Transformer Overcurrent Protection

[necnotevenclose]

Could someone please check my work and let me know if I'm doing this right? Here is what I have:

225A, 480Y/277V panel that feeds a 45kva 480V:208Y/120V transformer that feeds a 125A, 208Y/120V panel

It is my understanding that unless you are feeding a single phase or three phase delta-delta transformer you would size the overcurrent protection for primary & secondary protection based on Table 450.3(b) of the NEC (I'm using 2005 NEC). Table 450.3(b) says I need to size the primary OCPD by 250% (if greater than 9 amps) and the secondary OCPD needs to be sized by 125%.

Following that logic, here is what I get:

Primary OCPD:
45kVA/.831=54.2A*250% (Table 450.3(b))=135.4A
Using the next size up, primary OCPD would be: 150A/3P CB

Secondary OCPD:
45kVA/.36=125A*125% (Table 450.3(b))=156.3A
Using the next size up, secondary OCPD would be: 175A/3P CB

Is this correct? If it is this would mean that I could not use a 125A 208Y/120V panelboard and that I would need to upsize it to a 200A panelboard.

Thanks in advance for your response.

 

[iwire]

Table 450.3(b) says I need to size the primary OCPD by 250% (if greater than 9 amps) and the secondary OCPD needs to be sized by 125%.

You do not have to, your allowed to.

On the secondary side I would use a 100 amp breaker supplying a 100 amp or larger panel.

On the primary side I would stay under 100 amps, probably about 70 amps.

 

[augie47]

I believe your math is correct,but from a cost standpoint as Bob states, you would not normally use that high a rated primary beaker for a single transformer.
On 45 kva , I normally see 70 to 100 amp primary and 100 or 125 amp secondary.
In responce to your question, if you do want to go to 175 amps, which I don't recommend, your panel would have to be rated at least 175.

 

[necnotevenclose]

You do not have to, your allowed to.

On the secondary side I would use a 100 amp breaker supplying a 100 amp or larger panel.

On the primary side I would stay under 100 amps, probably about 70 amps.

In reference to the 70 amps are you sizing the primary based on 450.5(2)? Also could you please explain why I do not have to follow Table 450.3(B)?

 

[augie47]

presumptious of me to answer for Bob, as I can in no way fill his shoes...

but if I may be so bold...
that is following 450.3(B).. the Table notes "Maximum rating". Most folks would rather purchase a 70 amp breaker and wire if it does the job, than a 150 amp installation.

 

[iwire]

Also could you please explain why I do not have to follow Table 450.3(B)?

Look at the name of the table

'Table 430.3(B) Maximum Rating or Setting of.......'

Those are the maximum values, the NEC does not require you use those values.

Regardless of how you protect the transformer and the conductors that transformer is only capable of drawing 54 and providing 125 amps.

So by using the maximum Table values all you have gained is costs. Larger OCP may be into the next frame size. The larger OCP results in larger conductors which results in larger raceways.


You do have to take into account starting inrush so you do not want to size the primary to tight.


Also I do agree with Augie47 that the panel on the secondary side should be a 125 not a 100.

 

[necnotevenclose]

I believe your math is correct,but from a cost standpoint as Bob states, you would not normally use that high a rated primary beaker for a single transformer.
On 45 kva , I normally see 70 to 100 amp primary and 100 or 125 amp secondary.
In responce to your question, if you do want to go to 175 amps, which I don't recommend, your panel would have to be rated at least 175.

Backing into this calculation, I can see how the OCPD are sized:

480V

Scenario #1

45kVA/.831=54.2A
54.2A*250%=135.4A
OCPD=150A

Scenario #2 (includes using only up to 80% of rated transformer load)

45kVA*80%=36kVA
36kVA/.831=43.3A
43.3A*250%=108.3A
Next size down instead of up=100A/3P

Scenario #3 (includes unsing only up to 80% of rated transformer load and assumes less than 9 amps are connected):

45kVA*80%=36kVA
36kVA/.831=43.3A
43.3A*167%=72.3A
OCPD=80A
Next size down instead of up=70A/3P

208V


Scenario #1

45kVA/.36=125A
125A*125%=156.3A
OCPD: 175A/3P

Scenario #2 (includes using only up to 80% of rated transformer load and no OCPD multiplier)

45kVA*80%=36kVA
36kVA/.36=100A
OCPD: 100A

Scenario #2 (includes using only up to 80% of rated transformer load and 125% multiplier)

45kVA*80%=36kVA
36kVA/.36=100A
100A*125%=125A
OCPD: 125A/3P

It is my understanding that you only need to provide primary OCPD as long as you are connecting either a single phase 2 wire transformer or if you are connecting a 3 phase delta-delta transformer with a 3 wire secondary. But for a 3-phase 480V:208V system 450.3 should be followed.

 

[augie47]

I'm not sure where your 80% is entering the picture.
From my vantage point:
45000 / 480 * 1.732 = 54 amps.
450.3 (B) 54 x 2.5 = 136 with "next over" = 150
BUT that is MAXIMUM. Any size UP TO that may be allowed. Taking inrush into account, 70 to 100 is commonplace.

45000/ 208 *1.732 = 125. Secondary protection, again Maximum

as to your last sentence. I believe you will find from both Art 450 and 240 that you must always provide OCP. Art 450 is OCP for the transformer, Art 240 is OCP for the conductors. 240.4(F) does allow primary OCP to allow secondary OCP on 2 wire or 3 wire delta transformers.

 

[necnotevenclose]

I'm not sure where your 80% is entering the picture.
From my vantage point:
45000 / 480 * 1.732 = 54 amps.
450.3 (B) 54 x 2.5 = 136 with "next over" = 150
BUT that is MAXIMUM. Any size UP TO that may be allowed. Taking inrush into account, 70 to 100 is commonplace.

45000/ 208 *1.732 = 125. Secondary protection, again Maximum

as to your last sentence. I believe you will find from both Art 450 and 240 that you must always provide OCP. Art 450 is OCP for the transformer, Art 240 is OCP for the conductors. 240.4(F) does allow primary OCP to allow secondary OCP on 2 wire or 3 wire delta transformers.

By using 80% of the rated FLA of the transformer I was trying to figure out how others may be sizing the OCPD's. By doing those calculations with some very helpful advice, I think I'm better understanding that the OCPD's should really be sized for what is being connected and how the equipment/devices on the 208V side are being started. For a 208V/120V panel that has general purpose devices such as lighting and receptacles sizing the OCPD from anywhere from 100-125% should not cause any problems. However, if I'm looking at installing a large motor load or two that starts up with full voltage then I may want to look at increasing my primary OCPD to handle that inrush current.