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Is 725 the correct section for our application?

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C

Cabledesigner

Member
Location
Cary, NC, USA
We offer a approximately 2 amp rectifier-derived 54 dc volt power via two 12 awg wire (black and white) CL3R PLTC with signal optical fiber cables of up to 3000+ ft runs to campus cameras in a plug an play system. I see that Osha considers 42 volts and 75 ma as possibly fatal. We do not offer a third ground conductor per NEC clause 250.162 no 2. Are we interpreting this correctly or should we include a third ground wire in our system. Is NEC section 725 the correct section for a campus wide 54 volt approx 2 amp camera power delivery network?
 
ron

ron

Senior Member
Location
New York, 40.7514,-73.9925
250.162 (A) Exceptions 2 says
A rectifier-derived dc system supplied from an ac system complying with 250.20 shall not be required to be grounded.

Is the AC system serving the rectifier grounded?
 
don_resqcapt19

don_resqcapt19

Moderator
Staff member
Location
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retired electrician
ron said:
250.162 (A) Exceptions 2 says
A rectifier-derived dc system supplied from an ac system complying with 250.20 shall not be required to be grounded.

Is the AC system serving the rectifier grounded?
Click to expand...
But that doesn't address the EGC issue. Just because a system is not required to be grounded does not change the grounding and bonding requirements. It would be my opinion that the general rules in 250.4(B) require an equipment grounding conductor for this system.
 
don_resqcapt19

don_resqcapt19

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Location
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retired electrician
Cabledesigner said:
We offer a approximately 2 amp rectifier-derived 54 dc volt power via two 12 awg wire (black and white) CL3R PLTC with signal optical fiber cables of up to 3000+ ft runs to campus cameras in a plug an play system. I see that Osha considers 42 volts and 75 ma as possibly fatal. We do not offer a third ground conductor per NEC clause 250.162 no 2. Are we interpreting this correctly or should we include a third ground wire in our system. Is NEC section 725 the correct section for a campus wide 54 volt approx 2 amp camera power delivery network?
Click to expand...
I am not sure that Article 725 applies, as the circuit does not meet the requirement of a Class 1 power limited circuit and Class 1 circuits with a voltage exceeding 30 volts appear to be limited to remote control and signaling circuits. I don't see the power supply circuit to the cameras as a remote control and signaling circuit.
 
hbiss

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
don_resqcapt19 said:
But that doesn't address the EGC issue. Just because a system is not required to be grounded does not change the grounding and bonding requirements. It would be my opinion that the general rules in 250.4(B) require an equipment grounding conductor for this system.
Click to expand...

I assume that the "rectifier derived" DC power is first derived via a transformer? If so, equipment grounding isn't going to provide any safety factor because of the isolation from the 120v mains provided by the transformer. Also, because the DC side is floating, even if a piece of equipment faults to one of the power conductors there would be no shock hazard.

But the question in my mind is what should this power supply be classified as.

-Hal
 
G

GrayHair

Senior Member
Location
Nashville, TN
The 54VDC power supply sounds like POE, but 12 gauge sure doesn't! Running 3,000+ feet of #12 to minimize DC voltage drop doesn't seem cost effective compared to multiple remote power supplies.

If any cameras are outside, I think you are setting yourself up for damage to multiple cameras when a single camera takes a hit from lightning. I've seen it more than once; in one instance our cost for repair/replacement equipment was over $12,000 (no labor). All because they had someone else install another outdoor camera wired directly to security control room equipment (fiber module cost may be why we didn't get the job).

To indicate to a customer that I'd seen it before, I would point to my arm and say, "This is the scar, right here". Your video side may be "protected" by fiber modules, but regardless of what NEC Articles might or might not apply, I do not recommend what you propose. You don't want any scars.
 
don_resqcapt19

don_resqcapt19

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Staff member
Location
Illinois
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retired electrician
hbiss said:
I assume that the "rectifier derived" DC power is first derived via a transformer? If so, equipment grounding isn't going to provide any safety factor because of the isolation from the 120v mains provided by the transformer. Also, because the DC side is floating, even if a piece of equipment faults to one of the power conductors there would be no shock hazard.

But the question in my mind is what should this power supply be classified as.

-Hal
Click to expand...
I am not suggesting that the addition of an EGC would provide any benefit, just suggesting that the code requires one.

As far as what the power supply is, I don't see it fitting anything in Article 725, so it is requires Chapter 3 wiring methods.
 
C

Cabledesigner

Member
Location
Cary, NC, USA
Reply

Reply

Class 3 power supply is ac with ground to dc output 400 watt. (54 volts at approximately 2 amps)
Our CL3R listing allows up to 12 awg
I need something in the code that I can bring up, that rules out using section 725.
The control circuit mention is a good start, however is control circuit defined in more detail so as to exclude powering camera somewhere?
Thank you all for your input so far.
I will study it and reread any references mentioned. (past replies or future replies)
 
J

JFletcher

Senior Member
Location
Williamsburg, VA
Cabledesigner said:
Class 3 power supply is ac with ground to dc output 400 watt. (54 volts at approximately 2 amps)
Our CL3R listing allows up to 12 awg
I need something in the code that I can bring up, that rules out using section 725.
The control circuit mention is a good start, however is control circuit defined in more detail so as to exclude powering camera somewhere?
Thank you all for your input so far.
I will study it and reread any references mentioned. (past replies or future replies)
Click to expand...

Not sure what I'm missing but 400W output would be almost 8A @ 54V.
 
hbiss

hbiss

EC, Westchester, New York NEC: 2014
Location
Hawthorne, New York NEC: 2014
Occupation
EC
Well, this isn't 10-15 years ago. Although I have never seen a Class 3 listed power supply (everything has been either CL2 or 1) I have no doubt they exist. If the power supply that the OP is talking about is listed as CL3 then case closed.

-Hal
 
C

Cabledesigner

Member
Location
Cary, NC, USA
Correction to power supply label

Correction to power supply label

Correction. The cable used is class 3 or CL3R and CL3P, however the power supply is in fact Class 2. Please reply if that correction changes your advice. Thank you.
 
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