X-frmr calculation

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eager2learn

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I have a 400 amp buss that is a 3 phase 4 wire system rated for 277/480 volts. This application is suppose to feed a 150kva step down transformer = 480-120/208 volts 3 phase 4 wire. I'm having diffuculties trying to figure out the feeder conductors aswell as over current protection for this particular installation. Where do I start?
 
My suggestion,,, Start by verifying the load ;) Sometimes mistake are made.
Then, as trying hard suggests, look at Table 450.3(B) which covers transformer protection. Be sure to select a method that will protect your transformer.
Taking into account the load and the overcurrent device you have selected from 450.3 you can size your wire.

A very important step that is often missed is making sure to satisfy 240.21 (C) on your transformer secondary conductors.

A common primary OCP for a 150 kva would be 225 or 250 amp.

150 kva is a bit unusual in this atra..are you feeding a single panel ?
 
Start with the load and work backwards to the transformer feeder.

For example, based on NEC-2008.
1. Size the secondary panel (if used) per 408.30.
2. Provide a panel board main device per 408.36 (may be mounted remote from the panel)
3. Size and 'locate' the transformer secondary conductors to the panel per 240.4(F) and 240.21(C). NOTE: the device protecting these conductors may also be used to protect the panel.
4. Choose the primary protection for the transformer based on 450.3(b) at a not more than 125% FLA, or you can go up to 250% if the device used in step 3 is appropriately sized (may be mounted remote from the transformer).
5. Size the primary conductors based on the primary protective device in step 4.
6. Protect the primary conductors; based on 240.4 and located using 240.21(B). NOTE: the device protecting these conductors may also be used to protect the transformer primary.
 
xformer said:
Am I wrong for calculating the primary current as 180.43 amps? So wouldnt that require a 200 amp breaker??
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Yes, the primary current is 180.43 Amps.

So the primary c/b could be as large as 250A for "primary only protection," or
400A for "primary & secondary protection" per T450.3(B).
 
So for a transformer with Primary and Secondary OCPD, The primary OCPD is based upon the 125 % of the Primary Current withe the option to go up to 250 % as permitted in the second half of of table 450.3(B) ? And if this is the case, is this expansion in percentage the cure for inrush? Must I then resize my primary conductors?
 
xformer said:
So for a transformer with Primary and Secondary OCPD, The primary OCPD is based upon the 125 % of the Primary Current withe the option to go up to 250 % as permitted in the second half of of table 450.3(B) ? And if this is the case, is this expansion in percentage the cure for inrush? Must I then resize my primary conductors?
Click to expand...
You are looking for this XFMR?s ampere capacity which is fine but don?t lose sight of the most important issue (as others have pointed out early in this post) that is your 208v load this is what you are supplying ? you?re not supplying a XFMR.

The 208v secondary load capacity is 150kva (about 416A), if this secondary feeds a panelboard this panelboard will need to be rated 400A and include 400A protection.

If your XFMR includes the thermal overloads by the manufacture the OCPD can be upsized and I believe the primary circuit conductors can remain rated at the thermal overload rating, see the manufactures specs if this is your case, but you'll need to comply with all of note 3 in Table 450.3(B).
 
tryinghard said:
The 208v secondary load capacity is 150kva (about 416A), if this secondary feeds a panelboard this panelboard will need to be rated 400A and include 400A protection.
Click to expand...

Or you could have at panelboard rated 600A protected by a 500A or 600A OCPD.

tryinghard said:
If your XFMR includes the thermal overloads by the manufacture the OCPD can be upsized and I believe the primary circuit conductors can remain rated at the thermal overload rating, see the manufactures specs if this is your case, but you'll need to comply with all of note 3 in Table 450.3(B).
Click to expand...

I don't see anything that allows the primary conductors to be protected by an "upsized" OCPD. The primary conductors must be protected in accordance with 240.4.
 
david luchini said:
Or you could have at panelboard rated 600A protected by a 500A or 600A OCPD.
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If you sized your load first and it revealed the need for 600A panelboard would you use 150kva XFMR to supply?

david luchini said:
I don't see anything that allows the primary conductors to be protected by an "upsized" OCPD. The primary conductors must be protected in accordance with 240.4.
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You don't believe the thermal overload protection in note 3 accomodates this?
 
tryinghard said:
If you sized your load first and it revealed the need for 600A panelboard would you use 150kva XFMR to supply?
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No, if I sized my load first and it revealed the need for a 150kVA xfmr to supply it, the code would allow me to use a 600A panelboard on the secondary side for a xfmr protected with "primary and secondary" protection per T450.3. (Or even larger if the xfmr is protected with "primary only" protection.)


tryinghard said:
You don't believe the thermal overload protection in note 3 accomodates this?
Click to expand...

I don't see anything in Table 450.3, and specifically in note 3 that mentions protection of conductors at all. T450.3 is for protection of transformers, not conductors. The FPN in 450.3 says to see 240.4, etal. for protection of the conductors.
 
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david luchini said:
No, if I sized my load first and it revealed the need for a 150kVA xfmr to supply it, the code would allow me to use a 600A panelboard on the secondary side for a xfmr protected with "primary and secondary" protection per T450.3. (Or even larger if the xfmr is protected with "primary only" protection.)
Click to expand...
So if the XFMRs rated current is 180A the maximum "primary & secondary" OCP could be 1000A rather than 400A?
 
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tryinghard said:
So if the XFMRs rated current is 180A the maximum "primary & secondary" OCP could be 1000A rather than 400A?
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I'm not sure I understand your question.

If the 150kVA transformer is protected "primary only" per T450.3, then the max primary OCPD size would be 250A. The secondary OCPD could be any size you wanted it to be, including 1000A. The secondary conductors would have to have an ampacity not less than the rating of the secondary OCPD.

If the 150kVA transformer is protected "primary & secondary" per T450.3, then the max primary OCPD would be 400A. The max secondary OCPD would be 600A. The secondary conductors would have to have an ampacity not less than the rating of the secondary OCPD.
 
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