiller runs on 480V, 3 phase. The service is 208V.

Don't connect anything to the 208v neutral terminal; let it float.

If this is a step-down, make sure it is suitable to be reverse connected.

 

[petersonra]

Primary 250% max

Secondary 125% max

Size conductor ampacity to ocpd

Can only skip the secondary ocpd if it is Delta Delta or single phase and you have wye Delta.

 

[infinity]

The full load amps @ 208V are 127. So 127 x 1.25 = 158.75 amps continuous on the 208V side.
The full load amps @ 480V are 55. So 55 x 1.25 = 68.79 amps continuous on the 480V side.

So the chiller only draws 55 amps? What is the MaxOCPD size for the chiller? Do you plan on using the secondary protection for the transformer as the MaxOCPD for the chiller?

 

[jim dungar]

There are four things you must accomplish.

Protect the load, such as panels requiring a main device per 408.36
Protect the secondary side conductors, typically per 240.21(C).
Protect the transformer primary, based on conditions in 450.3
Provide properly sized primary side conductors.

I find it easiest to do them in this order.

 

[d0nut]

Does the chiller have a drive? If so, it probably won't like the 480V delta you are proposing to use.

 

[sokbok]

Don't connect anything to the 208v neutral terminal; let it float.

If this is a step-down, make sure it is suitable to be reverse connected.

thanks for the reply.
The chiller has been running off this transformer for many years and I am now rewiring it in a new location.
From what I can tell by the whip that came with the transformer, they fed the 208Wye side with no neutral. There are (3) 2/0 CU and a ground feeding the 208V side of the transformer. If I dont run a neutral (as it was installed previously), then can I consider the transformer a delta > delta? and only use primary OCPD ?

This transformer appears to be intended to be used as a step down from 480 to 208. We are using it in a reverse configuration.

 

[sokbok]

Does the chiller have a drive? If so, it probably won't like the 480V delta you are proposing to use.

I dont know, it has all sorts of weird stuff going on. Manual attached for the 208V model, ours is the 480V model, otherwise identical.
The chiller has been running off this 480V transformer for many years so it should work.

 

[sokbok]

So the chiller only draws 55 amps? What is the MaxOCPD size for the chiller? Do you plan on using the secondary protection for the transformer as the MaxOCPD for the chiller?

Yes at 480V the chiller should draw 55 amps max. Id say the OCPD size should be 60Amps. I plan on using the secondary protection for the transformer to also be the OCPD for the chiller. Its frusturating because there is no manual for his 480V version of the chiller, made in 2006, the only manual that is available is for the 208V version. which is attached. Id ideally like to not run a neutral on the 208V side and only use the primary OCPD on the 208V side as the only OCPD for the whole thing, but Im not sure that is possible.

 

[augie47]

As Larry noted in Post #2, you do NOT want to connect the neutral on the 208v side.

 

[don_resqcapt19]

...
I have a 208 wye > 480V Delta 75kva transformer. The chiller runs on 480V, 3 phase. The service is 208V.
...

thanks so much for any help.

You will have to make the 480 volt side a corner grounded system or provide ground fault detection on the 480 volt side.
Some equipment will not like a 480 volt corner grounded system.

 

[don_resqcapt19]

Yes at 480V the chiller should draw 55 amps max. Id say the OCPD size should be 60Amps. I plan on using the secondary protection for the transformer to also be the OCPD for the chiller. Its frusturating because there is no manual for his 480V version of the chiller, made in 2006, the only manual that is available is for the 208V version. which is attached. Id ideally like to not run a neutral on the 208V side and only use the primary OCPD on the 208V side as the only OCPD for the whole thing, but Im not sure that is possible.

Not possible because your transformer is a wye/delta, and you can only do that with delta/delta transformer.

 

[LarryFine]

Not possible because your transformer is a wye/delta, and you can only do that with delta/delta transformer.

Not true; that's what the leave-the-neutral-terminal-floating comments are about.

 

[infinity]

Not true; that's what the leave-the-neutral-terminal-floating comments are about.

I believe that Don's comment was about only using primary protection for the transformer.

 

[LarryFine]

I believe that Don's comment was about only using primary protection for the transformer.

Ah, I missed that.