Voltage Drop % Definition

[lalver1]

Hi,
I have a question about how voltage drop %, per the National Electrical Code (NEC), should be calculated for the attached circuit. Would A or B be the correct calculation? My main doubt is if the voltage drop due to the feeder cable should be divided by the voltage of the secondary side of the transformer (480v) or if it should be divided by the voltage of the utility source voltage (240v). I'm not clear which one of the two the NEC considers the 'source' voltage.

A) [ul]
[li]Feeder % voltage drop = 5.4v / 480v = 1.13%[/li]
[li]Branch % voltage drop = 0.4v / 120v = 0.33%[/li]
[li]Total % voltage drop = 1.13% + 0.33% = 1.46%[/li]
[/ul]

B)[ul]
[li]Feeder % voltage drop = 5.4v / 240v = 2.25%[/li]
[li]Branch % voltage drop = 0.4v / 120v = 0.33%[/li]
[li]Total % voltage drop = 2.25% + 0.33% = 2.58%[/li]
[/ul]

Thanks for your help!

 

[waross]

The Canadian code makes it clear.
(a) 3% in a feeder or branch circuit; and
(b) 5% from the supply side of the consumer’s service (or equivalent) to the point of utilization.
5% on a 120 Volt circuit is 6 Volts.
The calculated voltage at the load must not be less than 114 Volts.
The rigourous solution would have the voltage drop in the branch circuit as 114 Volts / 97% x 3% = .353 Volts maximum voltage drop in the branch circuit.
Some inspectors may allow the simpler solution of 120 Volts x 3% = 3.6 Volts maximum voltage drop in the branch circuit.
Whichever method is used it does not change the 5% overall allowable voltage drop.
Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

Solution A would appear to be correct.

 

[lalver1]

Thanks for the input so far.
It seems that the Canadian code makes it clear, and according to the Canadian code it seems that the correct calculation would be B) since the voltage in the denominator of the voltage drop % is the voltage at the point of utilization.
However, it seems that for the US code, solution A could also would match the NEC's definition of voltage drop %, unfortunately I cannot find any reference in the code to verify this claim. To my knowledge, the NEC code only mentions 3% and 5% voltage drop for feeder and branch circuits, and this is clear, but there is no mention of what the denominator is to calculate this % voltage drop.

 

[LionelHutz]

5.4V is the voltage drop on wires carrying 480V so it only makes sense to use 480V in a percentage calculation. Doesn't matter what any code says, basic math still applies here.

Calculate your voltages through the system to the 120V point and see which answer that matches.

 

[waross]

Lionel is 100% correct.
However, there is a third way to calculate:
480 Volts with 5.4V drop = 474.6 Volts.
474.6 V x 120/480 V = 118.65 Volts
118.65 V - .4 v = 118.25 Volts
120 V / (120 V - 118.25 V) = 1.46%
The voltage drop in Volts is dependent on the current.
You don't have to re-calculate the current at the reduced voltage.
In the Canadian code this rule applies.
"When calculating currents that will result from loads, expressed in watts or volt amperes, to be supplied by a
low-voltage ac system, the voltage divisors to be used shall be 120, 208, 240, 277, 347, 416, 480, or 600 as
applicable."
There is so very little difference that in most real world installations the calculated voltage drop will not be such as to fall in between the two calculations.
Also, the simple calculation is slightly more conservative.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[FacEngrPE]

the a, b, and Canadian solution don't include the transformer regulation droop. Not enough information in the problem statement to guess what it is. Are we making the assumption that these are "Ideal" Transformers?
Fred

 

[waross]

I was under the impression that many transformers were slightly overwound so that at rated load, the voltage dropped to the nominal voltage.
That is why I did not include a factor for the transformers.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[lalver1]

Correct, I made the assumption that these are ideal transformers. The loads are fairly light and composed of electronic devices mostly (cameras, radar detectors, and network switches) so I'm thinking that droop may not be an issue and therefore can be left out of the calculation.

 

[cuky2000]

In contrast with other international codes, the NEC only provides a suggested total voltage drop to be not more than 5%. _{(Exceeding this value is not recommended but is no technical a code violation)}. In the US the ANSI C84.1 established in 1954, is the official utilization voltage regulation that includes not only the maximum but also the minimum allowable operational voltage. Most recently, the CBMA/ITIC provides a guide for utilization voltage for computer and electronic equipment.

Regarding your VD calculation, the most important aspect is to provide a utilization voltage at the load within the window recommended by the manufacturer, usually, ±5% of the nameplate rated voltage. It should be noted that at the transformer output, the voltage can be regulated with the LTC to maintain the source voltage in the optima operating range.

 

[FacEngrPE]

Cuky - you stated what I was hinting at - utilization voltage limits are the only thing the load cares about. So the as designed voltage drop needs to limit the total drop to the utilization window, regardless of what delivers the power to the point of utilization. And of course depending on transformer selection and sizing with respect to everything else, they can help or hurt.
The circuit drawn is not vastly different than "vertical distribution" circuits I have seen here previously.
Fred

 

[waross]

It may be difficult to find 240:480 volt and 480:120/240 volt transformers with OLTCs
I would guesstimate those transformers as 25 KVA or less.
BTW, in Canada the maximum voltage drop is mandatory.
There are exceptions.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[cuky2000]

Hi Waross,

I'm sure that you know that it is commonly available for small distribution transformer de-energized LTC usually with 2 to 4 –2 ½% manual taps per NEMA Std. The fix tap changers compensate for small voltage variations along with the distribution system and could be an option to minimize excessive cable sizing.

 

[itsmoked]

The loads are fairly light and composed of electronic devices mostly (cameras, radar detectors, and network switches) so I'm thinking that droop may not be an issue and therefore can be left out of the calculation.

Beware! If the loads are all rectifier front-ends without PFC you could be very surprised about a bunch of 'light loads' not working due to the voltage at the utilization point, especially with the resulting harmonic distortion running thru smaller transformers.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

For variable loads the voltage must be kept within a useable range.
If there is too great a voltage drop at full load, correcting by raising the transformer voltage may drive the voltage too high at light loads.
The utility allows the voltage to vary quite a bit.
Typically the voltage goes high during times of low consumption.
If you try to compensate for undersized conductors by raising the transformer voltage then when your light usage coincides with light usage on the grid, your circuits will be over voltage.
I can't see an inspector accepting a transformer in place of properly sized conductors.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

I'm sure that you know that it is commonly available for small distribution transformer de-energized LTC usually with 2 to 4 –2 ½% manual taps per NEMA Std.

Actually I did know that.

(b) 5% from the supply side of the consumer’s service (or equivalent) to the point of utilization.

By the time an installation is large enough to have primary metering, it is exempt from the voltage drop rule.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[lalver1]

Thanks for everyone's input so far. A few comments from what I've gathered:

Cuky - you stated what I was hinting at - utilization voltage limits are the only thing the load cares about. So the as designed voltage drop needs to limit the total drop to the utilization window, regardless of what delivers the power to the point of utilization.

So it seems that solution A) would be the correct calculation according to this argument. It makes sense to me because the argument is saying something like: "the transformer is providing 480v, 5.4v are being dropped due to a long conductor run, leaving 474.6v available for the load. The voltage drop due to the conductors is only 1.125%, ensuring that the load equipment will operate as expected." Of course, the voltage drop at the branch still holds, I just wanted to focus on the feeder voltage drop % definition.

It may be difficult to find 240:480 volt and 480:120/240 volt transformers with OLTCs
I would guesstimate those transformers as 25 KVA or less.

Correct, the transformers are 25kVA or less.

If there is too great a voltage drop at full load, correcting by raising the transformer voltage may drive the voltage too high at light loads.
The utility allows the voltage to vary quite a bit.
Typically the voltage goes high during times of low consumption.
If you try to compensate for undersized conductors by raising the transformer voltage then when your light usage coincides with light usage on the grid, your circuits will be over voltage.

The voltage drops due to the conductors (5.4v and 0.4v) are calculated assuming a full load. So it seems that if not all the load components are connected, the current at the load will be lower, and therefore the conductor voltage drops will be lower. It seems that this should be ok. Finally, yes, the reason for the transformers is to scale down the load current in the feeder conductors because these are fairly long conductors (about 4000 ft, 2000ft each direction) and without this scaling, the conductor sizes get too large.

Thanks again everyone.

 

[waross]

I know the feeling.
I did an installation where we were running #2 AWG for a long way.
The load was very small.
I advised the customer that if the load was ever to be increased, that then transforming up to 480 Volts would allow a 400% increase in the load.
A few years later they increased the load.
Instead of installing a pair of transformers they had a new service installed at many times more cost.
Why didn't they use transformers?
Because they didn't listen.
That's what customers do. (Or don't do.)

Bill
--------------------
"Why not the best?"
Jimmy Carter