NEC 725.48, etc

[Isaiah]

This client wants to run over 90 #14 control conductors in a single, 2" rigid galv steel conduit then tee off to 8 valves located at different tank locations in a large tank farm - this does not seem to fit the definition of functionally associated per NEC 725.48 even though they perform the same type of control operation. Am I correct on this?

 

[Dale001289]

This client wants to run over 90 #14 control conductors in a single, 2" rigid galv steel conduit then tee off to 8 valves located at different tank locations in a large tank farm - this does not seem to fit the definition of functionally associated per NEC 725.48 even though they perform the same type of control operation. Am I correct on this?

as long as they’re all control cable it doesn’t matter


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[don_resqcapt19]

I agree with Dale, as long as the raceway does not contain power conductors such as those for motors or other equipment, there is no restriction on the number of control circuits that can be in a common raceway. This is a 725.48(A) installation, not 725.48(B).

I typically run three 40 conductor bundles between the DCS and the field junction boxes for control wiring.

 

[Isaiah]

I agree with Dale, as long as the raceway does not contain power conductors such as those for motors or other equipment, there is no restriction on the number of control circuits that can be in a common raceway. This is a 725.48(A) installation, not 725.48(B).

I typically run three 40 conductor bundles between the DCS and the field junction boxes for control wiring.

That about heat? Wouldn’t that many conductors creat a huge reduction in ampacity?


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[infinity]

That about heat? Wouldn’t that many conductors creat a huge reduction in ampacity?

Control wires generally do not generate much heat and derating is not required to be applied.

 

[Dale001289]

As long as the ampacity of any of the conductors is less than 10% of the maximum rated continuous current then no adjustment is required for the number of conductors in the conduit. However ambient derating is required, where applicable.

 

[don_resqcapt19]

That about heat? Wouldn’t that many conductors creat a huge reduction in ampacity?
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Most of these circuits have one amp fuses, and most often the load is very small. Typical solenoid valve coils are about 15 watts. The biggest loads are most often motor starter coils, but even there, unless the starter is larger than a size 3, those loads are less than 100 watts. Also many of the conductors are feed back from field devices such as valve position limit switches...the loads for those are only what it takes to set the point in the DCS..a few mA. So there is no issue with heat.

 

[Dale001289]

If you have #16AWG's, all Class 1 circuits, according to 240.4(D)(2), the maximum rated overcurrent protective device allowed is 10 Amps but (1) below that states continuous loads cannot exceed 80%, which is 8 Amps. That's the general rule for ampacities of #16 AWG.
That rule is amended by Art 725. In 725.51(A), you dont have to derate the #16AWG's for your Class 1 circuits unless they carry more than 10% of the 10 Amps continuously, which would be 1Amp. So if you have 90 conductors each in a single conduit, Class 1 Remote Control and Signaling circuit conductors exposed to 40 Deg C ambient with TFFN insulation for example and all are carrying less than 1A, then: 10A x .091= 9.1Amps (adjustment for ambient temp). Each current carrying conductor in the conduit needs to be protected with a fuse or CB rated or set not more than 10A since 240.4(B) allows the next standard size CB

 

[don_resqcapt19]

... Each current carrying conductor in the conduit needs to be protected with a fuse or CB rated or set not more than 10A since 240.4(B) allows the next standard size CB

10 amps if it is a fuse, but 15 amps if it is a breaker as the lowest standard breaker rating is 15 amps.

 

[Isaiah]

If you have #16AWG's, all Class 1 circuits, according to 240.4(D)(2), the maximum rated overcurrent protective device allowed is 10 Amps but (1) below that states continuous loads cannot exceed 80%, which is 8 Amps. That's the general rule for ampacities of #16 AWG.
That rule is amended by Art 725. In 725.51(A), you dont have to derate the #16AWG's for your Class 1 circuits unless they carry more than 10% of the 10 Amps continuously, which would be 1Amp. So if you have 90 conductors each in a single conduit, Class 1 Remote Control and Signaling circuit conductors exposed to 40 Deg C ambient with TFFN insulation for example and all are carrying less than 1A, then: 10A x .091= 9.1Amps (adjustment for ambient temp). Each current carrying conductor in the conduit needs to be protected with a fuse or CB rated or set not more than 10A since 240.4(B) allows the next standard size CB

So what would happen if one of the 90, 16AWG conductors carries more than 1A?


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[Dale001289]

In that case, 310.15(C)(1) [2020 NEC) applies in addition to the ambient temperature correction. The maximum fuse or breaker rating protecting each ungrounded conductor would be 35% of the 10A, which is 3.5A x 0.91 (temp. correction), which would result in a 3A OCPD.

 

[Isaiah]

What if all the conductors are LV power (no controls) and they're going to small motors and receptacles in different locations? Can they be combined into one common conduit?

 

[Dale001289]

What if all the conductors are LV power (no controls) and they're going to small motors and receptacles in different locations? Can they be combined into one common conduit?

You can route LV power together to different locations but then you have to derate.
9 conductors in one conduit would mean 50% derate.


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[Isaiah]

Wouldn’t there be a diversity factor on the receptacles in order to avoid such a drastic derate?


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[don_resqcapt19]

Wouldn’t there be a diversity factor on the receptacles in order to avoid such a drastic derate?
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There could be some reductions for non-coincident loads if the control system is set up that way. Example, either load A, B, or C, but not more than one at the same time. In that case, you would only count the conductors for one load as current carrying.

 

[Isaiah]

There could be some reductions for non-coincident loads if the control system is set up that way. Example, either load A, B, or C, but not more than one at the same time. In that case, you would only count the conductors for one load as current carrying.

So if the loads are interlocked through the control scheme, as you’ve indicated only one load could be counted. But you’d still have to count the receptacles as continuously loaded even though they’re designated on the drawing as “convenience” correct?


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[don_resqcapt19]

So if the loads are interlocked through the control scheme, as you’ve indicated only one load could be counted. But you’d still have to count the receptacles as continuously loaded even though they’re designated on the drawing as “convenience” correct?


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Yes, the conductors that feed the receptacles will have to be counted as current carrying conductors.