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- Connecticut
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- Engineer
Remove the part in red. Then it would be right.
Voltage Drop Calculation for Conductors larger than 310.16
- Thread starter NorthwestPV
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Remove the part in red. Then it would be right.
T
T.M.Haja Sahib
Guest
But this is a non-linear circuit for which Ohm's law is not applicable.
T
T.M.Haja Sahib
Guest
Why?
- Location
- Connecticut
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- Engineer
Because ampacity is in no way related to the voltage drop of a conductor due to any load current through it. Period. No ifs, ands, buts, or "excepts."
Here's another example. #1THHN with 75 deg terminations has an ampacity of 130. Compare the voltage drop on two feeders using #1 THHN:
The first feeder has a load current of 130Amps. The second feeder has a load current of 120Amps. Both feeders have the same ampacity, yet they will have a different voltage drop because they have different load currents.
Now compare a feeder with a 120A load current (130A ampacity) with a feeder with a 120A load current that shares a conduit with another feeder (6 current carrying conductors.) The ampacity of the feeder in the shared conduit is 150*0.8=120. The feeders have a different ampacity, yet they will have the some voltage drop because they have the same load currents.
So different load current with same ampacity = different voltage drop. And different ampacity with the same load current = same voltage drop. So do you see a relationship between load current and voltage drop, or between ampacity and voltage drop?
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kwired
Electron manager
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- NE Nebraska
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- EC
does that matter? ampacity of the conductor has not changed, the voltage drop has, which does result in a change in current if everything else remains the same. Voltage drop is caused by any impedance not just resistance.
T
T.M.Haja Sahib
Guest
You are correct. I simply meant this.
#1 size THHN conductor votage drop at 80A load current= (X) V
So resistance of conductor= (X)/80 Ohms
So the voltage drop at its ampacity i.e at 110 A= 110*[(X)/80] V.
So if voltage drop at any load current is known, voltage drop at the ampacity of a conductor can be calculated and vice versa.
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- Connecticut
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- Engineer
Again, ampacity has nothing to do with it.
What you are saying is that if you know the voltage drop for a conductor at a given distance for a given load current, you can calculate the voltage drop for the same conductor at the same distance for any other load current
T
T.M.Haja Sahib
Guest
Why I am insisting voltage drop at the ampacity of a conductor is that it is the maximum voltage drop that a conductor can safely attain at a given temperature rating of its insulation.
- Location
- Connecticut
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- Engineer
The concept of a maximum voltage drop that a conductor can "safely attain" doesn't make any sense. There are no "safety" issues with voltage drop (other than relating to fire pumps) in the Code.
If I were to ask what is the maximum voltage drop that #1 THHN conductors with 75 deg terminations (ampacity = 130) can attain for a load current of 130Amps, you wouldn't be able to answer that question.
T
T.M.Haja Sahib
Guest
You mean impedance of #1 THHN conductor at 75 deg per metre not available?
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- Connecticut
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- Engineer
No, I'm not saying that. I'm saying there is no maximum allowable voltage drop for a given conductor.
If you calculate the voltage drop for 130A on #1 THHN at 100' it will be less than at 200' or at 500' or at 1000'. So what is the "maximum voltage drop that #1 THHN can safely attain at a 75deg rating?"
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
No. Ampacity dictates what the maximum current a given conductor can carry without compromising its insulation, if it has any. At that current, there will be a calculable voltage drop. But it's not the voltage drop that hits a maximum, it's the current, and it's different for different types of conductors of the same size, even though for a given current the voltage drop is the same. On top of that, voltage drop is proportional to conductor length as well, which has nothing whatsoever to do with ampacity; if a wire can carry 100A, it can be a foot long or a mile long, it can still carry 100A. The ampacity is the same. The voltage drop, however, will be very different.
Ampacity and voltage drop are dependent variables which, although the sets of independent variables from which they are derived have some variables in common, do not derive from each other. It may sound a bit harsh, and I apologize for that, but it seems to me that you are just struggling to find a way to look at this so that you can be "right".
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Nicely put with brevity and clarity.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I'd bet he is willing to try to answer it though
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Or even simpler, strip the insulation off the conductor.
No - that leaves the voltage drop the same and changes the ampacity:happyyes:
ggunn
PE (Electrical), NABCEP certified
- Location
- Austin, TX, USA
- Occupation
- Consulting Electrical Engineer - Photovoltaic Systems
Yes, of course. I meant as a way to change one without changing the other. That's my story and I'm sticking to it.
T
T.M.Haja Sahib
Guest
I did not mean that way. The maximum allowable voltage drop for any given circuit is fixed, because the ampacity of the circuit is fixed. So to your question "what is the maximum voltage drop that #1 THHN can safely attain at a 75deg rating?", the answer begins with ''Where is the circuit?''
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
There is a maximum voltage drop based on maximum ampacity, but if you are overloaded the voltage drop will continue to increase. Again ampacity for insulated conductors is limited by the insulation - the conductor itself can carry more current.
- Location
- Connecticut
- Occupation
- Engineer
No, that is not correct. The maximum "allowable" voltage drop based on the ampacity of the circuit is limited by the source voltage, not the ampacity of the conductor. It is dependent on the length of the circuit, the resistance of the conductor, and the load current on the circuit. Ampacity has nothing to do with it.
What is the voltage drop on #1 THHN with 75 deg terminations with a 130A load at 200'? And what is it at 500'? And what is it at 5000'? The voltage drop changes for this circuit depending on the length, but the ampacity of the circuit is fixed. Each of these circuits has the same ampacity because they are using the same conductor, but their voltage drop is different. So what is the maximum voltage crop for the circuit ampacity? It changes with length.
Now imagine the voltage drop on #1 THHN with 75 deg terminations with a 130A load at 200'. Now increase the load to 140A. What will happen to the voltage drop? It will increase, because the load current increased. But how is it possible to get a greater voltage drop on a circuit than the "maximum allowable based on the ampacity"? Simple, ampacity has nothing to do with voltage drop.
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