NEC 240.21 (C) (1)

[AKsparky1]

Can anybody explain to me the physics behind why a xfmr having a 2-wire (single voltage) secondary like the example in mike Holts understanding the NEC page 199 bottom right, can have secondary conductors of any length?hmy: why does the primary protection work in this situation but not in the other one's listed?

 

[kwired]

Can anybody explain to me the physics behind why a xfmr having a 2-wire (single voltage) secondary like the example in mike Holts understanding the NEC page 199 bottom right, can have secondary conductors of any length?hmy: why does the primary protection work in this situation but not in the other one's listed?

With two wires in - two wires out your primary and secondary are always in proportion to one another.

Examples:

lets take a 5kVA transformer with 480 volts in, full rated load is 10.42 amps on primary.

If we have 120 volt two wire secondary full rated output is 41.66amps. The ratio of primary to secondary current is 1 to 4 and stays constant. if secondary is pulling 10 amps primary will see 2.5, since this ratio is constant we can monitor overcurrent of both sides via protection on primary side only.

Now take same transformer and configure it for 120/240 three wire on the secondary, and if you should happen to place all 5 kVA of load on only one half of the secondary the primary still sees 5 kVA, but the one half of the secondary is only rated for 2.5 kVA, the secondary winding (the half being used) will overheat, but the primary protection will not see any overload and will never open.

This is why we almost always must have secondary protection with multiwire secondary circuits.

 

[Carultch]

Can anybody explain to me the physics behind why a xfmr having a 2-wire (single voltage) secondary like the example in mike Holts understanding the NEC page 199 bottom right, can have secondary conductors of any length?hmy: why does the primary protection work in this situation but not in the other one's listed?

See 1:30 in the following video:
https://www.youtube.com/watch?v=tNVoJbhH764

When you have a single phase transformer, the fault currents will align winding-to-winding, with proportionality to one another via the ratio of transformer turns (voltage ratio). Consider a 100A breaker on the primary. An overcurrent of 300A on the secondary of a 240V to 120V single phase transformer, will transform directly into a 150A overcurrent on the primary. And a 100A breaker on the primary will catch this 150A, and thus protect the 200A circuit on the secondary by proxy.

This same concept is also extended for DELTA-TO-DELTA transformers, where fault current also lines up winding-to-winding-to-winding. In otherwords, A-phase faults on the secondary do not get distributed onto the B-phase and C-phase of the primary, thus going undetected. A-phase faults on the secondary pass directly to the A-phase of the primary, on a DELTA-DELTA transformer. In DELTA-DELTA transformers, all windings of either half of the transformer are in series with one another. Current from one winding cannot divide onto the remaining two windings.

When you have a WYE system, faults can add up, and be distributed onto two different phases. Causing it to go undetected by the primary OCPD. So we're talking the mixed topology WYE-DELTA / DELTA/WYE, or perhaps WYE-WYE, where you would find this.

 

[AKsparky1]

See 1:30 in the following video:
https://www.youtube.com/watch?v=tNVoJbhH764

When you have a single phase transformer, the fault currents will align winding-to-winding, with proportionality to one another via the ratio of transformer turns (voltage ratio). Consider a 100A breaker on the primary. An overcurrent of 300A on the secondary of a 240V to 120V single phase transformer, will transform directly into a 150A overcurrent on the primary. And a 100A breaker on the primary will catch this 150A, and thus protect the 200A circuit on the secondary by proxy.

This same concept is also extended for DELTA-TO-DELTA transformers, where fault current also lines up winding-to-winding-to-winding. In otherwords, A-phase faults on the secondary do not get distributed onto the B-phase and C-phase of the primary, thus going undetected. A-phase faults on the secondary pass directly to the A-phase of the primary, on a DELTA-DELTA transformer. In DELTA-DELTA transformers, all windings of either half of the transformer are in series with one another. Current from one winding cannot divide onto the remaining two windings.

When you have a WYE system, faults can add up, and be distributed onto two different phases. Causing it to go undetected by the primary OCPD. So we're talking the mixed topology WYE-DELTA / DELTA/WYE, or perhaps WYE-WYE, where you would find this.

Thank you so much! I gues I knew that if I just sat down and drew it out. Who has time for that. Lol

 

[Carultch]

Thank you so much! I gues I knew that if I just sat down and drew it out. Who has time for that. Lol

Glad I could help.

I almost forgot. The rule about delta-delta transformers, only applies when it is 3-wire delta on both sides.

High leg delta systems require OCPD on both sides. When in doubt, it is probably best to put OCPD on both sides anyway.

 

[AKsparky1]

Glad I could help.

I almost forgot. The rule about delta-delta transformers, only applies when it is 3-wire delta on both sides.

High leg delta systems require OCPD on both sides. When in doubt, it is probably best to put OCPD on both sides anyway.

Yes I can see that that it was explained. Thank you. You guys are doing an awesome job!