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Control Transformer Capacity
- Thread starter twinds
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:ashamed: Regarding the Industrial Control Panel to NFPA79 (UL508A), could anyone tell me whether there is the capacity limitation of the control circuit transformer or not ?
Could you clarify your question?
For example how would your question relate to a .5kva CPT?
[FONT=ＭＳ Ｐゴシック][/FONT]Thankyou templdl,
[FONT=ＭＳ Ｐゴシック][/FONT]NowI?m planning to use a 1 phase more than 40kVA control transformer supplying120V for a huge machine system. But 40kVA sounds unusual as a controltransformer, so I?m checking if this capacity would meet NFPA and / or ULrequirements, if this would violate them or not. And I find the clause 9.1.1.1and 9.1.1.3 from NFPA79, which seem to have relations with my question. HereNFPA79 has set the criteria for the need of transformer to available shortcircuit current 1000A rms. 40kVA transformer of course should supply far morethan 1000A in short circuit, and taking 120V 1000A available short circuitcurrent, the control transformer rating would be more or less 5 to 6kVA at a maximum. Meanwhile I also find in the NEC theArticle 725 concerning Class 1 circuit, here in clause 725.41(B) Class 1Remote-Control and Signaling Circuits, it says ?The power output of the sourceshall not be required to be limited? Taking this, the control transformercapacity has no limitation literally. I?m not sure how to think about this, socould you advise me ?
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[FONT=ＭＳ Ｐゴシック][/FONT]
[FONT=ＭＳ Ｐゴシック][/FONT]NowI?m planning to use a 1 phase more than 40kVA control transformer supplying120V for a huge machine system. But 40kVA sounds unusual as a controltransformer, so I?m checking if this capacity would meet NFPA and / or ULrequirements, if this would violate them or not. And I find the clause 9.1.1.1and 9.1.1.3 from NFPA79, which seem to have relations with my question. HereNFPA79 has set the criteria for the need of transformer to available shortcircuit current 1000A rms. 40kVA transformer of course should supply far morethan 1000A in short circuit, and taking 120V 1000A available short circuitcurrent, the control transformer rating would be more or less 5 to 6kVA at a maximum. Meanwhile I also find in the NEC theArticle 725 concerning Class 1 circuit, here in clause 725.41(B) Class 1Remote-Control and Signaling Circuits, it says ?The power output of the sourceshall not be required to be limited? Taking this, the control transformercapacity has no limitation literally. I?m not sure how to think about this, socould you advise me ?
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40kva isn't a CPT; it?s a distribution transformer, ref NEC art 450. Being UL is of no consequence because reputable manufacturers will manufacturer their product to meet UL requirements and label then as such.
Also, common 1ph transformers in that capacity range will be 37.5 or 50kva with an impedance of about 4.5 to 6.5%z. As such you would be able to compute the available fault current.
The percent regulation should be about 25% at 100%pf and5.5-6.5% at 80%pf.
I don't believe the fault current with this small of a transformer should be an issue but if you are going to supply control circuits as you Should you have provided a proposed schematic instead is providing a bits and pieces description I'm sure any response that I provide of that others give would be more relevant and to the point.
You may be envisioning one thing and the rest of would be envisioning a completely different scenario based upon what would be our best guest as best.
Also, common 1ph transformers in that capacity range will be 37.5 or 50kva with an impedance of about 4.5 to 6.5%z. As such you would be able to compute the available fault current.
The percent regulation should be about 25% at 100%pf and5.5-6.5% at 80%pf.
I don't believe the fault current with this small of a transformer should be an issue but if you are going to supply control circuits as you Should you have provided a proposed schematic instead is providing a bits and pieces description I'm sure any response that I provide of that others give would be more relevant and to the point.
You may be envisioning one thing and the rest of would be envisioning a completely different scenario based upon what would be our best guest as best.
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