Xformer question

[mjr10]

Hi all-
Sorry, but I'm a little rusty here. I was wondering if anyone could check my reasoning and make sure I'm not off track?
I'm installing a 30kVA 3-phase dry-type transformer at a warehouse on a 480/277 Y system. The transformer is 480 delta primary to 120/208 Y secondary. The mfr told me I can use this so long as I don't make a neutral connection on the primary side; just bond the neutral (X0) to ground on the secondary side. Does this seem correct?
Secondly, I wanted to make the entire capacity of the xformer available for use in the building. I calculated a primary circuit breaker size of 50A (30kVA/{480x1.73} plus 125%) and 125A on the secondary side (30kVA/{208x1.73} plus 125%) which is 104, but I went up to the next size?
Lastly, do I need primary and secondary circuit breaker protection? I thought I did only because of the bonding of X0 and ground on the secondary side.

Any thoughts, advice, opinions would be welcomed. I'll read Article 450 in the meantime. Thanks!!

 

[augie47]

Hi all-
Sorry, but I'm a little rusty here. I was wondering if anyone could check my reasoning and make sure I'm not off track?
I'm installing a 30kVA 3-phase dry-type transformer at a warehouse on a 480/277 Y system. The transformer is 480 delta primary to 120/208 Y secondary. The mfr told me I can use this so long as I don't make a neutral connection on the primary side; just bond the neutral (X0) to ground on the secondary side. Does this seem correct?

Correct, No neutral connection on the primary

Secondly, I wanted to make the entire capacity of the xformer available for use in the building. I calculated a primary circuit breaker size of 50A (30kVA/{480x1.73} plus 125%) and 125A on the secondary side (30kVA/{208x1.73} plus 125%) which is 104, but I went up to the next size?

Good logic, but the next "size" is 110 amps (210.4)

Lastly, do I need primary and secondary circuit breaker protection? I thought I did only because of the bonding of X0 and ground on the secondary side.

You need both. See 240.21(C)(1)

Any thoughts, advice, opinions would be welcomed. I'll read Article 450 in the meantime. Thanks!!

 

[david luchini]

You will require an OCPD on both sides of the transformer to protect the conductors but not necessarily to protect the transformer.

The transformer can be protected as primary only, with the primary OCPD being not more than 125% of primary rated current, using the next standard size up. In this case, the secondary OCPD can be any size you want, there is no maximum rating requirement. Keep in mind that the secondary conductors must have an ampacity of at least the rating of the secondary circuit breaker (next standard size up not allowed.)

The transformer can also be protected as primary/secondary. In this case, the secondary OCPD should be not more than 125% of the secondary rated current (using the next standard size up) and the primary OCPD can be rated not more than 250% of the primary current.

Also, you note a secondary current of 104A, and using a 125A c/b. If my memory is correct, a 110A OCPD would be the next standard size up for the 104A that you list.

 

[mjr10]

Thanks for the input from both of you!
So correct me if I'm wrong, but from what I see I can't omit secondary OCP since I have a multiphase secondary according to 240.21(C)(1)?

Also, am I correct in assuming that there's no restriction to install primary and secondary conductors in the same conduit (sized properly, of course)?

 

[jeremysterling]

Good topic and Q&A.

I'm not sure if you need secondary conductor protection if your conductors are less than 10 feet long. 240.21(C)(2).

However, the FPN in that section leads you to 408.36(A),(B), & (E). There is also 408.36(D). These sections seem to me to be the requirement for secondary OCPD. Is this correct?

 

[david luchini]

from what I see I can't omit secondary OCP since I have a multiphase secondary according to 240.21(C)(1)?

That is correct, only a delta-delta (3 ph) transformer would allow the secondary conductors to be protected by the primary OCPD, in accordance with 240.21(C)(1)

Also, am I correct in assuming that there's no restriction to install primary and secondary conductors in the same conduit (sized properly, of course)?

I don't know anything that specifically prohibits this, but why would you want to run then in the same conduit? They should be coming from/going to different locations.

I'm not sure if you need secondary conductor protection if your conductors are less than 10 feet long. 240.21(C)(2).

A secondary OCPD would be required to protect the conductors that are less than 10 feet.

 

[mjr10]

That is correct, only a delta-delta (3 ph) transformer would allow the secondary conductors to be protected by the primary OCPD, in accordance with 240.21(C)(1) I don't know anything that specifically prohibits this, but why would you want to run then in the same conduit? They should be coming from/going to different locations.


Normally, I wouldn't, but there is a bus duct running under the 480 panels and the only location where I can install the 120/208 panelboard (we've inherited this job from another EC who was dismissed).

 

[LarryFine]

MJ, welcome to the forum!

The transformer is 480 delta primary to 120/208 Y secondary. The mfr told me I can use this so long as I don't make a neutral connection on the primary side;

A Delta primary would have no X0 anyway. If you were using this unit in reverse, or any unit with a wye primary, the warning would apply, of course.

 

[skeshesh]

It's been mentioned that a secondary is necessary for conductor protection if the conductor is not less than 10 feet. Actually, you may also use the 25 feet rule per 240.21(C)(6), if the secondary conductors have a min. ampacity of no less than of value of primary-to-secondary voltage ration multiplied by 1/3 of the rating of primary OCPD, the conductors terminate in a single CB or set of fuses limiting the current, and that the conductors are protected from physical damage.
Using that rule, if you have a 50A primary OCPD and the voltage ratio is 2.3ish (480/208), then your allowed ampacity is 50 x 2.3 x 1/3 = 38.46. So as long as the conductor connected to the xformer secondary has an ampacity greater than, lets say 39A you don't have to provide secondary OCPD. Keep in mind this rule applies to individual conductor taps from the xformer secondary, meaning if you have one set of conductors that exceeds the value, you can provide an enclosed CB no more than 10 ft. away and then run to the load and at the same time have smaller conductors connected to the xformer secondary terminal that don't need secondary protection and can be routed and connected directly to your load.

 

[david luchini]

Using that rule, if you have a 50A primary OCPD and the voltage ratio is 2.3ish (480/208), then your allowed ampacity is 50 x 2.3 x 1/3 = 38.46. So as long as the conductor connected to the xformer secondary has an ampacity greater than, lets say 39A you don't have to provide secondary OCPD. Keep in mind this rule applies to individual conductor taps from the xformer secondary, meaning if you have one set of conductors that exceeds the value, you can provide an enclosed CB no more than 10 ft. away and then run to the load and at the same time have smaller conductors connected to the xformer secondary terminal that don't need secondary protection and can be routed and connected directly to your load.

Skeshesh, I'm not sure that I understand what you are saying here, can you elaborate?

It seems to me that 240.21(C)(2), (C)(3), (C)(4), (C)(5) and (C)(6) all require an overcurrent protection device to protect the secondary conductors, some at 10ft, some at 25ft, or unlimited length for outside conductors.

Only 240.21(C)(1) for a single phase 2wire secondary transformer, or 3 phase delta-delta 3wire secondary transformer alllows the secondary conductors to not have an OCPD, as they are considered to be protected by the primary OCPD if they meet the requirements of that section.