According to the chart 450.3B, If primary and secondary protection is provide, the primary and secondary protection is limited to 250%. However if I leave out the secondary protection I can bump the primary protection up to 300%. Am I reading this correct? If every thing is sized appropriately and the primary fuse is blowing I can increase the size of the primary by removing the secondary protection?
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Transformer OCP less than 2amps
- Thread starter hughb3
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Re: Transformer OCP less than 2amps
If you are limiting your conclusion to this, yes.
However, you didn't describe the actual application as this may influence the answer.
If for motor control Motor control circuits is covered in section VI of art. 430, Motor Control Circuits, 430.72 Over Current Protection.
Also, refer to NFPA79, Electrical Standard for Industrial Machinery. (My copy may be older as it's 1997.) Art 8.12 Control Transformer. This section covers both primary and secondary protection.
If you are limiting your conclusion to this, yes.
However, you didn't describe the actual application as this may influence the answer.
If for motor control Motor control circuits is covered in section VI of art. 430, Motor Control Circuits, 430.72 Over Current Protection.
Also, refer to NFPA79, Electrical Standard for Industrial Machinery. (My copy may be older as it's 1997.) Art 8.12 Control Transformer. This section covers both primary and secondary protection.
richwaskowitz
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- Shelby Twp. MI, near Detroit
Re: Transformer OCP less than 2amps
Could you expound on your application please? We design and build a lot of huge motor center / control panels and every one of them has a "control transfomer" to step the 480 down to 120.
NFPA 798.11.2 and 8.12, not NFPA 70 used to be the rulebook in these cases. Ther was a table provided there. When the 2002 edition was released, things changed. Now, NFPA 79 at 7.2.7, refers back to the NEC (NFPA 70) to article 430 and 450. Go figure.
Could you expound on your application please? We design and build a lot of huge motor center / control panels and every one of them has a "control transfomer" to step the 480 down to 120.
NFPA 798.11.2 and 8.12, not NFPA 70 used to be the rulebook in these cases. Ther was a table provided there. When the 2002 edition was released, things changed. Now, NFPA 79 at 7.2.7, refers back to the NEC (NFPA 70) to article 430 and 450. Go figure.
richwaskowitz
Senior Member
- Location
- Shelby Twp. MI, near Detroit
Re: Transformer OCP less than 2amps
Oops. Forgot to finish with what I really wanted to say. Make sure you are using the right type of fuse for that primary. At one time we were using Bussman LPSRK fuses across the board in some of our panels and experienced frequent fuse blowing on the control transformer primaries due to inrush. Consulted Bussman and was told not to use the faster LPSRKs and went back to FRS and teh problem went away.
Oops. Forgot to finish with what I really wanted to say. Make sure you are using the right type of fuse for that primary. At one time we were using Bussman LPSRK fuses across the board in some of our panels and experienced frequent fuse blowing on the control transformer primaries due to inrush. Consulted Bussman and was told not to use the faster LPSRKs and went back to FRS and teh problem went away.
Re: Transformer OCP less than 2amps
The application is a simple control transformer, stepping 480 volt to 120 volt. The only items on the control circuit are a relay and LED pilot light. The transformer size is a 75va transformer. 450.3B shows protection on transformer less than 2 amps with primary and secondary protection of 250%, the primary fuse is limited to a .4 amp fuse by (75va/480volt)*250%=.390 which you round up to .4 amps. Now by the description of the protection required in 450.3B if you do not have secondary protection you can change the primary protection by (75va/480volt)*300%=.46 amps this can be now rounded up to .5 amps.
The application is a simple control transformer, stepping 480 volt to 120 volt. The only items on the control circuit are a relay and LED pilot light. The transformer size is a 75va transformer. 450.3B shows protection on transformer less than 2 amps with primary and secondary protection of 250%, the primary fuse is limited to a .4 amp fuse by (75va/480volt)*250%=.390 which you round up to .4 amps. Now by the description of the protection required in 450.3B if you do not have secondary protection you can change the primary protection by (75va/480volt)*300%=.46 amps this can be now rounded up to .5 amps.
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