Table 9 Calculation Examples

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Natfuelbilll

Senior Member
The term "circuit length" is used.

Is "circuit length" the one way length of the conduit or is "circuit length" the sum of the length of the line and neutral conductors being used in the solution?

Please don't guess at your answer.

Thanks
 
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Natfuelbilll

Senior Member
The Calculation Example 1 and Calculation Example 2 on NECHB 2014 page 1078 use a the term circuit length. What exactly is "circuit length" - one way or two way?

I have seen in other circuit analysis questions that length is to be the sum of the line and the neutral conductor lengths.
 
luckylerado

luckylerado

Senior Member
Location
Lake of the Ozarks
Natfuelbilll said:
The Calculation Example 1 and Calculation Example 2 on NECHB 2014 page 1078 use a the term circuit length. What exactly is "circuit length" - one way or two way?

I have seen in other circuit analysis questions that length is to be the sum of the line and the neutral conductor lengths.
Click to expand...

It looks like it would be 2 way but then step 5 does not make sense.
 
Dennis Alwon

Dennis Alwon

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Location
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Retired Electrical Contractor
I do not have a handbook but the circuit length is back and forth . I do not know what formula you are using as VD formulas usually use the distance one way
 
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Carultch

Senior Member
Location
Massachusetts
Natfuelbilll said:
The Calculation Example 1 and Calculation Example 2 on NECHB 2014 page 1078 use a the term circuit length. What exactly is "circuit length" - one way or two way?

I have seen in other circuit analysis questions that length is to be the sum of the line and the neutral conductor lengths.
Click to expand...

In Example 1, "circuit length" is the 1-way length of the circuit.

When you calculate percentage voltage drop based on line-to-neutral voltage, your formula is as follows, when you only consider pure resistive effects:
Vdrop = I*r*L/Vpn

Where:
I = per phase current
r = resistance per unit length from the table, divided by 1000 ft to make the units consistent.
L = 1-way length
Vpn = phase-to-neutral voltage

The volts measured phase-to-phase is equal to sqrt(3) * the volts measured phase-to-neutral. Percentage voltage drops are equal, no matter which voltage you base it off of, albeit calculated with a slightly different formula. Therefore, your percentage voltage drop formula is as follows, when you input the phase-to-phase voltage instead. You can derive this by multiplying the previous formula by 1 in a fancy way:
1 = Vpp/Vpp
Vpp = sqrt(3)*Vpn
1 = sqrt(3)*Vpn/Vpp

Vdrop = I*r*L/Vpn

I*r*L/Vpn * 1 = I*r*L/Vpn * (sqrt(3)*Vpn/Vpp)

Cancel Vpn, which gives us the resulting formula:
Vdrop = I*r*L*sqrt(3)/Vpp

The way to think about this is that when the phases are balanced, the remaining two phases carry the return current of the first phase, without any additional current. So each phase wire only "sees" its outgoing current, and the voltage driving this current to flow, is the phase-to-neutral voltage.

This is the principle advantage of three phase power. That the remaining phases carry the return current, without additional current imposed on them. If all phase currents are balanced, the neutral is not needed (in concept). So instead of each phase having a line and a neutral wire, you simply have one line wire for each phase, and the remaining phases' line wires serve the purpose of its neutral. In practice, we often still have the neutral for numerous reasons. So what would take the equivalent total KCMIL to do in single phase, you can do with only 2/3rds of the total KCMIL in 3-phase for the same load and the same voltage drop.
 
Last edited:
C

Carultch

Senior Member
Location
Massachusetts
Carultch said:
In Example 1, "circuit length" is the 1-way length of the circuit.

When you calculate percentage voltage drop based on line-to-neutral voltage, your formula is as follows, when you only consider pure resistive effects:
Vdrop = I*r*L/Vpn

Where:
I = per phase current
r = resistance per unit length from the table, divided by 1000 ft to make the units consistent.
L = 1-way length
Vpn = phase-to-neutral voltage

The volts measured phase-to-phase is equal to sqrt(3) * the volts measured phase-to-neutral. Percentage voltage drops are equal, no matter which voltage you base it off of, albeit calculated with a slightly different formula. Therefore, your percentage voltage drop formula is as follows, when you input the phase-to-phase voltage instead. You can derive this by multiplying the previous formula by 1 in a fancy way:
1 = Vpp/Vpp
Vpp = sqrt(3)*Vpn
1 = sqrt(3)*Vpn/Vpp

Vdrop = I*r*L/Vpn

I*r*L/Vpn * 1 = I*r*L/Vpn * (sqrt(3)*Vpn/Vpp)

Cancel Vpn, which gives us the resulting formula:
Vdrop = I*r*L*sqrt(3)/Vpp

The way to think about this is that when the phases are balanced, the remaining two phases carry the return current of the first phase, without any additional current. So each phase wire only "sees" its outgoing current, and the voltage driving this current to flow, is the phase-to-neutral voltage.

This is the principle advantage of three phase power. That the remaining phases carry the return current, without additional current imposed on them. If all phase currents are balanced, the neutral is not needed (in concept). So instead of each phase having a line and a neutral wire, you simply have one line wire for each phase, and the remaining phases' line wires serve the purpose of its neutral. In practice, we often still have the neutral for numerous reasons. So what would take the equivalent total KCMIL to do in single phase, you can do with only 2/3rds of the total KCMIL in 3-phase for the same load and the same voltage drop.
Click to expand...


If interested in Volts Voltage drop, instead of percent, your formulas are as follows:
Vdrop-pn = I*r*L
Vdrop-pp = I*r*L*sqrt(3)
 
C

Carultch

Senior Member
Location
Massachusetts
Dennis Alwon said:
I do not have a handbook but the circuit length is back and forth . I do not know what formula you are using as VD formulas usually use the distance one way
Click to expand...

That's true for single phase and DC. The round trip length is back and forth, and if you base it off the 1-way length, your formula has a 2 in it to account for how 1-way length converts to round trip length.

"Circuit length" is an ambiguous term in this context. It depends on how the person is intending to use it. On all of my drawings, I specify the estimated 1-way length, as this equals the amount of conduit and EGC length you'd need. The amount of current-carrying conductor you'd need, is an integer multiple of this depending on circuit topology (DC=2, single phase=2, split phase=3, three phase delta=3, three phase wye=4).

%Vdrop = 2*L*r*I/V

In three phase, instead of a round trip length, you have what I like to call an "effective round trip length". On a phase-to-neutral basis, the current in each phase only travels one way, and without trying, the remaining phases carry it back. So your phase-to-neutral effective round trip length, is the 1-way length. On a phase-to-phase basis, the effective round trip length is 1-way length multiplied by sqrt(3) instead of 2.
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Consulting Electrical Engineer - Photovoltaic Systems
Natfuelbilll said:
The term "circuit length" is used.

Is "circuit length" the one way length of the conduit or is "circuit length" the sum of the length of the line and neutral conductors being used in the solution?

Please don't guess at your answer.

Thanks
Click to expand...

The numbers in Table 9 are AC resistance of single conductors. If you do your Vd calculations from first principles (which I recommend over using formulas when you don't understand how they are derived or what their terms mean), i.e., Vd=IR, for a two wire circuit, R= (2)(L/1000')(resistance per 1000') where L is the one way distance in feet. %Vd is just Vd/nominal system voltage X 100%.

It gets a little more complicated for three phase circuits.
 
Last edited:
Dennis Alwon

Dennis Alwon

Moderator
Staff member
Location
Chapel Hill, NC
Occupation
Retired Electrical Contractor
Carultch said:
That's true for single phase and DC. The round trip length is back and forth, and if you base it off the 1-way length, your formula has a 2 in it to account for how 1-way length converts to round trip length.

"Circuit length" is an ambiguous term in this context. It depends on how the person is intending to use it. On all of my drawings, I specify the estimated 1-way length, as this equals the amount of conduit and EGC length you'd need. The amount of current-carrying conductor you'd need, is an integer multiple of this depending on circuit topology (DC=2, single phase=2, split phase=3, three phase delta=3, three phase wye=4).

%Vdrop = 2*L*r*I/V

In three phase, instead of a round trip length, you have what I like to call an "effective round trip length". On a phase-to-neutral basis, the current in each phase only travels one way, and without trying, the remaining phases carry it back. So your phase-to-neutral effective round trip length, is the 1-way length. On a phase-to-phase basis, the effective round trip length is 1-way length multiplied by sqrt(3) instead of 2.
Click to expand...


My only point was to show that depending on the formula it can be either one. I could have been clearer, I guess.
 
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