voltage drop 3% include transformer or not?

[binwork91]

Do you consider the voltage drop in transformers?

In my opinion, we need to take the voltage drop in transformers into account.

However, I've heard different opinions where people only consider the voltage drop on feeders.

What is your opinion, and why?

 

[charlie b]

What matters is the voltage at the load. You will get some VD in the transformer and more in the wires. But you can adjust the secondary voltage using the primary taps, is VD is a major concern. That, of course, riske an overvoltage condition when the running load is low.

Short answer to your question: I calculate VD assuming rated secondary voltage at the transformer secondary terminals.

 

[binwork91]

What matters is the voltage at the load. You will get some VD in the transformer and more in the wires. But you can adjust the secondary voltage using the primary taps, is VD is a major concern. That, of course, riske an overvoltage condition when the running load is low.

Short answer to your question: I calculate VD assuming rated secondary voltage at the transformer secondary terminals.

I believe calculating voltage drop is important to ensure equipment operates correctly under all conditions and to avoid running equipment at undervoltage or overvoltage.

I don’t understand why risking an overvoltage condition might be considered acceptable while undervoltage is not.

 

[don_resqcapt19]

Given the difference in the nominal system voltages and the rated voltage of the equipment there is a lot of room for voltage drop. For example motors used on a 480 volt system almost always are rated at 460 volts.

 

[Julius Right]

According to ANSI C84.1 /2016 the Utilization voltage may be 105% maximum for 100% rated and 95% minimum. So, if you may supply 105% and if the current will be 0, the farthest point will get 105% and it is still good.
If the current rises up to Imax, then, at the farthest point the voltage will be still 92%.
From ANSI C84.1 /2016:
Table for Acceptable Industry Standard Nominal Voltage.
Service Entrance Voltage High Range A 105%
Service Entrance Voltage Low Range A 95%
Utilization High Voltage Range A 105%
Utilization Low Voltage Range A 90%

 

[ramsy]

From ANSI C84.1 /20??

Nice information. Thanks for sharing.

Would you be familiar with any adoptions of this standard.

The NEC is only showing C84.1 in informational notes for Art. 100 Voltage Nominal, 694.66, & 705.45(A), but no mandatory adoption.

 

[binwork91]

According to ANSI C84.1 /2016 the Utilization voltage may be 105% maximum for 100% rated and 95% minimum. So, if you may supply 105% and if the current will be 0, the farthest point will get 105% and it is still good.
If the current rises up to Imax, then, at the farthest point the voltage will be still 92%.
From ANSI C84.1 /2016:
Table for Acceptable Industry Standard Nominal Voltage.
Service Entrance Voltage High Range A 105%
Service Entrance Voltage Low Range A 95%
Utilization High Voltage Range A 105%
Utilization Low Voltage Range A 90%

From the picture, it seems like voltage drop on transformer is also considered.

Then what is your opinion about 3% voltage drop, do you consider the voltage drop in transformers?

 

[binwork91]

Given the difference in the nominal system voltages and the rated voltage of the equipment there is a lot of room for voltage drop. For example motors used on a 480 volt system almost always are rated at 460 volts.

If there is a lot of room for voltage drop, why do we need to set a 3% limit?

 

[don_resqcapt19]

If there is a lot of room for voltage drop, why do we need to set a 3% limit?

Even if the equipment is within its operating range, voltage drop is wasted energy and the energy codes set limits on that.

But in general, given the utilization equipment nameplate voltages as compared to the nominal system voltages, I don't see any electrical concern over a voltage drop that does not reduce the voltage at the equipment below the nameplate voltage.

The NEC other than 2 or 3 specific installations does not have voltage drop requirements....only unenforceable Informational Notes.

 

[binwork91]

But in general, given the utilization equipment nameplate voltages as compared to the nominal system voltages, I don't see any electrical concern over a voltage drop that does not reduce the voltage at the equipment below the nameplate voltage.

But the nameplate voltage is a specific number. The voltage you measure or calculate after accounting for voltage drop may not be the same as the nameplate voltage; it could be higher or lower. So, how can you determine the operating voltage range?
At least the 3% guideline helps me ensure that my system will operate correctly.

Another concern for me is the transformer. I have encountered a scenario where 208V is stepped up to 4160V, then stepped down to 480V, and finally stepped down to 208V. If we only consider the feeder voltage drop, it might be within 3%. However, if we also account for the voltage drop across the transformers, it can be quite significant, especially since there are three transformers involved.

 

[don_resqcapt19]

But the nameplate voltage is a specific number. The voltage you measure or calculate after accounting for voltage drop may not be the same as the nameplate voltage; it could be higher or lower. So, how can you determine the operating voltage range?
At least the 3% guideline helps me ensure that my system will operate correctly.

Another concern for me is the transformer. I have encountered a scenario where 208V is stepped up to 4160V, then stepped down to 480V, and finally stepped down to 208V. If we only consider the feeder voltage drop, it might be within 3%. However, if we also account for the voltage drop across the transformers, it can be quite significant, especially since there are three transformers involved.

Most manufacturers specify an operating voltage range as a percentage of the nameplate voltage.

In my opinion the only voltage that matters is the nameplate voltage and its permitted range. The design should try to place the voltage at the equipment terminations within that range.

For a transformer, I start over at the secondary terminals as that is a new system. If the nominal secondary voltage of the transformer is 208, the transformer taps should be set to obtain that voltage.

 

[binwork91]

For a transformer, I start over at the secondary terminals as that is a new system. If the nominal secondary voltage of the transformer is 208, the transformer taps should be set to obtain that voltage.

An interesting point is that the NYC Electrical Code Revision & Interpretation Committee (E.C.R.I.C.) has stated that transformer taps should not be used to adjust voltage drop.

Is there an official code or standard that addresses how to handle voltage drop when there is a transformer in the riser?

 

[PD1972]

The only enforceable code that limits your voltage drop (for a circuit that isn't a fire pump) is the energy code (NYC 2020 ECC C405.9). There is nothing in the NEC that says you need to limit your voltage drop to 3% (not sure where you are getting 3%), but the energy code requires 5% total across your branch circuits/feeders. The rationale behind the voltage drop is to save energy through minimizing wire losses.

As stated by other posters, equipment is rated for a voltage range, so there shouldn't be concerns with under/overvoltage as long as the voltage drop requirement of the energy code is met.

An interesting point is that the NYC Electrical Code Revision & Interpretation Committee (E.C.R.I.C.) has stated that transformer taps should not be used to adjust voltage drop.

I don't think that means what you think it means. All that is trying to say is transformer taps cannot be used to "zero-out" the voltage drop that occurs before/after a transformer.

For example, if the voltage drop on the feeder to the primary of a transformer is calculated to be 2.5%, the taps may not be used to "reset" the voltage drop to 0%. In that specific scenario, all wires on the secondary of the transformer must be sized to limit voltage drop to the 2.5% that is left from the 5%. Note that the voltage drop calculation on the secondary of the transformer would use the nominal voltage of the secondary of the transformer (because transformer taps). However, this specific scenario also assumes the feeder for the transformer is coming from a source that is at 0% voltage drop. Hopefully you see why you don't need to consider the transformer?

 

[binwork91]

The only enforceable code that limits your voltage drop (for a circuit that isn't a fire pump) is the energy code (NYC 2020 ECC C405.9). There is nothing in the NEC that says you need to limit your voltage drop to 3% (not sure where you are getting 3%), but the energy code requires 5% total across your branch circuits/feeders. The rationale behind the voltage drop is to save energy through minimizing wire losses.

The New York City 2011 Electrical Code (Local Law 39 of 2011)
SECTION 215.2 Subsection 215.2(A)(1) - Add two new sentences at the end of the first paragraph, before the Exception, to read as follows: Feeder conductors shall be sized so that the maximum voltage drop at the last overcurrent device does not exceed 3 percent and the total maximum voltage drop of feeder and branch circuit conductors to the last outlet does not exceed 5 percent. The minimum feeder size feeding a dwelling unit shall be 8 AWG copper or 6 AWG aluminum or copper-clad aluminum conductors.


Thank you for the explanation and examples. They are very helpful.

 

[kingpb]

Use the transformer taps to adjust voltage on LV side, you can either adjust to the LV terminals, or adjust to the desired voltage at the bus/load. Either way, you are including the transformer winding VD. I typically adjust taps t get a voltage on the LV side that is close or slightly above the nominal when loaded. Under lightly loaded, or no load conditions just make sure the voltage on the bus is not greater that 110% of rated.

 

[kingpb]

The only enforceable code that limits your voltage drop (for a circuit that isn't a fire pump) is the energy code (NYC 2020 ECC C405.9). There is nothing in the NEC that says you need to limit your voltage drop to 3% (not sure where you are getting 3%), but the energy code requires 5% total across your branch circuits/feeders. The rationale behind the voltage drop is to save energy through minimizing wire losses.

As stated by other posters, equipment is rated for a voltage range, so there shouldn't be concerns with under/overvoltage as long as the voltage drop requirement of the energy code is met.


I don't think that means what you think it means. All that is trying to say is transformer taps cannot be used to "zero-out" the voltage drop that occurs before/after a transformer.

For example, if the voltage drop on the feeder to the primary of a transformer is calculated to be 2.5%, the taps may not be used to "reset" the voltage drop to 0%. In that specific scenario, all wires on the secondary of the transformer must be sized to limit voltage drop to the 2.5% that is left from the 5%. Note that the voltage drop calculation on the secondary of the transformer would use the nominal voltage of the secondary of the transformer (because transformer taps). However, this specific scenario also assumes the feeder for the transformer is coming from a source that is at 0% voltage drop. Hopefully you see why you don't need to consider the transformer?

I'm not following what you are trying to convey. A transformer fed from a feeder, the VD on the HV side of the transformer has a 2.5% drop, so be it. On the LV side transformer terminals, assuming nominal tap, the VD will be 2.5% + the VD through the transformer so lets say 3%. You can easily select the appropriate HV side tap to adjust the voltage back to or near the rated nominal value. Further, if you have an additional VD to the bus from the LV side of the transformer, for example, another 2.5%, you can adjust the taps such that you are back to near nominal.

The taps simply change the turns ratio of the transformer. As an example - given a 13.8KV HV side and a 480V LV side, the turns ratio is HV/LV = 28.75. Standard taps are +2.5%, +5%, nominal, -2.5% and +5%. Typically stated as +/- 2 x 2.5%. Transformers can be ordered with other tap changes and tap changer devices. The taps are typically on the higher voltage side because higher voltage for the same KVA/MVA means less current, so cheaper to build. Thus when you adjust the tap to the -2.5% you are now saying the transformer essentially has a HV side of 13.46KV. The turns ration now becomes HV/LV = 28.05, so when you put the rated 13.8KV on the HV side the voltage on the LV side (no load) is 492V. But the VD from the feeders to the LV terminals is 3%, (in our example) so the voltage is going to measure around 492/1.03 = 477.7V. To account for the 2-3% from the LV terminals to the Bus/load you can use the -5% tap, which means; doing the calc, voltage at the Bus/Load would be around 477.4V, very desirable on a 480V system.

Using taps is very practical but often overlooked outside of industrial facilities. In the design of a power generation facility we typically would buy transformers with a nominal voltage of of 492V or even 504V to account for VD. Then use taps to fine tune the voltage.

Hope this helps some understand this a little better.

 

[wwhitney]

I'm not following what you are trying to convey. A transformer fed from a feeder, the VD on the HV side of the transformer has a 2.5% drop, so be it. On the LV side transformer terminals, assuming nominal tap, the VD will be 2.5% + the VD through the transformer so lets say 3%. You can easily select the appropriate HV side tap to adjust the voltage back to or near the rated nominal value. Further, if you have an additional VD to the bus from the LV side of the transformer, for example, another 2.5%, you can adjust the taps such that you are back to near nominal.

This analysis only applies to a steady state current. As the current on the system can go from 0 to the maximum load, if you use a tap to adjust the voltage at maximum load, the voltage at 0 current will be correspondingly higher. For extreme cases, or with multiple transformers in series, it could be too high.

So I think the point you were responding to was saying that in enforcing the energy code maximum 5% total voltage drop from the service to the load, the voltage drops on each conductor in the chain are added, without respect to the transformers and their tap settings. Which makes sense for an energy code, as any voltage drop on a conductor represents I²*R heating, even if it later compensated for.

Someone earlier commented that 5% seems conservative as the window between allowed service voltage and allowable utilization voltage is wider. But if the 5% represents only the conductors, then the extra width of that voltage window may used for voltage drop in transformers and other components.

As to the 3% voltage drop guideline, that comes from NEC 210.19(A) Informational Note 4 and 215.2(A) Informational Note 2. They each suggest that the branch circuit and the feeder should individually be sized for at most 3% voltage drop, with a total of at most 5% voltage drop between them.

Cheers, Wayne

 

[binwork91]

if you use a tap to adjust the voltage at maximum load, the voltage at 0 current will be correspondingly higher. For extreme cases, or with multiple transformers in series, it could be too high.

This is what I worry about. It seems that many people do not consider the transformer and instead use the tap to adjust the voltage.

As to the 3% voltage drop guideline, that comes from NEC 210.19(A) Informational Note 4 and 215.2(A) Informational Note 2. They each suggest that the branch circuit and the feeder should individually be sized for at most 3% voltage drop, with a total of at most 5% voltage drop between them.

And they are FPN, many people said we don't need to follow it.

The transformer also cause voltage drop is main issue that keeps bothering me. And 3% 5% voltage drop guide line in NEC are FPN.

And the worst issue is that the voltage drop guideline in the NYC local code is not a Fine Print Note (FPN).

 

[wwhitney]

And they are FPN, many people said we don't need to follow it.

Correct, but other codes may incorporate them as mandatory measures.

The transformer also cause voltage drop is main issue that keeps bothering me. And 3% 5% voltage drop guide line in NEC are FPN.

Hence my comments that 5% does not use up the entire practical voltage drop window, leaving room for some voltage drop on some transformers.

And the worst issue is that the voltage drop guideline in the NYC local code is not a Fine Print Note (FPN).

Well there you go, they are mandatory measures for you. But if they refer to "branch circuit" and "feeder" conductors, then that does not include the transformers themselves.

Cheers, Wayne

 

[kingpb]

This analysis only applies to a steady state current. As the current on the system can go from 0 to the maximum load, if you use a tap to adjust the voltage at maximum load, the voltage at 0 current will be correspondingly higher. For extreme cases, or with multiple transformers in series, it could be too high.

So I think the point you were responding to was saying that in enforcing the energy code maximum 5% total voltage drop from the service to the load, the voltage drops on each conductor in the chain are added, without respect to the transformers and their tap settings. Which makes sense for an energy code, as any voltage drop on a conductor represents I²*R heating, even if it later compensated for.

Someone earlier commented that 5% seems conservative as the window between allowed service voltage and allowable utilization voltage is wider. But if the 5% represents only the conductors, then the extra width of that voltage window may used for voltage drop in transformers and other components.

As to the 3% voltage drop guideline, that comes from NEC 210.19(A) Informational Note 4 and 215.2(A) Informational Note 2. They each suggest that the branch circuit and the feeder should individually be sized for at most 3% voltage drop, with a total of at most 5% voltage drop between them.

Cheers, Wayne

Very easy to check. You do have to watch the voltage at no load. For a 480V system you don't want the no load voltage to be greater than 10% over, thus 528V. So, after you adjust the taps you need to confirm you are not above that. Any load added will draw that down. Using taps is not something you do blindly. In reality there are a lot of reasons that go into the recommendations of VD limits in the NEC. It has to do with performance and equipment ratings.

Keeping the VD from Service to load needs to be 5% or less, and no transformer in the circuit, then not much you can do to adjust other than increase cable size/qty. If there is a transformer, the VD of the transformer itself is inconsequential to the VD of the load, you can utilize taps, thus the VD to transformer is 2.5%, and then use taps to adjust, the VD to load is going to be under 5%, your good to go. The VD across the transformer is ignored.