Electrical/Comm. cable separation?

[djkross]

Electrical and comm. cable being serviced in 4" pvc conduit. Shouldn't there be separation between the two cables? Electrical cables coming from 300kVA transformer.

 

[dpc]

You have not provide much information to go on - voltage levels, type of communications circuit, type of cable, etc, what country, what codes, etc.

In general, there is no NEC requirement for separation below 1000V, as long as all circuit are insulation for the highest voltage in the conduit. So, it is not illegal to put a 24 V dc analog signal cable in the same conduit with a 600 V power circuit, as long as the signal cable has 600 V insulation.

This does not mean that there won't be interference problems.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[avogel]

Maybe you can better define what electrical and comm are in this applicaton, but strictly speaking, if you are mixing conductors with differenet insulation ratings (ie: 300V and 600V) it's not permittted off the bat.

Also, if you dig into Article 800, it is also not permitted to mix power-limited with normal power circuits. There are exceptions pertaining to circuits which are soley to provide power supply to communications equipment. Or if there is a permanent barrier.

 

[davidbeach]

if you are mixing conductors with differenet insulation ratings (ie: 300V and 600V) it's not permittted off the bat

Not true, the power could be from a 208Y/120V system using 600V insulation and 300V insulated control wiring would be just fine. The way that dpc stated the rule is much more accurate.

 

[avogel]

yeah um ok, semantics. in any case. i still would check out article 800, because you will find its not allowed, even if your cable is 300v rated.

 

[controlsdude]

If fiber optic with your Hi voltage cable, why not? It should work fine

Any other communications cable type your just making noise for the comunications running them in same conduit. I guess this would be just be bad practice but still lawful within NEC or whatever spec that would allow it.

 

[dpc]

You must also pay close attention to the definition of a "Communications circuit" in Article 800.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[rob46]

The UTP cable should be installed some distance away from an EMI sources. Cabling pathways standard, EIA-569 table 4.8-5 provides those guidelines (source LANs/cabling-faq that refers to Northern Telecom, doc # IBDN-UM-9105, 1991):

Minimum Separation Distance
from Power Source at 480V or less
CONDITION <2kVA 2-5kVA >5kVA
Unshielded power lines or
electrical equipment in proximity
to open or non-metal pathways 5 in. 12 in. 24 in.
(12.7 cm) (30.5 cm) (61 cm)
Unshielded power lines or
electrical equipment in proximity
to grounded metal conduit pathway 2.5 in. 6 in. 12 in.
(6.4 cm) (15.2 cm) (30.5 cm)
Power lines enclosed in a grounded
metal conduit (or equivalent
shielding) in proximity
to grounded metal conduit pathway - 6 in. 12 in.
- (15.2 cm) (30.5 cm)

Transformers & electric motors ------- 40-in (1.02 m) -----

Fluorescent lighting ------- 12-in (30.5 cm) ----


NEC 2001, article 800.52

Communications cables can't be bundled in the same sheath with Class 1 circuits. Class 2 and Class 3 circuit conductors can be in a sheath with communications circuits, if the Class 2 and Class 3 circuits are contained in listed communications cable or multipurpose cable.

Communications conductors can't run in any raceway, compartment, outlet box, junction box, or similar fitting with conductors of electric light, power, or Class 1 circuits, but there are exceptions.

*

Separate the conductors from the power or Class 1 conductors by a barrier.
*

Introduce power circuit conductors only to connect to the communications equipment. The power circuit conductors require at least 0.25 in. of separation from communications circuit conductors.

In other applications, you must separate communications conductors by at least 2 in. from any electric light, power, or Class 1 circuit conductors, unless you install those conductors per a Chapter 3 wiring method, such as raceway, metallic or nonmetallic sheath, or UF cable.

 

[rcw retired EE]

Another issue - Small communication or control cables can be damaged during installation if they are in the same conduit with heavy power cables, especially if they are pulled into the conduit at the same time. The power cable can withstand much higher pulling tension than teh small cable. The pulling tension needed to get the heavy wires through the conduit can damage the small wires.