Primary protection on tap rule transformer

[TMMK]

Current situation: a 3 phase, 75KVA 480 - 208/120 transformer fed from 480V bus duct with 100amp fuses on primary with #2 THHN wire and the secondary has 2 taps made at the transformer, (1) tap #1/0 THHN has 60' feet of wire then terminating on a 150 amp breaker at a machine, the other tap is also 60' #1/0 THHN terminating on another 150 amp breaker. Does this meet the NEC for "transformer primary protection only" with a maximum of 100' tap? ref: Table 450.3(B) , 240.92 (C), and does 240.4(F) apply? NOTE : The conductors are protected meeting 240.92(C)(3). Can 90 degree table be used for calculating 240.92(C)(1)(1) although the termination are 75 degree. One last comment can 460V be used in place of 480 for the calculation in 240.92(C)(1)(1)?

Thank You
Tom

 

[teco]

240.4. F. says you need to provide secondary protection. IMO the secondary overcurrent devices can be from two up to six. 450.3B note 2. But they have to be grouped together in one location. Most always by the transformer. I'm assuming this is a 3 phase 4-wire transformer because you say 120/ 208. If was a delta to delta 3-wire secondary the primary overcurrent device may serve.

 

[jim dungar]

Current situation: a 3 phase, 75KVA 480 - 208/120 transformer fed from 480V bus duct with 100amp fuses on primary with #2 THHN wire and the secondary has 2 taps made at the transformer, (1) tap #1/0 THHN has 60' feet of wire then terminating on a 150 amp breaker at a machine, the other tap is also 60' #1/0 THHN terminating on another 150 amp breaker. Does this meet the NEC for "transformer primary protection only" with a maximum of 100' tap? ref: Table 450.3(B) , 240.92 (C), and does 240.4(F) apply? NOTE : The conductors are protected meeting 240.92(C)(3). Can 90 degree table be used for calculating 240.92(C)(1)(1) although the termination are 75 degree. One last comment can 460V be used in place of 480 for the calculation in 240.92(C)(1)(1)?
This is a Delta to Wye transformer
Thank You
Tom

450.3(B) is not applicable to sizing secondary conductor protection. This section only allows you to change the size of the primary IF secondary protection is applied.

240.90 says all section of 240 apply unless they are modified by part VIII (which does not address 240.4(F). Therefore 240.4(F) is applicable and will not allow this installation because a multi-voltage output can not be protected by the primary only. If 240.4(F) was not applicable you would need special protective relaying per 240.92(B)(1).

You should probably be looking at the tap rules of 240.21(C)(4).

Per 110.4 you need to use the nominal system voltage of 480V.


all references are to NEC2005.

 

[teco]

Jim, If you don't mind. I'd love the chance to pick a PEs brain. I think the question was transformer protection. I not sure if that matters But I think what you pointed out is very interesting and now I'm not sure I understand 450.3(B). Maybe you wouldn't mind explaining that table a little more? Why wouldn't note 2 apply? Where secondary protection is required. Thanx

 

[TMMK]

Thank You Jim & Teco. I am trying to determine if under supervised installations 240.92 which allow me to extend the location of the secondary overcurrent protection to 100' if I meet the requirements 240.92(C) which requires I meet all the conditions of 290.92(C)(1), (C)(2) & (c)(3).
My calculations for 290.92(c)(1) is 100 amp fuses < 150amps * (.433) * 150% which would require me to reduce the primary fuses to 90 amps.
290.92(c)(2)(1) I meet for a single over current device ?
290.92(c)(3) I meet the physical protection installed in rigid conduit.
I must use 480 in my calculations but can I use the 90 degree table which would then make the calculations for 290.92(c)(1) obtainable?
or have I completely missed something?
Thank You
NEC2008

 

[jim dungar]

Jim, If you don't mind. I'd love the chance to pick a PEs brain. I think the question was transformer protection. I not sure if that matters But I think what you pointed out is very interesting and now I'm not sure I understand 450.3(B). Maybe you wouldn't mind explaining that table a little more? Why wouldn't note 2 apply? Where secondary protection is required. Thanx

All conductors are protected using article 240.

Table 450.3(B) says the primary of a 600V transformer must be sized at not more than 125%.

Note 2 does not require secondary protection, it a condition for the primary device to be sized differently. If there is some secondary protection at not more than 125% then the primary may be sized larger.

 

[jim dungar]

Thank You Jim & Teco. I am trying to determine if under supervised installations 240.92 which allow me to extend the location of the secondary overcurrent protection to 100' if I meet the requirements 240.92(C) which requires I meet all the conditions of 290.92(C)(1), (C)(2) & (c)(3).
My calculations for 290.92(c)(1) is 100 amp fuses < 150amps * (.433) * 150% which would require me to reduce the primary fuses to 90 amps.
290.92(c)(2)(1) I meet for a single over current device ?
290.92(c)(3) I meet the physical protection installed in rigid conduit.
I must use 480 in my calculations but can I use the 90 degree table which would then make the calculations for 290.92(c)(1) obtainable?
or have I completely missed something?
Thank You
NEC2008

Are the transformer conductors protected? I believe 290.92 is for branch circuits unless special differential relaying is used on transformers.

It depends on where the conductors are going, there are no UL Listed overcurrent protective devices with terminations rated for conductors sized using the 90? column.

 

[petersonra]

There seems to be a fair amount of confusion regarding OCPD for transformers. the important thing to remember is that the transformer primary and secondary, and the transformer primary and secondary conductors have to be protected.

It is often convenient to use the one OCPD to protect the primary and primary conductors, and another to protect the secondary and secondary conductors.