XFMR protection 450

[don_resqcapt19]

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.

You can't protect the secondary conductors with the primay OCPD in other than two wire to two wire or 3 wire delta-delta installations. You can protect the secondary of the transformer itself with the primary OCPD as long as the rating of that device does not exceed 125% of the rated primary current.
The rules in Article 450 only apply to the protection of the transformer itself and the rules in Article 240 apply to the protection of the conductors.

 

[yired29]

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.

Protection of the secondary conductors is really never present. For example if a fault occured on the supply side of the secondary OCPD the 40 amp breaker would not trip. Thats why we have the tap rules. Most commonly used 10 foot rule. And thats why I said to use 250.102 (C) EBJ on supply side not just table 250.122.

 

[don_resqcapt19]

Protection of the secondary conductors is really never present. For example if a fault occured on the supply side of the secondary OCPD the 40 amp breaker would not trip. Thats why we have the tap rules. Most commonly used 10 foot rule. And thats why I said to use 250.102 (C) EBJ on supply side not just table 250.122.

The secondary conductors are protected from overload and not from short circuits or ground faults just like any tap conductor or like the service conductors.

 

[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.

Transformer protection: 15kva / 480v 3ph = 18a, x 125% = 22.5a, 20a breaker for this XFMR primary only, 450.3(B) row 1 of table, secondary overcurrent protection ?Not Required?

Panelboard protection: 15kva / 240v 3ph = 36a, #8 cu to 125a rated panel with up to 125a OCPD, 408.36 ?OCPD having a rating not greater than that of the panelboard?

XFMR Secondary Feeder Protection: 240.21(C ) ?shall be permitted?without overcurrent protection at the secondary?as specified in (C )(1) through (C )(6)? this is not saying ?all of the following. One of these can be used and comply.

All 75? terms, in this light sure seems like I only need to verify 240.21(C ). Again this is a mute point if I install a 40a main in the 120/240v panel. But if I don?t have a main at all it may still be a compliant application ? this is what I?d like to know.

 

[yired29]

All this being said still 20 amp primary and 40 amp secondary with # 8AWG would be a code compliant install. Anything over 45 amps on secondary IMO would be a violation.

 

[don_resqcapt19]

... Anything over 45 amps on secondary IMO would be a violation.

As long as the conductors are protected per the tap rules, you can put any size OCPD on the secondary side as protection of the transformer secondary is not required in this case.

 

[yired29]

240.21 (C) (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.

This is not a delta/delta single voltage secondary. Primary OCPD does not protect secondary conductors...

Not trying to have a pissing contest here but if I can learn something new this is how I do it.

 

[don_resqcapt19]

240.21 (C) (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.

This is not a delta/delta single voltage secondary. Primary OCPD does not protect secondary conductors...

Not trying to have a pissing contest here but if I can learn something new this is how I do it.

You said that the maximum size permitted size of the OCPD on secondary side of this transformer is 45 amps. That is not correct. The size of the OCPD on the secondary of this transformer is based on the size of the secondary conductors that are installed. If I choose to install 600kcmil conductors for this secondary, then I would be permitted to use a 400 amp OCPD.

 

[yired29]

You said that the maximum size permitted size of the OCPD on secondary side of this transformer is 45 amps. That is not correct. The size of the OCPD on the secondary of this transformer is based on the size of the secondary conductors that are installed. If I choose to install 600kcmil conductors for this secondary, then I would be permitted to use a 400 amp OCPD.

If we only have a 15kVA transformer 120/240 volts 3 phase and we put the secondary conductors on a 100 amp OCPD with #3 conductors. What is protecting the secondary conductors? When 240.21 (C) (1) says the secondary conductors are not considered to be protected by the primary OCPD when it is a delta 3 wire to delta 4 wire. (this example is not single voltage 3 wire secondary)
When does secondary protection from 450.3 (B) take effect?

 

[tryinghard]

If we only have a 15kVA transformer 120/240 volts 3 phase and we put the secondary conductors on a 100 amp OCPD with #3 conductors. What is protecting the secondary conductors? When 240.21 (C) (1) says the secondary conductors are not considered to be protected by the primary OCPD when it is a delta 3 wire to delta 4 wire. (this example is not single voltage 3 wire secondary)
When does secondary protection from 450.3 (B) take effect?

yired29, back up from 240.21(C)(1) and read its section (C) "Transformer Secondary Conductors" paragraph. You are selecting out one of its options, there are five others and if criteria is met they each comply. All 6 do not have to be met only one of em.

 

[tryinghard]

...When does secondary protection from 450.3 (B) take effect?

From what I now understand when none of the 6 options of 240.21(C) comply, and in my opinion this is often.

 

[yired29]

yired29, back up from 240.21(C)(1) and read its section (C) "Transformer Secondary Conductors" paragraph. You are selecting out one of its options, there are five others and if criteria is met they each comply. All 6 do not have to be met only one of em.

On a 15 kVA transformer with a FLC of 36 amps on the secondary if we install a 100 amp CB in the panelboard because 408.36 requires this. What is protecting the windings of the transformer? possible 100 amp load on windings with a FLC of 36 amps. Every book I read show OCPD on the secondary based on Table 450.3 (B). I see the problems with using a OCPD larger than the transformers secondary can handle.

 

[tryinghard]

On a 15 kVA transformer with a FLC of 36 amps on the secondary if we install a 100 amp CB in the panelboard because 408.36 requires this. What is protecting the windings of the transformer?

According to 450.3 the 20a primary breaker. 408.36 requires the breaker to not be more then the panel rating, read it carfully.

Every book I read show OCPD on the secondary based on Table 450.3 (B). I see the problems with using a OCPD larger than the transformers secondary can handle.

Read 450.3(B) carfully as well, its not required really - read far right of row 1.

A short on the secondary will overload the primary OCPD in cases compliant with 240.21(C)

 

[tryinghard]

So I really could use #2 cu secondary with no main breaker - 240.21(C)(2)(1)b - the rest complies. This case really doesn?t meet the other 6 options of 21(C).

If so this is my comparable for value and application: #2 cu no main breaker or #8 cu with 40a breaker.

I think we?ll go with the 40a main.

 

[yired29]

According to 450.3 the 20a primary breaker. 408.36 requires the breaker to not be more then the panel rating, read it carfully.

Read 450.3(B) carfully as well, its not required really - read far right of row 1.

A short on the secondary will overload the primary OCPD in cases compliant with 240.21(C)

Yet once again I will go to 240.21 (C) (1) which states primary OCPD will not protect secondary on a 3 wire to 4 wire delta/delta transformer... So if primary does not protect secondary than row 2 of table applies to secondary protection. The way I understand it is the general rule of 240.24 (C) just says OCPD is not needed at the transformer but when using 240.21 (C) (2) - (6) it is required after the taps based on table 450.3 (B)...

240.21 (C) Transformer Secondary Conductors. A set of conductors feeding a single load, or each set of conductors feeding separate loads, shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary, as specified in 240.21(C)(1) through (C)(6). The provisions of 240.4(B) shall not be permitted for transformer secondary conductors.

 

[yired29]

So I really could use #2 cu secondary with no main breaker - 240.21(C)(2)(1)b - the rest complies. This case really doesn?t meet the other 6 options of 21(C).

If so this is my comparable for value and application: #2 cu no main breaker or #8 cu with 40a breaker.

I think we?ll go with the 40a main.

I agree we could go to a MLO panel because 240.21 (C) (2) does not state that the secondary conductors have to land on a single circuit breaker or single set of fuses. But we still have to limit the load to that of the transformer capacity. All the other rules 240.21 (C) (3) - (6) require the secondary conductors to land on a single circuit breaker or single set of fuses.

Still have not answered why we would land a transformers secondary conductors on a breaker larger than than what the transformer can handle.

 

[tryinghard]

...So if primary does not protect secondary than row 2 of table applies to secondary protection...

Row 2 of table 450.3(B) only applies if 250.21(C):

• (2) Transformer Secondary Conductors Not over 10'
• or (3) Industrial Installation Secondary Conductors Not over 25'
• or (4) Outside Secondary Conductors
• or (5) Secondary Conductors from a Feeder Tapped Transformer
• or (6) Secondary Conductors Not over 25'
• or (1) Protection by Primary Overcurrent Device

cannot be met.

 

[tryinghard]

Still have not answered why we would land a transformers secondary conductors on a breaker larger than than what the transformer can handle.

Actually have a few times the primary OCPD provides this.



But only in cases that cannot meet the criteria of 240.21(C)



408.36 does not play a role in transformer protection



Its all down to 240.21(C)

 

[yired29]

Primary OCPD can not provide protection on a multivoltage secondary of a transformer..............Clear as day in 240.21 (C) (1) last paragraph. So if primary cant provide it than secondary protection is required. Hense row 2 table 450.3 (B).

 

[yired29]

I also forgot to mention 240.4 (F)

When you have a multivoltage secondary OCPD is required!!