450.3B 125% OR 250& PRIMARY WHICH 0R WHEN ?

[jrvazzer]

When sizing the primary ocpd for transformer using 450.3b what applications do you use 125% and 250%. Every time I see examples on M Holts they never use 250%. However when instructed in Dolittle school of engineeering they always use 250% for primary protection.

 

[jim dungar]

When sizing the primary ocpd for transformer using 450.3b what applications do you use 125% and 250%. Every time I see examples on M Holts they never use 250%. However when instructed in Dolittle school of engineeering they always use 250% for primary protection.

You must size the protection based on the 125%.
However, you are allowed to increase the primary side, up to 250%, when proper secondary protection has been supplied.

So, what secondary protection was used in the examples?

 

[jrvazzer]

You must size the protection based on the 125%.
However, you are allowed to increase the primary side, up to 250%, when proper secondary protection has been supplied.

So, what secondary protection was used in the examples?

In Dolittle college example they always use the 125% secondary for panels because that's what is allowed. My question is why not use the 125% primary and forget the secondary. You don't need it. Is that right?

 

[jim dungar]

You don't need it. Is that right?

Maybe you do and maybe you don't. don't forget you should consider transformer inrush as well as full load current

Say you have a 45kVA 480-208Y/120V transformer with a primary of 54A. You take 125% and then round up to the next largest standard breaker - 54 x 1.25 = 67.5 ->70A. Now a typical 70A breaker will have an instantaneous trip point of about 8X. This equates to about 10.37X the FLA of the transformer. If the transformer has an inrush of 14X, this breaker may trip on power up.

If you assume the above transformer has an, all but ridiculous inrush, of 22x, then you need a breaker with a trip point of 1188A. Using the above estimate of 8X for a trip point, your breaker would need to be a 148.5A, which would round up to 150A. Using the max possible breaker size of 250%, you could protect the transformer with a 135A breaker which would round up to 150A.

So to prevent having to know anything about the actual transformer, using 250% will almost always guarantee the transformer will 'start up'. Gee, this sounds like a good design practice, until you realize that oversizing the primary protection, means the primary conductors and raceways are also oversized. Good design = waste of money??

 

[don_resqcapt19]

In Dolittle college example they always use the 125% secondary for panels because that's what is allowed. My question is why not use the 125% primary and forget the secondary. You don't need it. Is that right?

You may not need secondary protection for the transformer, but you will most likely require protection for the secondary conductors. You have to protect the secondary conductors per the rules in 240.21(C). The rules in Article 450 only cover the protection of the transformer itself.

 

[templdl]

Maybe you do and maybe you don't. don't forget you should consider transformer inrush as well as full load current

Say you have a 45kVA 480-208Y/120V transformer with a primary of 54A. You take 125% and then round up to the next largest standard breaker - 54 x 1.25 = 67.5 ->70A. Now a typical 70A breaker will have an instantaneous trip point of about 8X. This equates to about 10.37X the FLA of the transformer. If the transformer has an inrush of 14X, this breaker may trip on power up.

If you assume the above transformer has an, all but ridiculous inrush, of 22x, then you need a breaker with a trip point of 1188A. Using the above estimate of 8X for a trip point, your breaker would need to be a 148.5A, which would round up to 150A. Using the max possible breaker size of 250%, you could protect the transformer with a 135A breaker which would round up to 150A.

So to prevent having to know anything about the actual transformer, using 250% will almost always guarantee the transformer will 'start up'. Gee, this sounds like a good design practice, until you realize that oversizing the primary protection, means the primary conductors and raceways are also oversized. Good design = waste of money??

Very well stated!! Excellent!

 

[fmtjfw]

If the transformer has more than two secondary connections then the primary fuse can not protect the secondaries (in the general case). It can only protect the secondaries if it is sized for the smallest ampacity secondary element (multiplied by the turns ratio)

For example a 480V primary with a 120V (2 connection) secondary can protect the secondary using reasonable primary OCPD
For example a 480V primary with a 120/240V (3 connection) secondary can not protect the secondary using reasonable primary OCPD because a 120V portion of the secondary can be overloaded without drawing a overload current in the primary.

 

[jrvazzer]

Maybe you do and maybe you don't. don't forget you should consider transformer inrush as well as full load current

Say you have a 45kVA 480-208Y/120V transformer with a primary of 54A. You take 125% and then round up to the next largest standard breaker - 54 x 1.25 = 67.5 ->70A. Now a typical 70A breaker will have an instantaneous trip point of about 8X. This equates to about 10.37X the FLA of the transformer. If the transformer has an inrush of 14X, this breaker may trip on power up.

If you assume the above transformer has an, all but ridiculous inrush, of 22x, then you need a breaker with a trip point of 1188A. Using the above estimate of 8X for a trip point, your breaker would need to be a 148.5A, which would round up to 150A. Using the max possible breaker size of 250%, you could protect the transformer with a 135A breaker which would round up to 150A.

So to prevent having to know anything about the actual transformer, using 250% will almost always guarantee the transformer will 'start up'. Gee, this sounds like a good design practice, until you realize that oversizing the primary protection, means the primary conductors and raceways are also oversized. Good design = waste of money??

I'm confused. If the load was less than 25', do you still need to increase the conductor size. I've seen examples where that was mention to justify the lesser size.

 

[BPoindexter]

If the total length of the primary plus the secondary conductors is less than 25 ft then look at 240.21(B)(3). All 5 conditions must apply.

 

[jim dungar]

I'm confused. If the load was less than 25', do you still need to increase the conductor size. I've seen examples where that was mention to justify the lesser size.

My examples was keyed to the discussion of possible reasons to use the 125% and 250% values found in table 450.3(b).

Any distance conditions are probably components of 240.21(C) or a related section in 240. These should be discussed in a separate thread.

 

[jrvazzer]

You may not need secondary protection for the transformer, but you will most likely require protection for the secondary conductors. You have to protect the secondary conductors per the rules in 240.21(C). The rules in Article 450 only cover the protection of the transformer itself.

According to 240.21(B)(3) scenario: I have 2000A MDP from it feeding a 100kva transformer 480v - 208/120 feeding a panel.

Primary Feeder 100KVA?1.73x480=120A x1.25 continuous load =150A table 310.15(15)(16) 75?C 3#1/0 primary.

Primary OCPD 403.3b 2.5x120A=300A NEC 240.6 =300A OCCP

So I can keep this the way it is if meets 240.21(B)(3). I don't have to match feeder with OCCP.

 

[don_resqcapt19]

According to 240.21(B)(3) scenario: I have 2000A MDP from it feeding a 100kva transformer 480v - 208/120 feeding a panel.

Primary Feeder 100KVA?1.73x480=120A x1.25 continuous load =150A table 310.15(15)(16) 75?C 3#1/0 primary.

Primary OCPD 403.3b 2.5x120A=300A NEC 240.6 =300A OCCP

So I can keep this the way it is if meets 240.21(B)(3). I don't have to match feeder with OCCP.

We would need information about the wire length of the primary conductors, the size and wire length of the secondary conductors and the rating of the OCPD that the secondary conductors terminate on before we could say that you can use 240.21(B)(3).

 

[jrvazzer]

I'm confused. If the load was less than 25', do you still need to increase the conductor size. I've seen examples where that was mention to justify the lesser size.

This is within the electrical room. nicely lined up. MPD board, Transformer and 120/208v Dist. panel. Transformer is 12' away from MDP 300a occp. secondary dist. panel is right next to transformer.