NFPA79 Branch Circuits/Field Devices

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Rocinante

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United States
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Engineer
Looking at NFPA79 for branch circuit requirements and I am a bit confused about 7.2.8. I commonly see Industrial Control Panels utilizing terminal blocks to feed many field devices (I.e, push buttons, inductive sensors, photo eyes, safety scanners). What I gather from this standard instead is that every single field device must have an OCPD rated for branch circuit protection. Is that correct?
 
Only if it is a branch, ie line power (not LV sensors etc.). But think that through a bit. WHERE is the power for these field devices coming from? A CPT? If so, is the secondary fused? If it is, there you go. If not, why not? If not a CPT, then a breaker somewhere else? If so, is that not a branch breaker?
 
Rocinante said:
Looking at NFPA79 for branch circuit requirements and I am a bit confused about 7.2.8. I commonly see Industrial Control Panels utilizing terminal blocks to feed many field devices (I.e, push buttons, inductive sensors, photo eyes, safety scanners). What I gather from this standard instead is that every single field device must have an OCPD rated for branch circuit protection. Is that correct?
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Only required for each branch circuit, not each field device. You can have as many field devices in a branch circuit as you need.
 
Jraef said:
Only if it is a branch, ie line power (not LV sensors etc.). But think that through a bit. WHERE is the power for these field devices coming from? A CPT? If so, is the secondary fused? If it is, there you go. If not, why not? If not a CPT, then a breaker somewhere else? If so, is that not a branch breaker?
Click to expand...
Hi, Jraef,

Thanks for your reply. The common circuit I see is sourced from a 24VDC listed power supply. It is protected at its output for the rating of the supply. For our example let’s assume it’s 10amps. From this feeder OCPD it is ran to a terminal block strip. From this strip, multiple conductors are tapped off the terminal strip and land on their respective branch circuit OCPDs sized at 3amps. From here they land on additional terminal block strips to provide power for field devices. My understanding is that each conductor originating from the final terminal block to provide power for the device (i.e, limit switch, inductive proximity switch, photo eye, etc.) would be considered a tap conductor. From 7.2.8 we see that each tap condition must end in a single branch circuit OCPD. This is what lead me to interpret that each device must now have its own personal OCPD, which seems ludicrous and expensive to me.
 
Rocinante said:
Hi, Jraef,

Thanks for your reply. The common circuit I see is sourced from a 24VDC listed power supply. It is protected at its output for the rating of the supply. For our example let’s assume it’s 10amps. From this feeder OCPD it is ran to a terminal block strip. From this strip, multiple conductors are tapped off the terminal strip and land on their respective branch circuit OCPDs sized at 3amps. From here they land on additional terminal block strips to provide power for field devices. My understanding is that each conductor originating from the final terminal block to provide power for the device (i.e, limit switch, inductive proximity switch, photo eye, etc.) would be considered a tap conductor. From 7.2.8 we see that each tap condition must end in a single branch circuit OCPD. This is what lead me to interpret that each device must now have its own personal OCPD, which seems ludicrous and expensive to me.
Click to expand...
It is my opinion that 7.2.4 applies. Assuming the conductors are 16 or larger, no additional OCPD is required.
 
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