45 kva Transformer questions

[Rogue305]

Hello,

I am currently installing a 208v delta to 480v wye transformer to supply a milling machine. I have done my homework but I am second guessing myself on this so I could use another opinion.

I have calculated my primary breaker to be 156 amps which I will raise up to 175a.
I will be using 2/0 as feeders
My secondary will be rated @ 70 amps and protected by a fused disconnect.
I will be using #6
My grounding will be a #6

Are my conductors overkilled?

This particular machine only needs 440 volts and needs a 40 amp circuit, the machine is also internally fused at 40 amps.

I will go from my equipment ground on the machine, to my disconnect then to the case on the transformer. I will bond the XO of my secondary side to the case of the transformer then building steel (right next to the transformer).

one of my concerns is when I pipe my primary to the transformer will this become another path to ground?

I have seriously confused myself.....maybe im just getting old

any help would be appreciated

 

[augie47]

You need equipment grounding on the primary to assure the transformer is grounded in the event of a primary ground fault. The added path is no problem.
The only question I have would be on the length of your secondary conductors. You are fine on the 10 ft tap rule.

 

[Rogue305]

You need equipment grounding on the primary to assure the transformer is grounded in the event of a primary ground fault. The added path is no problem.
The only question I have would be on the length of your secondary conductors. You are fine on the 10 ft tap rule.

Thank you so much for the fast reply,

The machine is only 5 foot from the transformer however, I will need to go up a column 110 foot, accross 10 foot then down another 8 foot to top feed the machine, si I seen that as 28 feet hence the secondary disconnect.

 

[david luchini]

Your secondary conductors (#6) are undersized for your 70A fuses per 240.21(C).

The ampacity of the secondary conductors needs to be not less than the rating of the OCPD at the termination of the secondary conductors.

 

[Rogue305]

Your secondary conductors (#6) are undersized for your 70A fuses per 240.21(C).

The ampacity of the secondary conductors needs to be not less than the rating of the OCPD at the termination of the secondary conductors.

Type-O
Should have read #4 (I should have looked before I posted):roll:

 

[Rogue305]

In thinking about this, it seems that the transformer is oversized, I am wondering if it is worth downsizing the OCP and the primary feeders to be more cost effective.

The machine needs 3 phase @ 440 volts and 40 amps.

Any suggestions?

 

[augie47]

few problems in my opinion
1st... Thank you, David, for catching the secondary ampacity... but the #4s took care of that
2nd .. Other than outside secondary conductors, I see no rule that allows you to exceed 25 ft. (with 3P4W or 1p3w secondary)

I woud think you need to install your secondary protection within 10 ft of the transformer.

 

[Rogue305]

Yes,

I will be mounting the secondary fused disconnect directly on the column next to the transformer (less that 5 foot)

 

[augie47]

With that in mind (a secondary conductor 25 ft or less), I see no problem.

One thing to keep in mind in downsizing you primary OCP device is inrush current. You don't want to choose a breaker that will trip on inrush.
The only way I know to avoid that for sure would be an engineering study comparing your transformer to breaker trip curves.
Zog, or another breaker guru, can possibly give us some guidance on that,

 

[zog]

Just be sure to get adjustable trips on your breaker. Poors mans coordination study, all setting to min, energize transformer. If it trips, increase delay, wash rinse repeat.

 

[Rogue305]

One last question,

If I leave the transformer and go directly into a fused disconnect (with in 5 foot), how far can I then travel with my load side and still be compliant? is it still only 25 feet?.

I looked at 240.21 but Im not clear if it means the taps just to the OCPD or the load leaving the OCPD.

Thanks again..

 

[kingpb]

The 440V machine on 480V system is questionable because that is 110% of rated.

If you have an over-voltage on your 208V system you could end up possibly damaging your machine. You will have very little VD because of your short cable distance so you can't count on that. Since you have a transformer there, you might want to think about using the taps on the Xfmr to pull your rated voltage down to 456V.

 

[Rogue305]

I most certainly will be adjusting my taps to lower the voltage closer to the 440v required....

Would anyone like to comment on my secondary question? like I said Im not clear on how it applies to 240.21

 

[david luchini]

One last question,

If I leave the transformer and go directly into a fused disconnect (with in 5 foot), how far can I then travel with my load side and still be compliant? is it still only 25 feet?.

I looked at 240.21 but Im not clear if it means the taps just to the OCPD or the load leaving the OCPD.

Thanks again..

After the fused disconnect switch, there is no limit on the length of the conductors to the load.

 

[jim dungar]

Would anyone like to comment on my secondary question? like I said Im not clear on how it applies to 240.21

Once you leave an OCPD you are dealing with 240.4 and 240.21(A) issues. See the definition of a Tap Conductor in 240.2 before trying to apply 240.21(B). Note how sections 240.21(C)(2) to 240.21(C)(6) all end at a protective device.

 

[Rogue305]

Thank you all very much for your input, it is greatly appreciated!