Low voltage cable in commercial building question? 3

[cajuntank]

I have a new application where a vendor ran some Romex to a outdoor enclosure for a IP camera. I saw where they had run the wire in conduit where visible, but I also saw above the drop ceiling where they did not continue with the conduit and have just Romex. The Romex splits into some AC adapters that then plug into some standard 110V outlets. According to the vendor, it's a 24V step down transformer to power the heater/blower for the outdoor camera enclosure that doesn't even run unless it's needed. He said they used the Romex wire just to have a heavier gauge cable to make sure they covered the distance they were running from my wiring closet to the camera enclosure.

So I know if this would have been a 110V or higher circuit, that code for commercial buildings would dictate that the cable would be completely housed in conduit or sheathed in metal. But since this is 24V, is this ok to have some of the Romex exposed or am I still breaking code? What's the magic number in volts that mandates conduit or a metal sheath?

Thanks.

 

[LkEErie]

Wow. I'm near Lake Erie and you are.....

Low voltage is a function (or not) of local codes. 100v sometimes applies in some areas, in others, all LV has to be in conduit (hint Chicago).

The last time I checked, the 24 volt power supply was adjustable and pretty forgiving. I don't know what distance you were talking, clearly less than 300', but I would think you could get your basic 24 volt supply to make the trip in 16 Ga stranded. I'm not sure, but I would think you could stand to lose 2-4 volts on a blower/heater and still function fine.

LkEErie

 

[misseditbythatmuch]

Unfortunately, there is more to it than a magic voltage (e.g. 30 volts). The type/class of supply plays a role as does the purposes of the circuit.

You need to assess your installation and compare to Article 725 (wiring class) and Article 300 (conduit / raceway / firestops, etc.)

Is the space above the ceiling a plenum space ? That will impact what type of cable and conduit can be run.

Is the cable resting on the tiles, or is it suspended, and if so, are the fixtures suspending it appropriate to the cable (generally single purpose load bearing hooks or suspension fixtures should be used, cable should be suspended, never "stretched" or supporting its weight load under tension)?

The NEC does not attempt to define low voltage circuits in general terms. Instead the Code delineates the rules for remote-control, signaling, power-limited and motor control circuits. Article 725 that contains most of the rules for low voltage and low power remote control, signaling and power limited circuits divides these circuits into Class 1, Class 2, and Class 3 circuits.

Class 1 circuits are further divided into two types, (1) power-limited, and (2) remote-control and signaling circuits. Class 1, power limited circuits are limited to 30 volts with a supply no greater than 1000 volt-amperes. Class 1 remote-control and signaling circuits are limited to 600 volts with no limitation on the size of the power supply.
Class 2 and Class 3 circuits are defined by rule for AC and separately for DC. In general, a 24 volt Class 2 circuit supplied from a listed power supply durably marked 'Class 2" and not exceeding 100 volt-amperes is the one most commonly used.

However, there are restrictive rules as to which class of conductors can and can not share a raceway.

Roughly,
CLASS 1
Power-Limited Circuits

Requirements: Shall be

1. 30 Volts or Less
2. 1000 Volt-amperes or Less
3. Transformers Protected per Article 450
4. Power Sources other than Transformers - limited to 2500 VA.
5. Power Sources other than Transformers Fault Current limited to 10,000 VA for 1 minute or less
6. Power Sources other than Transformers - overcurrent protection limited to 167 % VA of Source divided by Volts

Remote-Control and Signaling Circuits

Requirements: Shall be

1. 600 Volts or Less
2. No Limit on Power Source Volt-Amperes
3. Transformers per Article 450

CLASS 2 or 3

Separate rules for AC and DC


For Class 2 Circuits:
Power Supply generally limited to 100 Volt-amperes and maximum voltage of 30 volts.

For Class 3 Circuits:
Power Supply generally limited to 100 Volt-Amperes and maximum voltage of 100 Volts.

 

[misseditbythatmuch]

I neglected to mention, and LkEErie rightly points out, local code can be more restrictive than the NEC.

Some earthquake sensitive areas require utterly elaborate suspension devices for low voltage cables, for example.


Safest measure would be to consult with the Inspector of Wires in the jurisdiction of the installation.

 

[SYQUEST]

Don't forget Article 800, part of it may apply too...

....JIM....