Transformer sizing for deriving a neutral

[304sparky]

Hello,

I am needing to know the process to calculate my transformer size. I have 480V delta coming from a bus plug, I need to derive a neutral for a 200 amp disconnect panel. I know my neutral load is not huge, but how do I size a transformer for it’s sole purpose to derive a neutral?

Thanks.

 

[infinity]

Are you planning on installing a 3Ø, 480-480Y/277 volt transformer?

Welcome to the Forum.

 

[304sparky]

Yes, the transformer will be 480V delta to 480V wye.

 

[infinity]

I need to derive a neutral for a 200 amp disconnect panel

So you're load is going to be 200 amps? What size OCPD will there be on the secondary side of the transformer? Are you looking for the minimum neutral size because you want to use a neutral conductor that is smaller than the ungrounded conductors?

 

[304sparky]

My OCPD is 200 amps. I am looking for the transformer size. I only need to derive a neutral with the transformer. Will my transformer still need to be sized to my OCPD even though I’m just creating a neutral?

 

[infinity]

A 112.5 kva 480-480Y/277 transformer would have a secondary current of 234 amps, and a 75 kva would be 156 amps which is likely too small for your 200 amp load. Either one would have a neutral large enough for the small neutral load that you're adding.

 

[winnie]

While you say 'derive a neutral', the transformer being described (480V:480/277V delta:wye) is an isolation transformer powering the entire load. No special calculation needed.

If you just want to derive the neutral only, then you need something like a zig-zag transformer. But you are not allowed to use this to derive a neutral for a panel; you use (and size) such a transformer to ground an entire ungrounded electrical system.

If you have a load which requires 480/277V, and your supply is some other flavor of 480, then you need the delta:wye sized for the full load, even if the neutral current is small.

Jon

 

[304sparky]

While you say 'derive a neutral', the transformer being described (480V:480/277V delta:wye) is an isolation transformer powering the entire load. No special calculation needed.

If you just want to derive the neutral only, then you need something like a zig-zag transformer. But you are not allowed to use this to derive a neutral for a panel; you use (and size) such a transformer to ground an entire ungrounded electrical system.

If you have a load which requires 480/277V, and your supply is some other flavor of 480, then you need the delta:wye sized for the full load, even if the neutral current is small.

Jon

From what I am understanding, regardless of whether I’m just deriving a neutral or not, I still need to size my transformer to my load for a distributing panel.

 

[jim dungar]

From what I am understanding, regardless of whether I’m just deriving a neutral or not, I still need to size my transformer to my load for a distributing panel.

Yes, your transformer needs to be able to handle the anticipated load.
If you need a no load neutral then, as mentioned above, you could use a special grounding transformer, also called an artificial neutral, sized only to the expected ground fault current.

 

[electrofelon]

While you say 'derive a neutral', the transformer being described (480V:480/277V delta:wye) is an isolation transformer powering the entire load. No special calculation needed.

If you just want to derive the neutral only, then you need something like a zig-zag transformer. But you are not allowed to use this to derive a neutral for a panel; you use (and size) such a transformer to ground an entire ungrounded electrical system.

If you have a load which requires 480/277V, and your supply is some other flavor of 480, then you need the delta:wye sized for the full load, even if the neutral current is small.

Jon

Jon, isn't the transformer mentioned suitable for deriving a neutral as an autotransformer (not connecting anything to the delta side)?

 

[infinity]

If his existing OCPD is 200 amps and he installs a transformer, say 112.5 kva will nuisance tripping on start up be an issue?

 

[304sparky]

A 112.5 kva 480-480Y/277 transformer would have a secondary current of 234 amps, and a 75 kva would be 156 amps which is likely too small for your 200 amp load. Either one would have a neutral large enough for the small neutral load that you're adding.

How are you getting 234 amps out of a 112.5 kva transformer? My calculations give me 166 amps with a 112.5 kva transformer.

 

[wwhitney]

How are you getting 234 amps out of a 112.5 kva transformer? My calculations give me 166 amps with a 112.5 kva transformer.

Whereas for a 480Y/277V secondary I get 112.5 kVA / 480V / sqrt(3) = 135A.

Cheers, Wayne

 

[infinity]

Yup I left out the 1.73.

 

[304sparky]

Whereas for a 480Y/277V secondary I get 112.5 kVA / 480V / sqrt(3) = 135A.

Cheers, Wayne

With my calculations, with my OCPD on my distribution panel being 200 amps. My transformer needs to be rated at 225kva to be properly sized, correct?

 

[topgone]

How are you getting 234 amps out of a 112.5 kva transformer? My calculations give me 166 amps with a 112.5 kva transformer.

He was correct in saying your 100% neutral current requirement of 200A can be carried by a 112.5 kVA 480Y- 480 delta transformer. Just some kinks in the math.
Picture this: you are using a wye-grounded primary, delta secondary transformer with the delta secondary having no loading at all. The neutral is derived from the grounded wye-point of the primary. The possible full neutral current of 200A on your load side passing the neutral conductor and entering at the neutral point of the transformer will be shared equally by the three phases of the transformer, That 200A can be accommodated by a 112.5 kVA. See:

• current rating of the 112.5 kVA transformer = 112,500/(1.732 x 480)= 135A,
• That means the neutral current possible is = 3 x phase current rating = 3 x 135 = 405A
• 200A is very much smaller than 405A

A 75 kVA can carry a neutral of 271A if you choose to use a smaller transformer though.

 

[topgone]

With my calculations, with my OCPD on my distribution panel being 200 amps. My transformer needs to be rated at 225kva to be properly sized, correct?

Nope. 1/3 of that can suffice (75 kVA). See my reply above.

 

[304sparky]

He was correct in saying your 100% neutral current requirement of 200A can be carried by a 112.5 kVA 480Y- 480 delta transformer. Just some kinks in the math.
Picture this: you are using a wye-grounded primary, delta secondary transformer with the delta secondary having no loading at all. The neutral is derived from the grounded wye-point of the primary. The possible full neutral current of 200A on your load side passing the neutral conductor and entering at the neutral point of the transformer will be shared equally by the three phases of the transformer, That 200A can be accommodated by a 112.5 kVA. See:

• current rating of the 112.5 kVA transformer = 112,500/(1.732 x 480)= 135A,
• That means the neutral current possible is = 3 x phase current rating = 3 x 135 = 405A
• 200A is very much smaller than 405A

A 75 kVA can carry a neutral of 271A if you choose to use a smaller transformer though.

Nope. 1/3 of that can suffice (75 kVA). See my reply above.

To preface my situation, I am using coming off a bus line 480V delta, which is feeding a 200 amp distribution panel, however, the loads I have need a neutral. I have 8 chargers that charge electric fork trucks. The OCPD for those devices are 30 amps. The neutral load I metered for each charger was 6 amps.

I need to make sure if other circuits are added, my conductors are safe.

What are you suggestions for going about this?

 

[david luchini]

To preface my situation, I am using coming off a bus line 480V delta, which is feeding a 200 amp distribution panel, however, the loads I have need a neutral.

You want a 150kVA transformer

 

[topgone]

To preface my situation, I am using coming off a bus line 480V delta, which is feeding a 200 amp distribution panel, however, the loads I have need a neutral. I have 8 chargers that charge electric fork trucks. The OCPD for those devices are 30 amps. The neutral load I metered for each charger was 6 amps.

I need to make sure if other circuits are added, my conductors are safe.

What are you suggestions for going about this?

So, do you want to consider the harmonics of those chargers? Do you have the specs of the chargers? Is the manufacturer of the charger claiming it emits xx% y harmonics level?
Since you have measured a 6 A neutral current on the charger, 6 x 8 = 48A. If that's all you have, then size a transformer that can handle ~48A! For good measure, give it a 150% fudge factor should you happen to buy cheaper and more harmonics-laden ones, say you design for a total neutral current of 75A for the 8 units. That would be like a smaller grounding transformer of 75/3 x 480 x 1.732/1,000 = 20 kVA ~ 30 kVA, 3-phase, 480Y.