resistance "derating" for more than 3 cuurrent-carrying conductors in shared conduit

[niteflyer46]

resistance "derating" for more than 3 cuurrent-carrying conductors in shared conduit

We have been directed to adjust resistance values downward for voltage drop calculations on a municipal project to comply with NEC. I had already corrected ampacity values to account for more than 3 conductors in shared conduit. I have not found correction factors in NEC to adjust wire resistance. Where can I find this, assuming it is actually in the NEC?

 

[david luchini]

We have been directed to adjust resistance values downward for voltage drop calculations on a municipal project to comply with NEC. I had already corrected ampacity values to account for more than 3 conductors in shared conduit. I have not found correction factors in NEC to adjust wire resistance. Where can I find this, assuming it is actually in the NEC?

I think they might mean Chapter 9, Table 8, Note #2...

but the point of the ampacity adjustment it to make sure you don't exceed the 75degC, so I'm not sure what temperature adjustment you would make.

 

[infinity]

Voltage drop compensation is not required by the NEC except for a few limited applications such as fire pumps.

 

[Smart $]

We have been directed to adjust resistance values downward for voltage drop calculations on a municipal project to comply with NEC. I had already corrected ampacity values to account for more than 3 conductors in shared conduit. I have not found correction factors in NEC to adjust wire resistance. Where can I find this, assuming it is actually in the NEC?

Is this "direction" part of project documentation? If so, what is the exact wording?

As noted, the NEC does not require compensation for voltage. It is only suggested in the form of Informational Notes (not more than 3% for branch circuits, and not more than 5% for both feeders and branch circuits).

 

[ggunn]

I think they might mean Chapter 9, Table 8, Note #2...

but the point of the ampacity adjustment it to make sure you don't exceed the 75degC, so I'm not sure what temperature adjustment you would make.

I don't have my codebook handy, but isn't AC resistance always equal to or greater than DC resistance?

 

[Fulthrotl]

Is this "direction" part of project documentation? If so, what is the exact wording?

As noted, the NEC does not require compensation for voltage. It is only suggested in the form of Informational Notes (not more than 3% for branch circuits, and not more than 5% for both feeders and branch circuits).

the prints i'm seeing lately have voltage drop calcs on them.
they don't make it thru plan check without them, according
to my engineer... and it used to be you had 5% total, and
you could have 3% on feeders, and 2% on branch circuits,
or you could reverse those number, it it worked out better
in your situation, as long as you held it to 5% aggregate.

my EE is now having fits as feeders are limited to 2%.

 

[Smart $]

the prints i'm seeing lately have voltage drop calcs on them.
they don't make it thru plan check without them, according
to my engineer... and it used to be you had 5% total, and
you could have 3% on feeders, and 2% on branch circuits,
or you could reverse those number, it it worked out better
in your situation, as long as you held it to 5% aggregate.

my EE is now having fits as feeders are limited to 2%.

Not in the NEC. California or local amendments?

The NEC still "suggests" 3% on either but not more than 5% total..

 

[Julius Right]

As david luchini said : NEC Chapter 9 Tables:
Table 8 Conductor Properties Direct-Current Resistance at 75?C (167?F)
Table 9 Alternating-Current Resistance and Reactance for 600-Volt Cables, 3-Phase, 60 Hz, 75?C (167?F) ? Three Single Conductors in Conduit.
You have to recalculate the d.c. resistance at the required temperature, as David said.
For large conductor cross section-I'll say from 2 awg and up-in a.c. system- there are two factors : skin effect and proximity effect. Also the power losses in conduit are important.
Skin effect depends on dc resistance [at required temperature] and frequency.
Proximity effect depends on the distance between phase conductors, also.
For an exact calculation-in my opinion usually it is not necessary-you have to follow Neher&McGrath theory. See:
The Calculation of Temperature Rise and Load Capability of Cable Systems by J.H.Neher and M.H.McGrath

 

[Carultch]

We have been directed to adjust resistance values downward for voltage drop calculations on a municipal project to comply with NEC. I had already corrected ampacity values to account for more than 3 conductors in shared conduit. I have not found correction factors in NEC to adjust wire resistance. Where can I find this, assuming it is actually in the NEC?

There is no NEC requirement to calculate voltage drop according to a specific method, except in a very limited scope of applications.

For generic applications, the NEC only contains an informational note with recommendations. And there is no procedure for adjusting resistance values for conditions of use.

The resistance values in the NEC are based on 75C conductor temperature, which is well-representative of most of the worst-case conditions which any properly sized wire for ampacity and conditions of use will experience. For several reasons:
1. Most terminations need to be sized per the 75C column of the NEC, before even thinking about conductor type and conditions of use.
2. Temperature is not uniform inside a conductor. Even if the core of the metal achieves 90C, this temperature will decrease farther from the center. The true resistance of the conductor would need to consider the spatial variation of both temperature and the distribution of electrical current.
3. To achieve the high temperatures, it takes a time for a continuous current to accumulate resistive heating to the steady-state temperature. Continuous loads require a 1.25 safety factor on the wire ampacity. So a continuous load will not achieve the full 90C conductor temperature, even if the 90C column of the NEC is relied upon for ampacity calculations.

If you do need to document a calculation, you can apply a temperature correction to the resistance values from 75C to 90C, based on the column of the NEC you rely upon for your ampacity calculations. For instance, consider #8 Cu wire derated at 80% for bundling with a continuous load of 35A. You are relying upon the 55A value in the 90C column. Therefore you can derate copper's resistance of #8 wire from the given value at 75C to the worst-case scenario value at 90C. There is no documentation of this method, and it is very likely overkill to compensate for voltage drop due to this factor.

Anixter has tables which have resistance at 60C, 75C, and 90C:
https://www.anixter.com/content/dam/Anixter/Guide/7H0011X0_W&C_Tech_Handbook_Sec_07.pdf

When I have to upsize wires in extreme excess of their minimum local size due to distance and voltage drop curtailment, I commonly apply the 60C column resistances from the above document. Because the selected wire will likely be sized well within ampacities per the 60C column of the NEC.

 

[Carultch]

I don't have my codebook handy, but isn't AC resistance always equal to or greater than DC resistance?

That's correct. The reason is due to inductive effects, which increase with frequency. A stranded wire is an extremely long unintentional solenoid, and the magnetic/induction effects can come in to play, adding "inertia" to the current that do not exist for DC.

This is only significant for wires 1/0 and larger. The following document provides AC resistance data at 60 Hz:
https://www.anixter.com/content/dam/Anixter/Guide/7H0011X0_W&C_Tech_Handbook_Sec_07.pdf

 

[bob]

resistance "derating" for more than 3 cuurrent-carrying conductors in shared conduit

That has to be a mistake. There is nothing in the code that requires this.

 

[Smart $]

That has to be a mistake. There is nothing in the code that requires this.

Not specifically in that respect, but that is essentially part of the reason behind 310.15(B)(3)(a).

 

[mwm1752]

Commercial projects governed by the 2012 ASHREA 90.1 are limited to a 2% VD for feeders & 3% for branch circuits unless they are dedicated to emergency services.

If they require you to verify 310.15( C ) due to conduit placement of laterals it is unusual. They hire EE for that info & specs to install.

 

[bob]

Not specifically in that respect, but that is essentially part of the reason behind 310.15(B)(3)(a).

The posts says "resistance "derating" for more than 3 cuurrent-carrying conductors in shared conduit ".

The table you reference is for ampacity derating, not resistance derating unless I am missing your point.

 

[Smart $]

The posts says "resistance "derating" for more than 3 cuurrent-carrying conductors in shared conduit ".

The table you reference is for ampacity derating, not resistance derating unless I am missing your point.

I realize that, and you are missing my point. The physics underlying the table is to reduce the combined self-generated heat within the conductors, i.e. the temperature within the conduit. Conductor resistance varies [somewhat] proportionally to temperature change. Reducing a conductor's allowable ampacity is in effect the same as limiting the conductor's resistance rise.

 

[GoldDigger]

I realize that, and you are missing my point. The physics underlying the table is to reduce the combined self-generated heat within the conductors, i.e. the temperature within the conduit. Conductor resistance varies [somewhat] proportionally to temperature change. Reducing a conductor's allowable ampacity is in effect the same as limiting the conductor's resistance rise.

And what you appear to be missing is that the biggest effect from grouping the wires is an increase in insulation temperature because of the difficulty in getting rid of the increased heat, while the resistance and therefore voltage drop are almost independent of temperature in that range.
Certainly nowhere near the ampacity derate ratio!

 

[Smart $]

And what you appear to be missing is that the biggest effect from grouping the wires is an increase in insulation temperature because of the difficulty in getting rid of the increased heat, while the resistance and therefore voltage drop are almost independent of temperature in that range.
Certainly nowhere near the ampacity derate ratio!

I'm not missing anything. Wire resistance and temperature are interdependent, period. Change either, you change both. Cause of the change is irrelevant.

PS: I agree most certainly nowhere near ampacity derating ratio... but other than being based on the physics, I have no idea how the derating ratios were determined.

 

[kwired]

The problem is how it is worded and trying to interpret just what was intended by the request. Maybe OP needs to request more information from whoever is specifying this.

You generally do not adjust the resistance of a conductor, though with carefully controlled environment you maybe can rely on it being a certain resistance at a certain temperature. Instead we have heat developed because of the combination of load level, conductor size/resistance, number of conductors in a cable, raceway, other close proximity and ambient conditions that effect temperature as well. We need to consider all of those and select a conductor that will not be subjected to temperature that may compromise the integrity of the insulation.

The use of words "resistance adjustment" can be easily interpreted as seen in majority of posts so far to mean regulating voltage drop.

Voltage drop and conductor ampacity are two different things and OP needs to clarify which they are trying to address.

If you already have increased conductor size for ampacity adjustment reasons you already have made a step in the area of less voltage drop.

 

[Carultch]

The problem is how it is worded and trying to interpret just what was intended by the request. Maybe OP needs to request more information from whoever is specifying this.

You generally do not adjust the resistance of a conductor, though with carefully controlled environment you maybe can rely on it being a certain resistance at a certain temperature. Instead we have heat developed because of the combination of load level, conductor size/resistance, number of conductors in a cable, raceway, other close proximity and ambient conditions that effect temperature as well. We need to consider all of those and select a conductor that will not be subjected to temperature that may compromise the integrity of the insulation.

The use of words "resistance adjustment" can be easily interpreted as seen in majority of posts so far to mean regulating voltage drop.

Voltage drop and conductor ampacity are two different things and OP needs to clarify which they are trying to address.

If you already have increased conductor size for ampacity adjustment reasons you already have made a step in the area of less voltage drop.

I think our asker is taking a Murphy's law approach to calculating voltage drop. Even though the NEC doesn't require considering conditions of use in voltage drop calculations, the project requirements might require it, and our asker is wondering what the best approach would be.

I've explained how the 75C resistance values given in the NEC are already well-representative of the worst-case scenario temperature that will occur in any properly sized wire. The exception would be if the 90C ampacity rating of the wire is relied upon. With the 125% safety factor on any continuous load that would heat the wire up to 90C, your actual max cont current is only going to heat the wire to 75C.

 

[kwired]

I think our asker is taking a Murphy's law approach to calculating voltage drop. Even though the NEC doesn't require considering conditions of use in voltage drop calculations, the project requirements might require it, and our asker is wondering what the best approach would be.

I've explained how the 75C resistance values given in the NEC are already well-representative of the worst-case scenario temperature that will occur in any properly sized wire. The exception would be if the 90C ampacity rating of the wire is relied upon. With the 125% safety factor on any continuous load that would heat the wire up to 90C, your actual max cont current is only going to heat the wire to 75C.

But Op already mentioned he had made adjustments for more then 3 current carrying conductors in raceway. Though such adjustments do not always equate to an increase in conductor size, if they do voltage drop is changed. Voltage drop is independent of conductor ampacity. There are times when the run is short you can run a smaller conductor then required minimum ampacity and still satisfy any voltage drop requirements you may have. So one must determine minimum ampacity needed, then check to see if that conductor allows desired/required voltage drop allowances.

The confusion IMO is back in the OP where the term resistance derating and adjusting resistance are used. Nobody seems to know what that means, and the NEC doesn't use such terms. NEC does use terms like voltage drop and ampacity adjustment. Otherwise what is adjusting resistance? To me it is turning the knob on a potentiometer.:happyyes: