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#101969 04/29/04 12:19 AM
Joined: Nov 2001
Posts: 45
I wanted to get some input into running control wires and supply wires in the same conduit servicing a roof top unit. Is this a bad idea? If so, why? I am wiring a building that has a wrap around vapour barrier and roof penetrations are to be kept to a minimum. There is a spec restricting the roof penetrations to a certain kind of "gum cup". This "gum cup" allows only one conduit or liquidtite flex conduit through it. What are your thoughts guys?

Joined: Oct 2003
Posts: 147
Derating the conductors ampacity (Table 5C) might make it necessary to use larger supply conductors if there are a large number of control wires.

I am curious if anyone has any thoughts on rule 12-904(2) and 12-904(2),(c). Especially where the supply and control wiring are a different voltage, and in the same conduit.

Joined: Mar 2004
Posts: 947
twh Offline
I see control wires with power conductors quite often. 4-004(7)(a) excludes control wires from the derating calculations.

Some roof top units have ko's designed for entry through the return air. No need to go through the roof, at all.

There may be a derating factor for ambient if the feed runs across the roof. In Sask. its 50 Celsius, so I never bothered looking for it in the CEC.

crash, no thoughts besides that it is allowed. You have me curious, what are you thinking?

Joined: Oct 2003
Posts: 147
TWH: Good call on 4-004(7),(a). I never applied that rule to control conductors. I guess I was wrong on the derating.

On 12-904 there are two things i'm thinking about.

Last winter I had to deal with a RTU that was to be interlocked with several ventalation fans, and several exhaust fans. This lead to the conduit going to the RTU with circiuts from several different panels (panels being located in several different locations in the building). I didn't think this was a wise thing to do, but 12-904 seems to allow it.

The other thing I'm thinking about is the possibility of a higher voltage conductor (maybe 600/347) inducing a voltage on lower voltage (maybe 12V) conductor, that connects to a control board with sensitive electronic equipment on it.

I've never heard of any problems, but I'd like to here any ideas on the subject.

Joined: May 2001
Posts: 144
Rule 12-904(2)

No raceway or compartment of a multiple-channel raceway shall contain conductors that are connected to different power or distribution transformers or other different sources of voltage except where the conductors:

(a) Are separated by the metal armour or metal sheath of cable assemblies of the types listed in Table 19; or

(b) Are separated by a barrier of sheet steel not less that 0.0528 inch (No. 16 MSG) thick or a flame-retardant nonmetallic insulating material not less than 1.5 mm in thickness; or

(c) Are used for the supply and/or control of remote devices, are insulated for at least the same voltage as that of the circuit having the highest voltage, and none of the conductors of the circuits of lower voltages is directly connected to a lighting branch circuit.

Rationale and Intent for Rule 12-904(2).

We intend to prevent conductors of different systems from becoming crossed and impressing another voltage on a circuit. We intend that conductors from different transformers or other sources of voltage not be installed in the same raceway or compartment of a multi-channel raceway, except where the conductors are used for the supply and/or control of remote devices and are insulated for the highest voltage in the enclosure. We intend that the lower voltage conductors not be directly connected to lighting branch circuits. Conductors of different systems may be installed in the same raceway or compartment, provided they are separated by enclosure in an armoured cable or metal-sheathed cable or by a barrier of sheet steel. For example, the conductors of a 115 V control circuit may be installed in the same conduit supplying a 440 V, 3-phase motor, provided all the conductors insulation is rated 600 V.

Tony Moscioni
Electrical Inspector
Electrical Safety Authority

Joined: Mar 2004
Posts: 947
twh Offline
crash, I think most of us are uneasy about mixing systems in a conduit or control box, but sometimes it is unavoidable. I will add that when I open a box with multiple circuits and find installation notes written on the cover in jiffy marker, I don't care if the entire building is wired in extension cord, I still think the person who left the notes is the greatest electrician in the world.

As to circuits interfering with each other, the only time I saw a problem was where a contactor was mounted beside a circuit board. It was a system that counted pulses and the contactor would interfere with the count.

Hopefully, someone with more experience will stop by.

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