Voltage drop and derating corelation

[DTempleman]

I have a question that I have never gotten a complete answer on and I would like to know all the factors into making an intelligent descion on this question. Where did you find the articles in the code and if not there then where.
I am going to give a hypethetical situation to better illustrate my question at the end.
Lets say that I am feeding an assembly line of equipment. All the equipment is 208 volts and it is lets say 400 feet away. Each Unit draws lets say 30 amps and their are 9 of them. I know that this situation would probably never happen but i use these extremes to better illustrate my point. Now my question is Knowing that I will I have to upsize my wire to 125% of the F.L.A. and Upsize for Voltage drop. My question is does the fact that i upsized for the f.l.a. and upsized for voltage drop do i need to upsize again for deration {T. 310.13}? or could I already use those upsizing of conductors to cover the stipulation for derating to make a cleaner, cheaper, and less labor intensive instalation.

 

[Smart $]

...My question is does the fact that i upsized for the f.l.a. and upsized for voltage drop do i need to upsize again for deration {T. 310.13}?

Yes. Each is essentially a different component to the buildup of heat in the conductor.

 

[iwire]

Yes. Each is essentially a different component to the buildup of heat in the conductor.

I don't 'have to' do anything about voltage drop.

Size the conductors for 125% of FLA and number of conductors and your done as far as the NEC is concerned.

 

[infinity]

I would follow what Bob said. First find the conductor size with the 125% value. Next apply derating factors and increase the conductor size if needed. Now check your adjusted conductor size and see if it's large enough to compensate for the potential voltage drop. If so you're good. If not increase the conductor size until your voltage drop is less than your design requires. As Bob said voltage drop considerations are voluntary and not required by the NEC.

 

[Smart $]

I don't 'have to' do anything about voltage drop.

Size the conductors for 125% of FLA and number of conductors and your done as far as the NEC is concerned.

That is correct.

The word "need" in the question is subjective.

 

[iwire]

Re-reading my post I see where it looks like I was ignoring voltage drop. I was just trying to point out the rules.

I agree with Rob's post, after you take care of the NEC, run the figures and see where you are with voltage drop in relation to what you are willing to accept.

There are no black and white answers when dealing with voltage drop. You have motor loads, what current would you use to perform the voltage drop calculations?

• The expected true running load current?
• The nameplate FLA?
• The nameplate FLA + 25%?
• The start up current?

Now how much voltage drop are you willing to tolerate?

• 1%
• 3%
• 5%
• 10%
• other?

 

[Luketrician]

Would 430.52 come in to play here?

 

[iwire]

Would 430.52 come in to play here?

Not really sure what you mean. :smile:

 

[Luketrician]

If there are infact motors in this assembly line of equipment wouldn't you have to use 430.52 and upsize the OCPD for the inrush currents at start up?

 

[iwire]

If there are infact motors in this assembly line of equipment wouldn't you have to use 430.52 and upsize the OCPD for the inrush currents at start up?

Well kind of. :smile:

430.52 does not require us to up size the OCD, that section allows it.

However regardless of the OCD being sized at 100% or 300% or even 800% the conductor sizes and currents do not change.

 

[mikehughes8]

Art 210.19(a)(1)
FPN #4
Total voltage drop between branch and feeder circuit to furthest outlet should not exceed 5% <paraphrased>

So is this to be interrupted as a reccomendation or a rule??

 

[iwire]

Art 210.19(a)(1)
FPN #4
Total voltage drop between branch and feeder circuit to furthest outlet should not exceed 5% <paraphrased>

So is this to be interrupted as a reccomendation or a rule??

FPNs are just suggestions, see 90.5(C).

But even if we decide to follow that suggestion and try to maintain less then 5% drop we have to make a personal choice about what current we will use to determine the voltage drop.

 

[Luketrician]

Well kind of. :smile:

430.52 does not require us to up size the OCD, that section allows it.

However regardless of the OCD being sized at 100% or 300% or even 800% the conductor sizes and currents do not change.

Understood.:smile:


Would you agree that the code only gives us minimum guidelines to follow, and that it is up to us electricians to be able to interpret when and where we need to apply certain formulas to ensure a quality installation? Which would be why 430.52 allows us to upsize but doesn't require it?

 

[iwire]

Would you agree that the code only gives us minimum guidelines to follow, and that it is up to us electricians to be able to interpret when and where we need to apply certain formulas to ensure a quality installation? Which would be why 430.52 allows us to upsize but doesn't require it?

Yes, without a doubt. :smile:

But going back to the OPs voltage drop question.

I might well choose to size the OCPD at 300% prevent tripping on start up but I would never consider using that OCPD rating to calculate the voltage drop. :smile:

 

[Smart $]

Yes, without a doubt. :smile:

But going back to the OPs voltage drop question...

Another aspect concerning upsizing for both derating and voltage drop is the fact that you may have to upsize for derating purposes even though none of the circuits are conducting continuously near that ampacity. So upsizing for one may also cover the other.

For example you have 10-20 ccc's on 20A circuits in a conduit. Because of the number of ccc you end up using #10 THHN. However, your circuits are only loaded 50% (10A). Derating assumes maximum circuit loading. That difference can be attributed to upsizing for voltage drop (though it will involve a little more math than a basic Vd calculation).

 

[iwire]

Another aspect concerning upsizing for both derating and voltage drop is the fact that you may have to upsize for derating purposes even though none of the circuits are conducting continuously near that ampacity. So upsizing for one may also cover the other.

For example you have 10-20 ccc's on 20A circuits in a conduit. Because of the number of ccc you end up using #10 THHN. However, your circuits are only loaded 50% (10A). Derating assumes maximum circuit loading. That difference can be attributed to upsizing for voltage drop (though it will involve a little more math than a basic Vd calculation).

I have no idea what your talking about.:-?

 

[DTempleman]

More clarification on my question

More clarification on my question

My thought is to use as many ccc's as possible in one conduit and still be accepted by the NEC. I also thought that deration applied to overcurrent protection not load on the conductor. I just want to know where in the NEC it stipulates this and also the reasoning behind said stipulation. If not then I will use my interpretation of it for being the easyist and cheapest installation.

 

[Smart $]

I have no idea what your talking about.:-?

The basis for circuit derating is thermal transfer reduction due to an increased number of conductors in a conduit. That is, an increase in the number of current carrying conductors in a conduit reduces the ability of those conductors to disperse its heat as efficiently as when there are fewer conductors in the conduit.

The implementation of derating uses circuit ampacity as the cutoff point when determining when the conductor must be upsized. That is to say, derating assumes the amount of heat generated by the conductors is as if it is conducting at its rated ampacity (or perhaps I should say at 80% of its rated ampacity).

However, derating does not take into consideration that conductors of a circuit may not be loaded to the rated ampacity of the circuit. Thus less than the assumed amount of heat is generated and the expected temperature of the conductor is substantially less. This lower operating temperature also means the conductor has a lower resistance... and lower resistance translates into less voltage drop.

 

[don_resqcapt19]

My thought is to use as many ccc's as possible in one conduit and still be accepted by the NEC.

The only limit on the number of current carrying conductors in a raceway are the raceway fill rules.

I also thought that deration applied to overcurrent protection not load on the conductor.

The deratation applies to the ampacity of the conductor. It becomes the actual ampacity for both load and overcurrent protection purposes.

I just want to know where in the NEC it stipulates this and also the reasoning behind said stipulation. If not then I will use my interpretation of it for being the easyist and cheapest installation.

The rule in 310.15(B) specifies that the ampacity of the conductor is that as corrected or adjusted by the rules in (B)(1) through (B)(6). This ampacity is used for all purposes in the code rules just like you would use the ampacities in the Table if you were not required to apply any correction or adjustment factors.