Voltage Drop Single Phase 120/240 Sub Panels

[AWinston]

I am trying to calculate the voltage drop for various sub-panels in 17 different apartment buildings. Since there no major reactive loads (except the garbage disposal), can I just use the "Alternating-Current Resistance..." column in Table 9 and not worry about the "XL (Reactance)" column? If not, what is a general power factor value I can use when doing voltage drop calculations for dwellings?

 

[Fitzdrew516]

I am trying to calculate the voltage drop for various sub-panels in 17 different apartment buildings. Since there no major reactive loads (except the garbage disposal), can I just use the "Alternating-Current Resistance..." column in Table 9 and not worry about the "XL (Reactance)" column? If not, what is a general power factor value I can use when doing voltage drop calculations for dwellings?

I think you're over complicating things. I've attached some excel voltage drop calculators I made. Not sure if you're using copper or aluminum conductors so I attached both. The files are .xlsx format, but the forum will only let me attach .xls files so I changed the extension of the files to .xls to upload - you will need to change them back to .xlsx before opening them.

- Drew

 

[AWinston]

I think you're over complicating things. I've attached some excel voltage drop calculators I made. Not sure if you're using copper or aluminum conductors so I attached both. The files are .xlsx format, but the forum will only let me attach .xls files so I changed the extension of the files to .xls to upload - you will need to change them back to .xlsx before opening them.

- Drew

It looks like when I use your chart I get twice the voltage drop. I attached the spreadsheet I made when doing my calculations. I am using aluminum conductors.

 

[Fitzdrew516]

It looks like when I use your chart I get twice the voltage drop. I attached the spreadsheet I made when doing my calculations. I am using aluminum conductors.

I checked a couple of your numbers against mine. You are curiously getting half the voltage drop that I am on every one of the runs. I didn't check your formulas, but a guess would be you're forgetting the "2" in the formula, VD= ((2*K*I*D)/Cmil)).

 

[Fitzdrew516]

It looks like when I use your chart I get twice the voltage drop. I attached the spreadsheet I made when doing my calculations. I am using aluminum conductors.

I figured it out. Looks like you're just using the ohms law method, which is fine. However, you need to account for total circuit length. So if your unit is 100 ft. away you need to use 200ft of wire in your formula. This explains why your values are half of mine.

 

[Fitzdrew516]

He calculated the voltage drop at 120V rather than 240V. Therefore, half the voltage drop.

This is another mistake, but I made the same one when I ran the #'s. I was calculating at 120v just because I was looking at his chart and I was still getting twice the VD as he was because the total wire length wasn't considered. In this case he was calculating @ 120V, but forgot to account for twice the wire length so conveniently his calculations were correct. Two wrongs made a right in this particular instance. Not sure if any of that made sense lol. Anyway, yes, 240V should be the voltage it's calculated for sure.

 

[david luchini]

This is another mistake, but I made the same one when I ran the #'s. I was calculating at 120v just because I was looking at his chart and I was still getting twice the VD as he was because the total wire length wasn't considered. In this case he was calculating @ 120V, but forgot to account for twice the wire length so conveniently his calculations were correct. Two wrongs made a right in this particular instance. Not sure if any of that made sense lol. Anyway, yes, 240V should be the voltage it's calculated for sure.

Yes, I read the post too quickly.

 

[AWinston]

Yes, I read the post too quickly.

Thanks guys! Makes total sense!

 

[AWinston]

Yes, I read the post too quickly.

This brought up another question. How much voltage drop can you have from the output of the service provider's transformer to our switchboard. The attached spreadsheet shows 4.7%. Will that be an issue? I know there are only recommendations in the code.

 

[Fitzdrew516]

This brought up another question. How much voltage drop can you have from the output of the service provider's transformer to our switchboard. The attached spreadsheet shows 4.7%. Will that be an issue? I know there are only recommendations in the code.

The NEC does not require a certain percentage of voltage drop, however it does make a suggestion and a lot of energy codes have specific requirements. A general rule of thumb is that feeders should be less than or equal to 2% VD and branch circuits to be no more than 3% VD so that the total on any given circuit can't be more than 5%. If you are doing work in California as your profile suggests, this is what Title 24 requires so you are required to adhere to that standard. Keep in mind, you may have to show voltage drop for every circuit. I just got done with a California job for a 200,000 sq. ft. food manufacturing facility and the plans reviewer made me run voltage drop on every single circuit. It makes for a good time when making revisions to the drawings

- Drew

 

[AWinston]

The NEC does not require a certain percentage of voltage drop, however it does make a suggestion and a lot of energy codes have specific requirements. A general rule of thumb is that feeders should be less than or equal to 2% VD and branch circuits to be no more than 3% VD so that the total on any given circuit can't be more than 5%. If you are doing work in California as your profile suggests, this is what Title 24 requires so you are required to adhere to that standard. Keep in mind, you may have to show voltage drop for every circuit. I just got done with a California job for a 200,000 sq. ft. food manufacturing facility and the plans reviewer made me run voltage drop on every single circuit. It makes for a good time when making revisions to the drawings

- Drew

That sounds awful . I hope they don't make me do that :happysad:. My voltage drop from the pole to the main switchboard in the apartment is 4.7% using 2 sets of #500 conductors (4 conductors total). In order to get the voltage drop down to 2%, I would need an enormous conductor. Using a 1000 AWG conductor, I am still at 2.6%. What are my options?

 

[ActionDave]

That sounds awful . I hope they don't make me do that :happysad:. My voltage drop from the pole to the main switchboard in the apartment is 4.7% using 2 sets of #500 conductors (4 conductors total). In order to get the voltage drop down to 2%, I would need an enormous conductor. Using a 1000 AWG conductor, I am still at 2.6%. What are my options?

You can parallel four or five, or more sets of conductors you just need the right lugs to handle them.

 

[Fitzdrew516]

You can parallel four or five, or more sets of conductors you just need the right lugs to handle them.

True! Also switching to copper will help. Not sure about the cost/benefit analysis on that looks like though; that would be up to you.

- Drew

 

[AWinston]

You can parallel four or five, or more sets of conductors you just need the right lugs to handle them.

I realized in my original calculation,I didn't reduce the total resistance due to the parallel run of conductors. At 2 sets, the voltage drop was 2.4% so I added an additional set.

I was able to do 3 sets of 3#500-#1/0GND and the voltage drop was reduced to 1.6%. Thanks for all the help.

 

[AWinston]

True! Also switching to copper will help. Not sure about the cost/benefit analysis on that looks like though; that would be up to you.

- Drew

Another question: If you have a system (Power from Utility) ---------->(Switchboard)------->(Panels)---------->(Load)
Does the 2% rule apply all the way from (Power from Utility) to the (Panels) or just from the (Switchboard) to the (Panels)?

 

[Fitzdrew516]

Another question: If you have a system (Power from Utility) ---------->(Switchboard)------->(Panels)---------->(Load)
Does the 2% rule apply all the way from (Power from Utility) to the (Panels) or just from the (Switchboard) to the (Panels)?

Technically the energy code mentions feeders and branch circuits and says nothing about service conductors. However, in my opinion it is still good practice to design based upon 2% VD for the service conductors.