Voltage Drop Calculation for Conductors larger than 310.16

[T.M.Haja Sahib]

I'm afraid I don't quite understand his point either. As evidenced by...



But let's try this...

#1 THHN with 60deg terminations has an allowable ampacity of 110, #1 THHN with 75 deg terminations has an allowable ampacity of 130 and, #1 THHN with 90 deg terminations has an allowable ampacity of 150. An 80A load over a distance of 500 feet would have the same voltage drop for all three of these conductors, even though they have different ampacities.

Thanks for your work out, David. But I want to submit that the question is about the relationship between ampacity and voltage drop and not about a given load current less than the conductor ampacity, in this case 80A, and the resulting voltage drop. If the conductor is loaded to its ampacity at different temperature ratings, the resulting voltage drops will be different, thereby suggesting a close relationship between the two.

 

[david luchini]

Thanks for your work out, David. But I want to submit that the question is about the relationship between ampacity and voltage drop and not about a given load current less than the conductor ampacity, in this case 80A, and the resulting voltage drop. If the conductor is loaded to its ampacity at different temperature ratings, the resulting voltage drops will be different, thereby suggesting a close relationship between the two.

This is completely irrelevant. The ampacity of the conductor has no relationship on the voltage drop, as you suggest.

You have simply applied two different load currents to a conductor and found that the two different load currents produce two different voltage drops. This shows an relationship between load current and voltage drop. It is the level of current in the circuit, not the ampacity of the conductor that affects the voltage drop.

 

[kwired]

Ampacity of a conductor is a function of both the insulation material and the metal inside; conductors with less heat tolerant insulation have lower ampacities and conductors with higher cross sectional areas of metal have higher ampacities. Voltage drop is a function of the metal only; conductors with higher cross sectional area of metal have proportionately lower resistance and therefore less voltage drop irrespective of which insulation they have or even if they have insulation. A 1/0 wire has only one voltage drop but it can have more than one ampacity. They are somewhat related because the cross sectional area of the metal affects them both. But you know all that, right?

First choose a conductor size based on ampacity, and then upsize it to reduce voltage drop if needed. Never downsize a conductor for voltage drop. That's pretty straightforward.

I like that response. Insulation I suppose can have some influence by trapping heat around the conductor, but this heat is not normally even accounted for in basic ampacity or voltage drop calculations.

 

[T.M.Haja Sahib]

This is completely irrelevant. The ampacity of the conductor has no relationship on the voltage drop, as you suggest.

You have simply applied two different load currents to a conductor and found that the two different load currents produce two different voltage drops. This shows an relationship between load current and voltage drop. It is the level of current in the circuit, not the ampacity of the conductor that affects the voltage drop.

But ampacity means maximum allowable current through a conductor at different temperature ratings. The maximum allowable current i.e ampacity is different at different temperature ratings and so the resulting voltage drops, if the conductor is loaded to its ampacity.

 

[david luchini]

But ampacity means maximum allowable current through a conductor at different temperature ratings. The maximum allowable current i.e ampacity is different at different temperature ratings and so the resulting voltage drops.

I'm not sure how else to say it, but the maximum allowable current (ie ampacity) is not relevant to voltage drop. In its simplest form, voltage drop is calcuated as 2R * L * I for single phase circuits and 1.732R * L * I for three phase circuits, where R=resistance of the conductor in ohms/ft, L = the one way length of the circuit, and I = the current in the circuit. You can see that "ampacity" does not enter into the equation.

For the same size conductor, and the same circuit length, voltage drop is then proportional to the current flowing in the circuit. More current flowing means a larger voltage drop, less current flowing means a smaller voltage drop.

 

[T.M.Haja Sahib]

I'm not sure how else to say it, but the maximum allowable current (ie ampacity) is not relevant to voltage drop. In its simplest form, voltage drop is calcuated as 2R * L * I for single phase circuits and 1.732R * L * I for three phase circuits, where R=resistance of the conductor in ohms/ft, L = the one way length of the circuit, and I = the current in the circuit. You can see that "ampacity" does not enter into the equation.

For the same size conductor, and the same circuit length, voltage drop is then proportional to the current flowing in the circuit. More current flowing means a larger voltage drop, less current flowing means a smaller voltage drop.

I do not understand why current in the circuit i.e load current can not be equal to the maximum allowable current i.e ampacity of the conductor forming the circuit.

 

[david luchini]

I do not understand why current in the circuit i.e load current can not be equal to the maximum allowable current i.e ampacity of the conductor forming the circuit.

Because the current in the circuit is dependant on the electrical load that is connected to the circuit.

For instance, a 15HP - 460V motor has a full load current of 21 Amps.

Lets say we feed this motor with #10 THWN which has an ampacity of 35. The current flowing in the circuit is 21Amps, not 35. Now lets say we feed this motor with #6 THWN which has an ampacity of 65. The current flowing in the circuit is STILL 21Amps, not 65.

When figuring the voltage drop on a circuit, you use the actual load current flowing in the circuit, not the maximum current that is allowed to be on the circuit.

 

[T.M.Haja Sahib]

david luchini:
There is really no ban to load a conductor up to its ampacity with a load of suitable size, unless any derating factors of NEC prohibit it.
For example, #1 THHN with 60deg terminations has an allowable ampacity of 110, A load of 110A can be connected to it, provided no derating factors come into play.

 

[ggunn]

I do not understand why current in the circuit i.e load current can not be equal to the maximum allowable current i.e ampacity of the conductor forming the circuit.

The voltage drop in a conductor is normalized to 75 degrees C, and that is equal to the temperature of a conductor from the 75 degree column of the ampacity chart running at full capacity. So what? The voltage drop depends on the current, and to a first worst case approximation we calculate it based on whatever current we want to push through the lines assuming the temperature to be 75 degrees C. Unless that current is enough to raise the conductor temperature to 75 degrees C (or unless it gets there some other way), the ACTUAL voltage drop will be less.

But ampacity is simply a limit to the current we can push through a conductor without destroying its insulation, while the voltage drop is a measured quantity irrespective of insulation. Sure, the "current in the circuit i.e load current can ... be equal to the maximum allowable current i.e ampacity of the conductor forming the circuit", but it doesn't have to be. Voltage drop and ampacity are apples and oranges. They have some things in common (they are fruits, they are round, they are about the same size...), but they are not interdependent.

 

[david luchini]

david luchini:
There is really no ban to load a conductor up to its ampacity with a load of suitable size, unless any derating factors of NEC prohibit it.

And there is no ban on loading a conductor below its allowable ampacity. Again, this is completely irrelevant to voltage drop.

For example, #1 THHN with 60deg terminations has an allowable ampacity of 110, A load of 110A can be connected to it, provided no derating factors come into play.

This is exactly what I said in post #40. Yet you took exception to it in post #41?!?!

#1 THHN with 60deg terminations has an allowable ampacity of 110, #1 THHN with 75 deg terminations has an allowable ampacity of 130 and, #1 THHN with 90 deg terminations has an allowable ampacity of 150.

I could put a 110A load on #1 THHN with 60 deg terminations, 75 deg terminations, or 90 deg terminations. The voltage drop over the same distance would be the SAME for each over these installations, even though the circuit conductors would have a different allowable ampacity in each case.

 

[T.M.Haja Sahib]

The voltage drop of a conductor is proportional to the current flow through it up to the ampacity of the conductor for different temperature ratings.

 

[don_resqcapt19]

The voltage drop of a conductor is proportional to the current flow through it up to the ampacity of the conductor for different temperature ratings.

The ampacity of the conductor has nothing to do with the physics of the voltage drop caused by current flow through that conductor. The code rated ampacity of the conductor limits the conductor to applications where the current flow will not exceed the conductor ampacity. These are two independent issues.

 

[kwired]

And if you load the conductor beyond its determined ampacity the voltage drop continues to increase.

 

[T.M.Haja Sahib]

And if you load the conductor beyond its determined ampacity the voltage drop continues to increase.

in proportion to the current and the conductor insulation being damaged meanwhile.

 

[T.M.Haja Sahib]

The code rated ampacity of the conductor limits the conductor to applications where the current flow will not exceed the conductor ampacity.

The magnitude of voltage drop of a conductor due to a load current can not exceed that voltage drop which is due to the current equal to the ampacity of a conductor for any temperature rating.

 

[david luchini]

The magnitude of voltage drop of a conductor due to a load current can not exceed that voltage drop which is due to the current equal to the ampacity of a conductor for any temperature rating.

#1 THHN with 75 deg terminations has an ampacity of 130. So figure the voltage drop over a given distance for a 130A load current. Now increase the load current to 150A. The voltage drop will increase - beyond the voltage drop equal to the ampacity of 130.

 

[ggunn]

The magnitude of voltage drop of a conductor due to a load current can not exceed that voltage drop which is due to the current equal to the ampacity of a conductor for any temperature rating.

That is simply not true. The voltage drop is due to a physical phenomenon and doesn't care a whit about ampacity, which is a human imposed regulation. The calculation of what that voltage drop will be is more complicated if the current through the conductor is such that the temperature exceeds 75 degrees C, but it still goes up as current increases and one can calculate it. If you have the appropriate resistance data and temperature correction factor, you can figure out the voltage drop in a light bulb filament or a steel girder.

I really am starting to get confused about what point you are trying to make. Cross sectional area of the metal in a conductor and the temperature tolerance of its insulation are independent variables. Ampacity and voltage drop are dependent variables. Both dependent variables are derivative, but both independent variables influence ampacity while only the former influences voltage drop. Neither dependent variable directly influences the other; you can change ampacity without changing voltage drop and vice versa. Are voltage drop and ampacity related? Of course they are, but only because tweaking one of the independent variables (cross sectional are of the metal) changes them both.

 

[don_resqcapt19]

The magnitude of voltage drop of a conductor due to a load current can not exceed that voltage drop which is due to the current equal to the ampacity of a conductor for any temperature rating.

The voltage drop can be any amount of voltage that is less than the circuit voltage. The voltage drop is only based on the current flow and the impedance of the conductor. The ampacity has nothing to do with the votlage drop. Of course you have to look at the code ampacity when installing a system, but that does not have anything to do with the voltage drop on that conductor.

 

[kwired]

Wrap a conductor a few times around a steel core and you have changed the voltage drop but the ampacity has remained the same. Actual current flowing will be lower if nothing else changes but that does not mean the ampacity has changed.

Ampacity as used in Art 310 tables is how much current a conductor can carry without raising the insulation temperature to a level that is damaging to the insulation. I am not sure what the limiting factors are for a bare conductor, I would guess a conductor temperature has been selected that corresponds to when some other deteriorating effect of the conductor may start to happen, but is definitely much higher temperature than encountered with thermoplastic insulation types.

 

[T.M.Haja Sahib]

I really am starting to get confused about what point you are trying to make.

Sorry, for any confusion,ggunn.
Ampacity of a conductor is no way related to the voltage drop of a conductor due to any load current through it except by the proportionality factor per Ohm's law. Is this right?