208 to 480 xfrm OCP sizing

[Trickyflea]

I am installing power for a large printer that is not on site yet but the printer manufacturer has supplied a 35 kva 208 to 480 transformer.
I need some help sizing both the primary and secondary conductors and OCP.

 

[infinity]

Is this going to be a Deta-Wye step up transformer?

 

[Trickyflea]

Yes

 

[don_resqcapt19]

35 kVA is not a common transformer size.

The maximum size of the primary OCPD is 250% of the primary rated current. The primary supply conductors are sized to the primary OCPD, so I would keep the primary protection closer to 125%.
The secondary of the transformer will need protection at no more than 125% of the secondary rated current. This is most often provided by the secondary conductor protection required by 240.21(C)

XO of the secondary will need to be connected to a grounding electrode via a grounding electrode conductor.

 

[Trickyflea]

Thank you Don

 

[Jpflex]

I am installing power for a large printer that is not on site yet but the printer manufacturer has supplied a 35 kva 208 to 480 transformer.
I need some help sizing both the primary and secondary conductors and OCP.

Assuming 3 phase grounded system delta / Wye
35,000 VA / 208 x 1.732 = 97.15 i

Primary if max OCPD protection 97.15 x 250% = 243 i or (i think you cannot go up to next standard size breaker normally but this is a step up transformer?)


PRIMARY
so 225 ampere standard size primary side breaker with 4/0 THHN good for 230i at 75 degrees celcius



SECONDARY
Secondary 35,000 VA / 480 x 1.743 = 42.09 i amperes

42.09 i x 125% = 52.62 i or 53 amperes

Secondary OCPD 60 ampere maximum next size up standard breaker (if allowed) and a #6 AWG THHN good for 55 amperes at 60 degrees celcius (depends on equipment termination rating)

 

[Trickyflea]

Thank you Jp

 

[infinity]

Secondary OCPD 60 ampere maximum next size up standard breaker (if allowed) and a #6 AWG THHN good for 55 amperes at 60 degrees celcius

The next size up rule is not permitted for transformer secondary conductors. Since most terminals are rated at 75° C your #6 AWG THHN condcutors will have an ampacity of 65 amps so they would work with a 60 amp OCPD.

 

[Jpflex]

The next size up rule is not permitted for transformer secondary conductors. Since most terminals are rated at 75° C your #6 AWG THHN condcutors will have an ampacity of 65 amps so they would work with a 60 amp OCPD.

I don’t have my NEC book for reference but yes i was just going off what i could recall.

I couldn’t remember if it was the primary or secondary which could not have the OCPD bumped up to the next larger standard size breaker exceeding the ampacity of the conductor it protects.

My original thought was that the primary could have been bumped up but not secondary. Perhaps I had this reversed?

The termination of the secondary being under 100 amperes is why i selected a wire size based on 60 degrees c according to code rules. However, in case the equipment was listed for 75 degree c, i did make a note in brackets

 

[infinity]

My original thought was that the primary could have been bumped up but not secondary.

Correct.

 

[don_resqcapt19]

Assuming 3 phase grounded system delta / Wye
35,000 VA / 208 x 1.732 = 97.15 i

Primary if max OCPD protection 97.15 x 250% = 243 i or (i think you cannot go up to next standard size breaker normally but this is a step up transformer?)


PRIMARY
so 225 ampere standard size primary side breaker with 4/0 THHN good for 230i at 75 degrees celcius



SECONDARY
Secondary 35,000 VA / 480 x 1.743 = 42.09 i amperes

42.09 i x 125% = 52.62 i or 53 amperes

Secondary OCPD 60 ampere maximum next size up standard breaker (if allowed) and a #6 AWG THHN good for 55 amperes at 60 degrees celcius (depends on equipment termination rating)

I would not use anything larger than 125 amp OCPD on the primary. Why would I want to use 4/0 when I can use 1 AWG?

 

[winnie]

The reason to use larger OCPD on the primary side is to avoid nuisance tripping due to inrush current when energizing the transformer.

With that said, in my very limited experience, I've never had a problem with tripping on inrush even with primary OCPD <100%.

To the OP: do you know how much fault current is available at 208V? If your 208V supply has lots of impedance, you can't get much inrush, and 125% primary OCPD should be just fine.

Jon

 

[Jpflex]

I would not use anything larger than 125 amp OCPD on the primary. Why would I want to use 4/0 when I can use 1 AWG?

The decision you make for the size breaker on the primary can be just about any size under 250% of its rating. Whether you choose more or less depends on whether your system will have an in rush start up current problem and cause be usable breaker tripping.

I provided an example of maximum allowed OCPD but you can choose the size breaker and wire at your discretion

 

[Jpflex]

Usable breaker tripping = “nuisance” breaker tripping. Phone changed typing

 

[don_resqcapt19]

The decision you make for the size breaker on the primary can be just about any size under 250% of its rating. Whether you choose more or less depends on whether your system will have an in rush start up current problem and cause be usable breaker tripping.

I provided an example of maximum allowed OCPD but you can choose the size breaker and wire at your discretion

I have never seen a transformer breaker trip on inrush when sized at 125%, and one of my professional engineer friends would use a 100 amp primary OCPD for this.

 

[Jpflex]

I have never seen a transformer breaker trip on inrush when sized at 125%, and one of my professional engineer friends would use a 100 amp primary OCPD for this.

You would have to take this up with NEC. I did not make the rules or process of OCPD selection. The installer must follow manufacture listed instructions and NEC code when making a decision on what size wire and breaker to run, but once it’s installed the money is already spent.

 

[don_resqcapt19]

You would have to take this up with NEC. I did not make the rules or process of OCPD selection. The installer must follow manufacture listed instructions and NEC code when making a decision on what size wire and breaker to run, but once it’s installed the money is already spent.

Take what up with the NEC??? Any size primary OCPD that is 250% or less than the primary current is permitted by the code.
You will have to show me transformer instructions that actually specify the size of the primary OCPD. Installed hundreds from 1 kva to 2500 kva and have never seen such an instruction.

 

[Jpflex]

Any size primary OCPD that is 250% or less than the primary current is permitted by the code.

Isn’t this what I’ve already said?

 

[winnie]

The NEC _permits_ up to 250% of transformer primary current rating for the OCPD.

@don_resqcapt19 has said that he would not use more than 125% of transformer primary rating, and has installed lots of transformers. In your experience, have you ever experienced nuisance tripping on inrush when using a 125% OCPD?

I have much more limited experience (2 or 3 transformers in my lab) and have never had problems with inrush tripping, always using < 125% primary OCPD.

I've seen discussions of people having inrush tripping issues, but not many.

The fact that code permits higher primary current rating may be a historical artifact. Or maybe going larger than 125% OCPD is simply rarely actually needed.

-Jon

 

[Jpflex]

The NEC _permits_ up to 250% of transformer primary current rating for the OCPD.

@don_resqcapt19 has said that he would not use more than 125% of transformer primary rating, and has installed lots of transformers. In your experience, have you ever experienced nuisance tripping on inrush when using a 125% OCPD?

I have much more limited experience (2 or 3 transformers in my lab) and have never had problems with inrush tripping, always using < 125% primary OCPD.

I've seen discussions of people having inrush tripping issues, but not many.

The fact that code permits higher primary current rating may be a historical artifact. Or maybe going larger than 125% OCPD is simply rarely actually needed.

-Jon

It would likely depend on your breaker trip curve time and available fault current at transformer among other things.

What’s your transformer impedance value?