Voltage Drop for CATV installation

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Richard Franklin

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Hello,
I have been tasked to survey a job performed by an Electrical Contractor which I have a voltage drop concern.

The installation is as follows:
The scope of the work was to install 18 Flat Screen CATV Monitors on a total of 14 floors in a commercial office building. The wiring method was 10/2 MC cable and EMT conduit. There are two branch circuit home runs back to a junction box for each floor. The wiring continues back thru a Riser on each floor to a 3-phase 4-wire 120/208 volt sub-panel designated for the power of this equipment. The branch circuit MC cables have a one-way distance of 825.0 feet for cable #1 and 1025.0 feet for cable #2. Each Monitor has a 120 volt duplex receptacle, using a 2 wire factory cord and connector terminating into a power supply reducing the voltage to 13.5 volts for the CATV Monitor. Every Monitor has a load of 0.5 ampere and each branch circuit has a load of 4.5 ampere.

Using the NEC Article 210.19 (A)(1) FPN No. 4 there is a 3% voltage drop on Branch circuits and 2% voltage drop on Feeders.

The voltage drop calculation formula that I have always been taught to use is as follows:

VD=2KIL divided by CM
VD=2 times K times I times L divided by CM

(OR)

CM=2KIL divided by VD
CM=2 times K times I times L divided by VD

VD-Voltage Drop
K-Constant for the conductor material (12 for Copper and 18 for Aluminum)
I-Amperage of the circuit
L-Length of the circuit
CM-Circular mills of the conductor (10AWG has a 10380 circular mill area)

For Single phase 120 volt or Single phase 277 volt the total distance is measured from the (LINE) voltage source, to the connected (LOAD) of the circuit and measured back to the (NEUTRAL) connection. On multi-phase circuits the distance is measured (ONE-WAY) from the (LINE) to the (LOAD).

With this in mind the allowed 3% voltage drop on a 120 volt circuit is 3.6 volts.

The distance of cable #1 is 825.0 feet times 2 = 1650.0 feet.
The distance of cable #2 is 1025.0 feet times 2 = 2050.0 feet.

Using the above formula for cable #1:
2 times 12 times 4.5 times 1650 : divided by 10380 = 17.17 volt (total drop)

120 volt - 17.17 volt = 102.83 volt (measured at the farthest outlet)

17.17 volt divided by 120 volt = .15 (15% voltage drop)


Using the above formula for cable #2:
2 times 12 times 4.5 times 2050 : divided by 10380 = 21.33 volt (total drop)

120 volt - 21.33 volt = 98.67 volts (measured at the farthest outlet)

21.33 volt divided by 120 volt = .18 (18% voltage drop)

I checked the voltage at each Monitor location using a True RMS meter and measured the following:
Monitor Load "not connected" - a steady 120.7 volts AC
Monitor Load "connected" - a variable of 0.5 to 2.0 volt drop

The Question is:
1) Why did I not see the voltage drop as calculated by the above Voltage
drop Calculation?
2)Could it be because the connected load is for electronic/solid state
equipment using a step down power supply?
3)I forwarded this survey to my Supervisors who in turn forwarded the
information to an Electrical Engineer in charge of the office building. The
reply I received back was that NEC Article 210.19 (A)(1) FPN No. 4 was not
a mandatory requirement. The installation by the contractor for this
wiring is correct and meets code requirements. I see in the Code Forum
that this is correct. I also see that in the Building Energy Code
ANSI/ASHRA/IESNA Standard 90.1-2004 -Chapter 8 that this is a
mandatory requirement for installations using the National Electrical Code.
Does anyone have any information pertaining to this requirement?
4)For the NEC Article 647 - Sensitive Electronic Equipment I do not see a
definition for this Article. Could CATV Monitors installations be required to
meet the requirements of this Article?

Thank you for you input in the above concerns.
 
Q1. Why did I not see the voltage drop as calculated by the above Voltage
drop Calculation?

A1. Because the formula you used is way too liberal with the “K” rating of copper. Try something like 9 to 10 for a value of “K”. I have recorded hundreds of voltage drops and have never witnessed K to be 12.9. Personally I use anywhere from 10 for air conditioned spaces to high as 11.1 for hot locations, and even these are too high.

Q2. Could it be because the connected load is for electronic/solid state
equipment using a step down power supply?

A2. No, that has nothing to do with it. It has to do purely with the actual RMS load current. I would bet the value you used is too high, or if you measured it, your meter is in error.

Q3. I forwarded this survey to my Supervisors who in turn forwarded the
information to an Electrical Engineer in charge of the office building. The
reply I received back was that NEC Article 210.19 (A)(1) FPN No. 4 was not
a mandatory requirement. The installation by the contractor for this
wiring is correct and meets code requirements. I see in the Code Forum
that this is correct. I also see that in the Building Energy Code
ANSI/ASHRA/IESNA Standard 90.1-2004 -Chapter 8 that this is a
mandatory requirement for installations using the National Electrical Code.
Does anyone have any information pertaining to this requirement?

A3 the engineer is correct. Voltage drop is not an issue per NEC. The ANSI standard you refer to is only applied when and if the spec engineer requires it as part of job in most cases. In other words it is a design issue, and nothing to do with code or safety.

Q4. For the NEC Article 647 - Sensitive Electronic Equipment I do not see a
definition for this Article. Could CATV Monitors installations be required to
meet the requirements of this Article?

A4. Nope, not at all. Article 647 is an optional article with strict limitations. In fact you could not use this article for your application due to the requirement of 647.3(2) which states: “The system use is restricted to areas under close supervision by qualified personnel." Office space does not meet that requirement.
 
Last edited:
Richard Franklin said:
Each Monitor has a 120 volt duplex receptacle, using a 2 wire factory cord and connector terminating into a power supply reducing the voltage to 13.5 volts for the CATV Monitor. Every Monitor has a load of 0.5 ampere and each branch circuit has a load of 4.5 ampere.
Click to expand...
At which voltage is this current?
 
Richard Franklin said:
...
The voltage drop calculation formula that I have always been taught to use is as follows:

VD=2KIL divided by CM
...
L-Length of the circuit
...
The distance of cable #1 is 825.0 feet times 2 = 1650.0 feet.
The distance of cable #2 is 1025.0 feet times 2 = 2050.0 feet.
...
The Question is:
1) Why did I not see the voltage drop as calculated by the above Voltage
drop Calculation?
...
Click to expand...
First, there are at least two major flaws in your calculation method. The first is that "L" is one-way length of the circuit. The "2" in the formula is the length multiplier for single-phase loads. You multiplied you one-way length by 2 and multiplied by 2 again in the VD formula, thus obtaining a 2X result.

Second, the VD formula assumes the entire load is at the end of the circuit. Most likely, the monitors are distributed along the run of the circuit. The load decreases after each monitor outlet. With decreased load on the remaining circuit, the voltage drop for the same distance is reduced.

Additionally, the value of "K" varies with the current through the conductor, and is also dependent on ambient temperature and other factors.

The formula you are using is far from accurate and is used to calculate voltage drop under the most extreme usage of the circuit. Thus the calculated voltage drop will likely never occur.

A voltage drop calculaiton is for the purpose of pre-installation assurance. If the circuit is already installed and in use, don't bother with the calculation... simply measure the voltage at the furthest point in the circuit with all the equipment energized and be done with it.
 
Follow-up to Voltage Drop for CATV Installations

Follow-up to Voltage Drop for CATV Installations

Thank you for correcting me on the correct calculation of the Voltage Drop Formula....

It is obvious now that I used the incorrect figures by adding the total distance to the load and back to the source, then multiplying again by two. Also using the correct "K" rating for copper of 9 or 10 bring the calculated voltage drop to a realistic value for the circuit.

Since Voltage Drop is not an issue here per NEC, I still believe that the design and distance is a factor for the operation of these Monitors as notification of building evacuation and life safety.

Thank you again for your knowledge and expertise of this concern...

Richard Franklin
 
Richard, you may just be splitting hairs, and worring about nothing. If all monitors are turned on and working properly: What is the problem?
 
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