XFMR protection 450

[tryinghard]

I don?t have my ?08? book with me just ?05?, for some reason I thought I?d need a smaller main breaker in a subpanel ?B? directly below a 15kva XFMR? I do need to apply ?08?.

Specifically I have a 125a 480v 3ph 3w panel ?A? with a 20a breaker feeding a 15kva XFMR which is 480-120/240v feeding a 125a rated panel with a calculated load of 36a. As far as ?08? goes does 408.36(B) require a main breaker in panel ?B? no greater than 40a, I?m looking now and see the OCP for the XFMR at the primary 20a meaning a main may not even be required in panel ?B? at all?

Panel ?A? is physically next to the XFMR which is next to panel ?B?.

 

[raider1]

The overcurrent protective device for a panelboard must be on the load side of any transformer so the primary overcurrent device for the transformer can't be used to protect the panelboard.

Chris

 

[yired29]

The overcurrent protective device for a panelboard must be on the load side of any transformer so the primary overcurrent device for the transformer can't be used to protect the panelboard.

Chris

We don't always need overcurrent on secondary side of a transformer. Single voltage to single voltage 2 wire in 2 wire out or 3 wire delta/delta transformer. 240.21 (C) (1). In this case we have a delta wye configuration therefor secondary overcurrent protection is required. Sized based on Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 600 Volts and Less. Key word maximum size. You could undersize the protection.

 

[don_resqcapt19]

408.36 would be complied with if there was a 125 amp main in the panel. The issue is going to be the protection of the secondary conductors per the rules in 240.21(C). The only cases where you can use the primary OCPD to protect the secondary conductors is when you have a two wire to two wire transformer or a delta delta transformer.

 

[augie47]

yired, I agree with you on the Code sections you quoted, however, there is one you omitted:
408.36 Overcurrent Protection.
In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard.

(There are exceptions, of course)

 

[tryinghard]

The overcurrent protective device for a panelboard must be on the load side of any transformer so the primary overcurrent device for the transformer can't be used to protect the panelboard.

Chris

Chris, I'm back at my '08' book now and see there really is differance in 408.36. Anyway if I follow what your saying I can have a 125A main breaker in subpanel "B" because its rated up to 125A, is this correct?

 

[don_resqcapt19]

We don't always need overcurrent on secondary side of a transformer. Single voltage to single voltage 2 wire in 2 wire out or 3 wire delta/delta transformer. 240.21 (C) (1). In this case we have a delta wye configuration therefor secondary overcurrent protection is required. Sized based on Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 600 Volts and Less. Key word maximum size. You could undersize the protection.

With a 20 amp OCPD on the primary, the secondary of the transformer does not require any overcurrent protection, however protection for the secondary conductors must be per 240.21(C).

 

[tryinghard]

We don't always need overcurrent on secondary side of a transformer. Single voltage to single voltage 2 wire in 2 wire out or 3 wire delta/delta transformer. 240.21 (C) (1). In this case we have a delta wye configuration therefor secondary overcurrent protection is required. Sized based on Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 600 Volts and Less. Key word maximum size. You could undersize the protection.

My case is delta/delta

 

[yired29]

My case is delta/delta

480-120/240 is not a 3wire to 3wire single voltage transformer 240.21 (C) (1). I did not specify single voltage secondary but I did reference 240.21 (C) (1)

 

[yired29]

408.36 Overcurrent Protection.
In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard. You have a 125 amp panelboard so a 40 OCPD would be in compliance.

 

[tryinghard]

With a 20 amp OCPD on the primary, the secondary of the transformer does not require any overcurrent protection, however protection for the secondary conductors must be per 240.21(C).

Don, my case does not comply with (C)(1) because its 120/240 but it does with (C)(2) if I install a 40a main or is #2 cu without a main correct?

 

[tryinghard]

408.36 Overcurrent Protection.
In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply side of the panelboard. You have a 125 amp panelboard so a 40 OCPD would be in compliance.

And with my understanding of 408.36 so would a 125a breaker because its rating is "not greater than that of the panelboard" rating. If this is correct understanding then it really does go back to 240.21(C)

 

[yired29]

And with my understanding of 408.36 so would a 125a breaker because its rating is "not greater than that of the panelboard" rating. If this is correct understanding then it really does go back to 240.21(C)

125 amp breaker would be in violation Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 600 Volts and Less. I = (kVA x 1000) / (E x 1.73) 15 kVA FLA priamary would be 18.06 amps and secondary would be 36.13.
Primary max OCPD 18.06 x 250% = 45.15 240.6 45 amp OCPD max on primary.
Secondary max OCPD 36.13 x 125% = 45.16 45 amp OCPD max on secondary.
There is note 1 for secondary which allows to go to the next size OCPD. 45.16 fall on a standard size OCPD

I would use 25 on primary and 45 on secondary.

The values determined from table 450.3 (B) are maximum. We can go lower not higher than these values.

 

[tryinghard]

This case may comply with 240.21(C)(3) but I'll need to check the panel schedule, this really seems to be pushing the envelope though.

Interesting (C)(3) "sum of the ratings of the breakers" compared to (C)(2) "Not less then the combined calculated loads"

Regardless the essence is most often a main breaker in the sub. We had figured on a 40a main I was looking for support and appreciate what you guy's have given! I don?t have any good excuse for missing 240.21 [brain freeze]

 

[yired29]

Remember that 240.4 (B) does not apply to secondary conductors. Secondary conductors must have a ampacity equal to or greater than the secondary OCPD. and EBJ is sized based on 250.102 (C) using table 250.66

 

[tryinghard]

125 amp breaker would be in violation Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 600 Volts and Less. I = (kVA x 1000) / (E x 1.73) 15 kVA FLA priamary would be 18.06 amps and secondary would be 36.13.
Primary max OCPD 18.06 x 250% = 45.15 240.6 45 amp OCPD max on primary.
Secondary max OCPD 36.13 x 125% = 45.16 45 amp OCPD max on secondary.
There is note 1 for secondary which allows to go to the next size OCPD. 45.16 fall on a standard size OCPD

I would use 25 on primary and 45 on secondary.

The values determined from table 450.3 (B) are maximum. We can go lower not higher than these values.

yired29, this is really were I started but missed 240.21. I have the XFMR protected with the primary 20a breaker, I have the panelboard protected as per 408.36 up to 125a, this leaves the secondary feeder to account for and 240.21(C)(2) may fit as minimum.

If so 240.21(C)(2)b may cause me to have #2 cu without a main, I don't know if I understanding this correctly. I am most likely going to supply a 40a main in "B" I'm now just curious to know.

 

[yired29]

yired29, this is really were I started but missed 240.21. I have the XFMR protected with the primary 20a breaker, I have the panelboard protected as per 408.36 up to 125a, this leaves the secondary feeder to account for and 240.21(C)(2) may fit as minimum.

If so 240.21(C)(2)b may cause me to have #2 cu without a main, I don't know if I understanding this correctly. I am most likely going to supply a 40a main in "B" I'm now just curious to know.

240.21 (C) (2) b the rating of the device would be the OCPD not the panel board amperage. You can not have a OCPD larger than what is determined from table 450.3 (B) which I believe would be a 45 amp OCPD with #8 AWG wires. #8 = 50 amps at 75 degrees. You need to check the temperature of termination of the transformer if 60 degree than you would need # 6 on the 45 amp OCPD. Temperature of terminantion is from 110.14 (C). If you use 40 amp OCPD than #8 would be good for both temperatures.

 

[tryinghard]

240.21 (C) (2) b the rating of the device would be the OCPD not the panel board amperage. You can not have a OCPD larger than what is determined from table 450.3 (B) which I believe would be a 45 amp OCPD with #8 AWG wires. #8 = 50 amps at 75 degrees. You need to check the temperature of termination of the transformer if 60 degree than you would need # 6 on the 45 amp OCPD. Temperature of terminantion is from 110.14 (C). If you use 40 amp OCPD than #8 would be good for both temperatures.

I don?t know notice 240.21(C) "XFMR secondary conductors...shall be permitted to be connected to a XFMR secondary, without overcurrent protection at the secondary as specified in..."; and my case may comply with (C)(2)? Are you thinking I have to apply the second row of 450.3(B)? Because I don't believe I do when I have the secondary in compliance with either 408.36 or 240.21(C).

I?m saying my case is in compliance with 450.3(B) because this XFMR is protected with a primary 20a - don't miss note 1 - (again it?s a 15kva). We currently have a design including Article 220 calcs meaning the 36a calc load for panel ?B? is final and complete so 40a is more than adequate for the 120/240v load.

I?m really wondering if 240.21(C) can apply?

 

[yired29]

I don?t know notice 240.21(C) "XFMR secondary conductors...shall be permitted to be connected to a XFMR secondary, without overcurrent protection at the secondary as specified in..."; and my case may comply with (C)(2)? Are you thinking I have to apply the second row of 450.3(B)? Because I don't believe I do when I have the secondary in compliance with either 408.36 or 240.21(C).

I?m saying my case is in compliance with 450.3(B) because this XFMR is protected with a primary 20a - don't miss note 1 - (again it?s a 15kva). We currently have a design including Article 220 calcs meaning the 36a calc load for panel ?B? is final and complete so 40a is more than adequate for the 120/240v load.

I?m really wondering if 240.21(C) can apply?

You can not protect the secondary with primary OCPD when you have a delta/delta 3 wire to 4 wire.

1) Protection by Primary Overcurrent Device. Conductors supplied by the secondary side of a single-phase transformer having a 2-wire (single-voltage) secondary, or a three-phase, delta-delta connected transformer having a 3-wire (single-voltage) secondary, shall be permitted to be protected by overcurrent protection provided on the primary (supply) side of the transformer, provided this protection is in accordance with 450.3 and does not exceed the value determined by multiplying the secondary conductor ampacity by the secondary-to-primary transformer voltage ratio.
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors are not considered to be protected by the primary overcurrent protective device.

 

[don_resqcapt19]

125 amp breaker would be in violation Table 450.3(B) Maximum Rating or Setting of Overcurrent Protection for Transformers 600 Volts and Less. I = (kVA x 1000) / (E x 1.73) 15 kVA FLA priamary would be 18.06 amps and secondary would be 36.13.
Primary max OCPD 18.06 x 250% = 45.15 240.6 45 amp OCPD max on primary.
Secondary max OCPD 36.13 x 125% = 45.16 45 amp OCPD max on secondary.
There is note 1 for secondary which allows to go to the next size OCPD. 45.16 fall on a standard size OCPD

I would use 25 on primary and 45 on secondary.

The values determined from table 450.3 (B) are maximum. We can go lower not higher than these values.

A 20 amp OCPD was specified as the primary protection in the original post. With a 20 amp primary OCPD there is no requirement for protection of the transformer secondary. Even with a 25 amp OCPD on the primary there is no code requirement for protection of the transformer secondary.
That leaves the requirement for the protection of the secondary conductors and the panel. Size the secondary conductors to the load and the panel main to the size of the conductors. In this case #8s with a 40 amp main would do the job.