Table 450.3(B) Max rating of OCPD for xf 1000v or less

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nocketd

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I have a 750va transformer taped into a 480 volt feeder for 120 volt control power. I'm using Primary and secondary protection.

Pri max OCPD is 3.5 amps (calculated 3.91 amps)
Sec max OCPD is 10 amps (calculated 10.44 amps)

Can I use a 4 amp fuse on the Primary side of this transformer?
 
I don't understand. If I provide primary protection only, I can use a 4 amp fuse and don't need any protection on the secondary. But if I provide secondary protection I am limited to a 3 amp fuse on the primary. Can someone please explain this?
 
RB1 said:
I don't understand. If I provide primary protection only, I can use a 4 amp fuse and don't need any protection on the secondary. But if I provide secondary protection I am limited to a 3 amp fuse on the primary. Can someone please explain this?
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First of all, you have to understand that secondary conductor protection is a completely separate element of system protection. Just because you fuse the secondary doesn't mean you must use Primary and Secondary Protection under Table 450.3(B). You can use Primary Only Protection (4 amp fuse) coupled with Secondary Conductor protection and be compliant. See 240.21(C) for the latter.
 
Smart$

So if the OP removes the secondary fuses and uses No. 12 AWG control wires, a 4 amp fuse adequately protects the transformer and secondary conductors. But if he installs 10 amp fuses on the secondary in the same scenario, he would have to replace the 4 amp primary fuse with a 3 amp fuse to adequately protect the transformer. Do I understand it correctly?
 
RB1 said:
Smart$

So if the OP removes the secondary fuses and uses No. 12 AWG control wires, a 4 amp fuse adequately protects the transformer and secondary conductors. But if he installs 10 amp fuses on the secondary in the same scenario, he would have to replace the 4 amp primary fuse with a 3 amp fuse to adequately protect the transformer. Do I understand it correctly?
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Primary protection only would be a 3A fuse max. 125% multiplier + note 1 which allows next standard size up.
 
RB1 said:
Smart$

So if the OP removes the secondary fuses and uses No. 12 AWG control wires, a 4 amp fuse adequately protects the transformer and secondary conductors. But if he installs 10 amp fuses on the secondary in the same scenario, he would have to replace the 4 amp primary fuse with a 3 amp fuse to adequately protect the transformer. Do I understand it correctly?
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No. He would not have to change the primary fuses because the transformer protection schema is still Primary Only Protection... assuming he didn't add fuses for additional transformer protection... a test of one's own integrity.

Fusing on the secondary side can serve as 1) transformer secondary protection, 2) secondary conductor protection, or 3) both.

Note secondary conductors can only be protected by primary fusing for 1Ø 2-wire and 3Ø 3-wire delta secondaries... so in some scenarios one is forced to have secondary conductor protection... and this protection is not required to also protect the transformer secondary (though it can).
 
Smart $ said:
No. He would not have to change the primary fuses because the transformer protection schema is still Primary Only Protection... assuming he didn't add fuses for additional transformer protection... a test of one's own integrity.

Fusing on the secondary side can serve as 1) transformer secondary protection, 2) secondary conductor protection, or 3) both.

Note secondary conductors can only be protected by primary fusing for 1Ø 2-wire and 3Ø 3-wire delta secondaries... so in some scenarios one is forced to have secondary conductor protection... and this protection is not required to also protect the transformer secondary (though it can).
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I would note that the transformer secondary winding can be protected by the primary fusing only for 1Ø 2-wire and 3Ø 3-wire delta as well. In all other circumstances (i) the transformer secondary must be protected and (ii) the secondary conductors must be protected. It is legal to use the same OCPD for both purposes if it is mounted right at the transformer or to protect only the secondary winding with the first OCPD and run smaller secondary conductors from there as taps with downstream OCPD.
 
Smart $ said:
...Note secondary conductors can only be protected by primary fusing for 1Ø 2-wire and 3Ø 3-wire delta secondaries...
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In this case if primary is fused at 4A and no secondary fusing is utilized, the secondary conductor ampacity must equal or exceed the primary fuse rating times the voltage ratio, so 4A×480/120=16A* means using at least #12 Cu... or add secondary conductor protection fuses to use smaller gauge control wiring.

*represents equivalent secondary conductor protection rating.
 
GoldDigger said:
I would note that the transformer secondary winding can be protected by the primary fusing only for 1Ø 2-wire and 3Ø 3-wire delta as well. In all other circumstances (i) the transformer secondary must be protected and (ii) the secondary conductors must be protected. It is legal to use the same OCPD for both purposes if it is mounted right at the transformer or to protect only the secondary winding with the first OCPD and run smaller secondary conductors from there as taps with downstream OCPD.
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Where is this required? ...assuming you mean with a secondary-side OCPD. The whole transformer, primary and secondary is always required to be protected. The matter of how is all that can be questioned.

There is no such thing as secondary conductors after the first OCPD!
 
Smart $ said:
Where is this required? ...assuming you mean with a secondary-side OCPD. The whole transformer, primary and secondary is always required to be protected. The matter of how is all that can be questioned.

There is no such thing as secondary conductors after the first OCPD!
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I was just referring to the fact that if the secondary is not single phase two wire or three wire delta any primary protection that allows full use of the transformer rating does not protect the secondary windings.

With a single phase three wire winding, for example, you could have a fault on only one side of the secondary which pulled twice its rated current without tripping the primary OCPD.
With reference to secondary conductors, there is the argument that even the wires from the secondary terminals of the transformer to the first OCPD would need to be oversized compared to the secondary rating in order to be protected by the primary OCPD. The reasoning being that if they were not accordingly oversized and were only protected by the first OCPD on the secondary side they would be an illegal tap (secondary conductors) of a tap (the secondary winding).
Not everyone agrees with this, of course. :happyno:
 
GoldDigger said:
I was just referring to the fact that if the secondary is not single phase two wire or three wire delta any primary protection that allows full use of the transformer rating does not protect the secondary windings.

With a single phase three wire winding, for example, you could have a fault on only one side of the secondary which pulled twice its rated current without tripping the primary OCPD.
With reference to secondary conductors, there is the argument that even the wires from the secondary terminals of the transformer to the first OCPD would need to be oversized compared to the secondary rating in order to be protected by the primary OCPD. The reasoning being that if they were not accordingly oversized and were only protected by the first OCPD on the secondary side they would be an illegal tap (secondary conductors) of a tap (the secondary winding).
Not everyone agrees with this, of course. :happyno:
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You have a valid point, or should I say points. But there is no requirement to use primary and secondary transformer protection where the secondary is not 2-wire or 3Ø 3-wire delta. You will see however that secondary conductor protection is required. There is a gap in the protection requirements.

We need not speculate here as I believe this case is a 2-wire secondary...
 
Primary protection vs primary and secondary protection

Primary protection vs primary and secondary protection

In this example I have a control transformer that has a 3.5 amp fuse on the primary and a 10 amp fuse on the secondary. The xf is 2 wire on the primary and secondary.

When is it applicable to use Primary protection vs Primary and secondary protection. Is there a book or article that will provide good examples and explanation of these options and the pros and cons.


Thank you for your replies.
 
nocketd said:
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When is it applicable to use Primary protection vs Primary and secondary protection. Is there a book or article that will provide good examples and explanation of these options and the pros and cons.
...
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I don't know of any publications off hand. Others may chime in with a recommendation or two. Then of course, there is always Google (and similar).
 
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