NEC 725.48, etc

Isaiah

Senior Member
Location
Baton Rouge
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

Senior Member
Location
Georgia
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

Moderator
Staff member
Location
Illinois
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

Senior Member
Location
Baton Rouge
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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Dale001289

Senior Member
Location
Georgia
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

Moderator
Staff member
Location
Illinois
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

Senior Member
Location
Georgia
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
 

Isaiah

Senior Member
Location
Baton Rouge
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

Senior Member
Location
Georgia
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

Senior Member
Location
Baton Rouge
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

Senior Member
Location
Georgia
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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don_resqcapt19

Moderator
Staff member
Location
Illinois
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

Senior Member
Location
Baton Rouge
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

Moderator
Staff member
Location
Illinois
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.
 
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