208 to 480 xfrm OCP sizing

[don_resqcapt19]

Isn’t this what I’ve already said?

Then why did you tell me to take it up with the NEC???????????

 

[Jpflex]

Then why did you tell me to take it up with the NEC???????????

Based on what you said previously it appeard you were upset my calculated results for maximum wire size or breaker size. These were the results based on NEC rules and calculations. I believe you asked why should you use such a large wire and I told you, you didn’t have to but could be within 250% of primary current rating

 

[don_resqcapt19]

Based on what you said previously it appeard you were upset my calculated results for maximum wire size or breaker size. These were the results based on NEC rules and calculations. I believe you asked why should you use such a large wire and I told you, you didn’t have to but could be within 250% of primary current rating

not upset, just saying your sizing wastes money and it appeared that you were trying to say that the NEC requires the use of the 250% primary OCPD

 

[Jpflex]

not upset, just saying your sizing wastes money and it appeared that you were trying to say that the NEC requires the use of the 250% primary OCPD

My sizing would not waste money. I have a RANGE withinn 250% and it’s not my sizing it’s code calculation and compliance

 

[winnie]

Using 250% OCPD on the primary wastes money if a smaller OCPD would do the job.

Using 125% wastes money if it causes excessive nuisance tripping and needs to be replaced.

Both are permitted by the NEC. Probably worth a survey of members to see how often nuisance tripping on inrush has been a problem for them with modern transformers and breakers.

Jon

 

[augie47]

I'm with don (Posr #15). Ive installed a lot of transformers with 125% primary and don't recall any nuisance trip.

 

[TX+ MASTER#4544]

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 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.

Hello, Trickyflea
Here's your answer.

Assuming 3 phase 480 volts primary side
208/120 volts secondary side
35 kVA transformer
2023 NEC Section 450.3(B) Transformers 1,000 Volts or Less.....refers to use T.450.3(B).

Primary side first:
Step 1: 35 kVA x 1,000 = 35,000 VA
Step 2: 35,000 VA / 480 x 1.732 = 831.36.....................35,000 VA / 831.36 = 42.10 amps primary side amps
Step 3: Select primary OCPD.........250 % x 42.10 = 105.25 amps
Step 4: No Foot Note here, cannot round up to next size
Step 5: Because there's no Foot Note that allows us to round up, go to T.240.6 (A) Standard Ampere Ratings for Fuses and Inverse Time Circuit Breakers............There we select a 100 amp OCPD which is under the 105 amp which was our product of multiplying by 250 %

Now the secondary side:
Step 1: 35 kVA x 1,000 = 35,000 VA
Step 2: 35,000 VA / 208 x 1.732 = 360.25.................35,000 VA / 360.25 = 97.15 amps secondary side
Step 3: Select secondary OCPD.................125% x 97.15 = 121.44 amps
Step 4: We have a Foot Note # 1 that allows for round up of 125% thus......121.44 amps x 125% = 121.44 amps
Step 5: 121.44 amps and using T.240.6(A) and a Foot Note #1 found in T.450.3 (B) we round up to a 125 amp OCPD
A 125 amp OCPD for the secondary side protection.

There's no guess work involved here, it's straight out of the Code book.

Comments accepted
Thanks for reading
TX+MASTER#4544

 

[infinity]

Here's your answer.

Assuming 3 phase 480 volts primary side
208/120 volts secondary side

You have it backwards. This is a step up transformer. From the thread title:

208 to 480 xfrm OCP sizing​

 

[winnie]

[...]
Assuming 3 phase 480 volts primary side
208/120 volts secondary side
[...]
Step 3: Select primary OCPD.........250 % x 42.10 = 105.25 amps
[...]...There we select a 100 amp OCPD which is under the 105 amp which was our product of multiplying by 250 %
[...]
There's no guess work involved here, it's straight out of the Code book.

As noted the OP is looking at a 208V primary, 480V secondary.

@TX+ MASTER#4544 your calculations are exactly correct for the step down 480:208/120V transformer. But you've missed the key point being discussed.

The 250% factor is the maximum that the code allows, not a value that the code requires. If someone follows your procedure the result will be a safe and functional installation.

For most installations, a 125% factor for primary OCPD will also be safe, functional and more economical. For some rare installations, a 125% factor will result in tripping on inrush current. The consensus of people in this discussion with lots of transformer installation experience is that using a 125% factor is fine; a proper survey would be needed to determine if 125% is always or usually fine.

The 250% factor is a code mandated maximum. Using less than 250% is a code permitted design decision.

So no guess work, but some thinking involved.

-Jonathan

 

[Jpflex]

You have it backwards. This is a step up transformer. From the thread title:

That’s what i was going to say. His math had me confused as to why he was diving 35 kVA by 480 instead of 208 for the primary side

One thing i would like to mention is that it can seem complicated if converting and stepping up a single phase to a 3 phase higher voltage system

I have not installed such a set up yet but i would think that a single phase source could not be readily converted into a three phase separately derived system, without additional equipment. For example, maybe im unaware but I don’t think you could go from a single phase source to just feed a second three phase transformer to get three phases without extra equipment?

Wouldn’t you need some kind of a converter between two transformers before feeding a three phase transformer from a single phase source to do this? If you were to attempt to step up a standard single phase 120/240 volt 30 kVA transformer source to 480 three phase would you not first have to use a converter that could artificially creat 3 phases off a single phase system by using capacitors to creat a 120 degree phase displacement among 3 line leads? Then use this to supply the primary of a 3 phase transformer?


What happens to calculating primary current if feeding a three phase transformer with a single phase source? do you still use 1.732 multiplier or exclude it? How would you install such a set up. I have only installed single phase to single phase systems and three phase to single phase grounded and ungrounded systems but not yet single phase to three phase

 

[winnie]

To go from single phase to three phase, you must have some sort of additional conversion equipment, not just a transformer. This additional conversion equipment must include some sort of energy storage and conversion hardware.

The simplest such system is a rotary phase converter, which is basically a 3 phase motor fed with single phase to 2 terminals. The spinning rotor acts as the energy storage, and the interaction between the rotor magnetic field and the stator coils generates the 3rd leg of the 3 phase system. Additional hardware is needed to ensure the motor actually starts spinning on the input power, and additional components may be used to regulate voltage.

As far as calculating current in such a system, IMHO the best way is to separately calculate KW and KVA at each stage in the process, including factors for efficiency, and then use the appropriate single phase or three phase calculation for the stage being considered.

Say, for example that you have a 480V 10kVA 3 phase system supplied by a 240V single phase source. You'd probably have a 240V phase conversion system, followed by a 240V to 480V step up. You would use the 1.732 factor in calculating the current at 480V. You'd also use the 1.732 factor on the 240V three phase side. The phase converter probably has a power factor change; depending upon the technology you might actually get _improved_ power factor, but you also take an efficiency hit. So for simplicity we will _assume_ the same 10kVA input to the phase converter. Here you have 240V single phase, so for this stage you _don't_ use the 1.732 factor.

Assuming ideal systems and no change in power factor, I get:
1 phase 240V, 10kVA: 41.7A on each of 2 conductors, converted to ->
3 phase 240V, 10kVA: 24A on each of 3 conductors, converted to ->
3 phase 480V, 10kVA: 12A on each of 3 conductors


-Jon