Voltage drop calc

[Mavcomt]

Is the distance always doubled, or is the length only taken once? I've seen examples of both.

 

[Smart $]

Depends on what your distance and length values are measuring...

Simple rule: Account for length (L) of wire the electricity travels from source point to load point... and back.

With a 2-wire circuit, where formulas or methods consider distance to be the length of one wire from source point to load point, that'd be 2xD = L.

In the case of 3?, the current of one conductor is shared by two others as it goes out to or returns from the load. In those cases, 1.732xD = L.

 

[GoldDigger]

Actually, for a balanced three phase load the voltage drop, based on the current in a phase conductor not the nominal line to line load current, is just based on the and one run of wire.
The complications come when you look at percentage voltage drop, since you have to decide which voltage to use as the base.

 

[Smart $]

Actually, for a balanced three phase load the voltage drop, based on the current in a phase conductor not the nominal line to line load current, is just based on the and one run of wire.

Allow me to interpret or rather put in other terms for others.

What you are saying is the voltage measured from end to end of the same conductor is the voltage drop of that wire. True no matter how many wires are involved in the complete circuit.

When you have three such wires, each connected to one line of a 3? system, both source end and load end, the combined voltage drop does sum arithmetically. Say each wire contributed a 1 volt drop. The combined voltage drop would not be 2 or 3, but rather 1.732 because the current is shared among the three wires.

The voltage should always be the line-to-line voltage where using the 1.732 factor. If using the line-to-neutral voltage, one should either use 2xD or just 1xD and figure the neutral separately.

 

[GoldDigger]

What really bugs me is that online VD calculators "automatically put in 1X, 2X, or even 1.732X based on your choice of DC, 120/240, or three phase, but do not tell you what they are doing. And different ones make different choices!

 

[Tony S]

If you have the cables mV/A/M (miliVolt per Amp Metre) it is a simple calculation.

If anyone wants I?ll e-mail a calculations spreadsheet but you will have put the NEC figures in. It is written for BS7671 so the table in it refers to are our volt drop values, the calculations are the same wherever you are.

PM your e-mail address and I will send the spreadsheet. It covers a hell of a lot more than just volt drop.

 

[Carultch]

Is the distance always doubled, or is the length only taken once? I've seen examples of both.

In single phase and DC, you place a "2" factor in the numerator of the formula and multiply it with the 1-way circuit length.

In three phase, there are two ways of doing it:
Method 1: place a sqrt(3) factor in the numerator of the formula, multiply it with the 1-way circuit length, and place the phase-to-phase voltage in the denominator.
Method 2: place the it with the 1-way circuit length in the numerator of the formula with no additional dimensionless constants, and place the phase-to-neutral voltage in the denominator.

The reason for this is that in single phase and DC, the current has to travel from the source, to the load, and back to the source. Twice the distance.

However, in three phase, the other two phases are synchronized to carry the same current as is required for the return path. It is as if there are three out-of-phase loads, which are single phase loads connected phase-to-neutral, and the remaining two loads carries the return path of the first, without requiring any additional current other than that which is equivalent to their own outgoing current.

Mathematically, phase-to-phase voltage = sqrt(3) * phase-to-neutral voltage. So if you like to think in terms of phase-to-phase voltage, you simply multiply by 1 in a fancy way, which is sqrt(3)/sqrt(3). The numerator sqrt(3) remains there, and the denominator sqrt(3) gets absorbed when we replace phase-to-neutral voltage with phase-to-phase voltage.


As an example, consider a 24A load connected with #10 Cu wire, with a 20 ft 1-way length, in each of the following scenarios.

Common data:
I = 24A, r = 1.24 ohm/kft, L = 20 ft

#1: 48 Volts DC
Vd = 2*I*r*L/V/(1000 ft/kft) * 100%
Vd = 2*24*1.24*20/48/1000 * 100%
Vd = 2.48%

#2: 120 Volts single phase, line-to-neutral
Vd = 2*I*r*L/V/(1000 ft/kft) * 100%
Vd = 2*24*1.24*20/120/1000 * 100%
Vd = 0.992%

#3: 240 Volts single phase, line-to-line
Vd = 2*I*r*L/V/(1000 ft/kft) * 100%
Vd = 2*24*1.24*20/240/1000 * 100%
Vd = 0.496%

#4: 120/208 Volts three phase, calculated per the 120V value
Vd = I*r*L/Vpn/(1000 ft/kft) * 100%
Vd = 24*1.24*20/120/1000 * 100%
Vd = 0.496%

#5: 120/208 Volts three phase, calculated per the 208V value
Vd = I*r*L*sqrt(3)/Vpp/(1000 ft/kft) * 100%
Vd = 24*1.24*20*sqrt(3)/208/1000 * 100%
Vd = 0.496%

 

[Carultch]

What really bugs me is that online VD calculators "automatically put in 1X, 2X, or even 1.732X based on your choice of DC, 120/240, or three phase, but do not tell you what they are doing. And different ones make different choices!

What bothers me, is that the online calculators lock you in to a common voltage selection with the drop-down menus. Sometimes I might want a custom DC voltage to be 421V. Or I might want European AC voltage to be 230/400.

 

[Carultch]

If using the line-to-neutral voltage, one should either use 2xD or just 1xD and figure the neutral separately.

There is zero current on the neutral conductor for balanced linear three-phase loads.

When using line-to-neutral voltage, you use 1xD when calculating the effective length of the circuit.

 

[JRW 70]

I'll agree with Tony S. The BS7671 has been in some
ways more useful than the NEC tables for certian calculations
and design decisions over the past few years for me anyway.
Yes it takes on more of a guidance role for acceptability than
the NEC's more prescriptive approach. But it (BS7671) helped
with some abstract thinking that after working to NEC specs.
And now NESC regs. It is good to merge the thoughts together,
After all the electrons obey the same Physics regardless of the
governing athourity.

And I mean no disrespect to the views stated above.

JR

 

[Smart $]

There is zero current on the neutral conductor for balanced linear three-phase loads.

When using line-to-neutral voltage, you use 1xD when calculating the effective length of the circuit.

True, in the sense that when you have a balanced-linear-load MWBC, there is 0 current on the neutral and the load end of the neural is also at 0 volts relative to source end neutral.

However, as with many MWBC's, the three loads are not always balanced, or they are not always on at the same time. Say you have one or two of three identical linear loads on and one off. The Vd is the same as if each load was supplied by 2-wire circuits. Say you have 3 unbalanced loads, the Vd will be different for each load and equal to the line Vd plus the neutral Vd.

 

[Smart $]

What really bugs me is that online VD calculators "automatically put in 1X, 2X, or even 1.732X based on your choice of DC, 120/240, or three phase, but do not tell you what they are doing. And different ones make different choices!

What bothers me, is that the online calculators lock you in to a common voltage selection with the drop-down menus. Sometimes I might want a custom DC voltage to be 421V. Or I might want European AC voltage to be 230/400.

Shouldn't be too hard to make your own in a spreadsheet. Can even easily put it online, such as a Google sheet on Google Drive or an Excel file on OneDrive.

 

[Carultch]

Shouldn't be too hard to make your own in a spreadsheet. Can even easily put it online, such as a Google sheet on Google Drive or an Excel file on OneDrive.

I know that. It is just that I commonly use the online calculators to debug and double check my spreadsheets.

 

[Carultch]

I'll agree with Tony S. The BS7671 has been in some
ways more useful than the NEC tables for certian calculations
and design decisions over the past few years for me anyway.
Yes it takes on more of a guidance role for acceptability than
the NEC's more prescriptive approach. But it (BS7671) helped
with some abstract thinking that after working to NEC specs.
And now NESC regs. It is good to merge the thoughts together,
After all the electrons obey the same Physics regardless of the
governing athourity.

And I mean no disrespect to the views stated above.

JR

Can you provide an example, where BS7671 would be more useful than the NEC?

 

[JRW 70]

To answer your question directly no. I'm doing
calcations where I would be using multiple source
material to see how close we really are. When these
do come up, however I use a combination of
(a) 2012 NESC
(b) 2014 NEC
(c) IEEE 835-1994 (Standard power cable ampacities)
(d) IEEE 399-1997 (System analysis)
(e) BS7671 Seventeenth edition with the addenda

From all this is where we determine the system
safety. And use parts of each to arrive at an
acceptable installation (that still must meet the
internal company policies)

So rather than a single example, we use these
references together. I don't believe any design
was based solely on any of the references. And
because of that I can't give you a fair answer to
your question.

JR

 

[topgone]

Can you provide an example, where BS7671 would be more useful than the NEC?

Not going into specific examples but just to clear things up (defuse the BS versus NEC thing).

BS7671 calcs requires one to gather/refer to cable manufacturers published millivolt per ampere per meter figures. Those figures are based on the single-line to neutral effective resistance of the cable. A straightforward multiplication with the line amperes and the length will give you the voltage drop from source to the end of the line.

In the NEC way, you are going to use the cable constants (of the cable material, 12.5 for copper and 22 for aluminum), cable circular mils, cable length, the line amperes but with the additional factor that basically considers whether you are calculating two-wires (single-phase) voltage drop or calculating on a per phase basis (x 1.732, three-phase balanced).
Long story short, you are given two ways to kill the cat, they say. Cheers.