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Buck-Boost Transformers Secondary Protection
- Thread starter sadegh
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- PE (Retired) - Power Systems
Re: Buck-Boost Transformers Secondary Protection
Transformers connected in a buck-boost arrangement are really being used as autotransformers so 450.4 applies.
Transformers connected in a buck-boost arrangement are really being used as autotransformers so 450.4 applies.
Re: Buck-Boost Transformers Secondary Protection
There is never any OCPD place in the secondary of a B-B transformer. Protection is always provided on the line side. Should an OCPD be supplied on the load side it is possible for the load side voltage to increase in proportion to the high and low voltage winding ratio because the OCPD opens the secondary circuit which is always supposed to be closed in a B-B application.
There is never any OCPD place in the secondary of a B-B transformer. Protection is always provided on the line side. Should an OCPD be supplied on the load side it is possible for the load side voltage to increase in proportion to the high and low voltage winding ratio because the OCPD opens the secondary circuit which is always supposed to be closed in a B-B application.
john m. caloggero
Senior Member
Re: Buck-Boost Transformers Secondary Protection
There is no prohibition for the installation overcurrent devices in the output side conductors of a buck-boost autotransformer configuration. The prohibition is that an overcurrent device is no allowed to be installed in series with the shunt winding.
There is no prohibition for the installation overcurrent devices in the output side conductors of a buck-boost autotransformer configuration. The prohibition is that an overcurrent device is no allowed to be installed in series with the shunt winding.
Re: Buck-Boost Transformers Secondary Protection
I should clarify this. Secondary OCPD is not required so that implies that you can provide sec. OCP. This leaves it open to an error when locating the OCPD. If inadvertanly placed in the shunt winding circuit that's where the problem occurs.
I should clarify this. Secondary OCPD is not required so that implies that you can provide sec. OCP. This leaves it open to an error when locating the OCPD. If inadvertanly placed in the shunt winding circuit that's where the problem occurs.
Re: Buck-Boost Transformers Secondary Protection
Output protection would only be required if the output is serving outlets or to live within the limitation of the maximum overcurrent protection for an appliance.
Examples:
1. 30 amp 277 volt single phase branch circuit suppies a 750 va 240 volts by 32 volt buck boost transformer that has approximately 240 volts on the output. Primary to secondary voltage ratio is 1.133:1 meaning that the effective secondary overcurrent protection is 33.99 read 34 amps. This means that primary and secondary equipment grounding conductors need to be a minimum of #8 copper and the primary supply conductors need to be a minimum of #8 copper.
Output overcurrent protection in the amount of 30 amps required for a 30 amp receptacle or a 4,500 watt electric heater. No output overcurrent protection required for a 6,000 watt 240 volts heater.
2. Same 750 va 240 volt by 32 volt buck boost transformer is used to supply a 30 amp 240 volt outlet off of a 35 amp 208 volt branch circuit. Effective output overcurrent protection is 30.88 amps if input protection is 35 amps, output protection theoretically required. If input protection is 40 amps then effective output protection is 35.29 amps definitely requiring a 30 amp circuit breaker.
3. A 500 va 240 volt by 32 volt byuck boost transformer is used to supply a 15 amp 240 volt receptacle off of a 208 volt 20 amp circuit. Effective ouput protection is 17,64 amps requiring a 15 amp circuit breaker to protect the receptacle.
Gets very interesting after a while.
Output protection would only be required if the output is serving outlets or to live within the limitation of the maximum overcurrent protection for an appliance.
Examples:
1. 30 amp 277 volt single phase branch circuit suppies a 750 va 240 volts by 32 volt buck boost transformer that has approximately 240 volts on the output. Primary to secondary voltage ratio is 1.133:1 meaning that the effective secondary overcurrent protection is 33.99 read 34 amps. This means that primary and secondary equipment grounding conductors need to be a minimum of #8 copper and the primary supply conductors need to be a minimum of #8 copper.
Output overcurrent protection in the amount of 30 amps required for a 30 amp receptacle or a 4,500 watt electric heater. No output overcurrent protection required for a 6,000 watt 240 volts heater.
2. Same 750 va 240 volt by 32 volt buck boost transformer is used to supply a 30 amp 240 volt outlet off of a 35 amp 208 volt branch circuit. Effective output overcurrent protection is 30.88 amps if input protection is 35 amps, output protection theoretically required. If input protection is 40 amps then effective output protection is 35.29 amps definitely requiring a 30 amp circuit breaker.
3. A 500 va 240 volt by 32 volt byuck boost transformer is used to supply a 15 amp 240 volt receptacle off of a 208 volt 20 amp circuit. Effective ouput protection is 17,64 amps requiring a 15 amp circuit breaker to protect the receptacle.
Gets very interesting after a while.
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