Voltage Drop Calculations 1

[jerebear]

This question is in regard to how voltage drop is calclated for a circuit from a distribution panelboard. I know how the calculation is done and that it needs to be no more than 3%. My issue is an electrical engineer that does not understand how to calculate the voltage drop for a circuit. He looks at the individual circuit, in this case a 20 amp 120 volt special purpose circuit 250' from the panelboard the total calculated load on the circuit is to be 8.75 amps. Going upstream from the panelboard there are a couple of distribuition switchboards, a 4160 substation and 4160 switchgear at the main service. In stead of calculating just the 3% voltage drop of the circuit, he is calculating 5% voltage drop from the receptacle all the way back to the main service. Now you have to take into account that he has used this same philosophy to calculate the 2% voltage drop for the feeders to all switchboards and panelboards throughout the building.
That being said, the circuit in question per his calculation is to be 2 - #4 & 1- #8 grd. The hardest one to believe is a 120 volt 20 amp circuit that is even further from the main service and is specified to be 1/0 conductors. Is there some documentation somewhere other than the NEC, that is where he is getting is reasoning, where I can show him that he is calculating this incorrectly?

 

[rbulsara]

Change the engineer!

Rafiq Bulsara

http://www.srengineersct.com

 

[waross]

As per table D3 in the Canadian Electrical Code, you would need #8 AWG.
Our code is usually pretty close to the NEC.
Rafiq's suggestion is good.
Other options:
Remedial reading,
"Just mail my check!!"

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

 

[ProtectionEngineer]

Try this Excel Program.
Alex

 

[jerebear]

The question is not how to calculate voltage drop, I have a voltage drop calculator and I understand the formula for calculating it. I have an engineer of record who has miscalculated and an owner that won't allow a change without documentation to prove the engineer wrong. We have discussed this with other engineers and the electrical inspector, they all agree with us but won't put anything in writing since they are not the engineer of record.

Waross, I agree with your calculation, that is the I have size conductor I have come up with. This is just a misunderstanding of the code by the engineer.

 

[rbulsara]

jerebear:

Since you are not the Owner or the EOR, I would suspect you are the contractor. You pick your battles. Do you want to prove the engineer wrong or get paid for your work? Overkills are difficult to be proven as wrong. You also made your point to the Owner. I would just install what is asked for and for a good measure issue an RFI as to how would you terminate a #1/0 wire on a 20A breaker (even though you may know the answer). Have the EOR provide a detail, come up with a change order!.

Rafiq Bulsara

http://www.srengineersct.com

 

[DRWeig]

That's a good one, Rafiq. Made my day.

Goober Dave

 

[jghrist]

See NEC 210.19(A)(1)FN No. 4 for where the engineer gets his reasoning.

...where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent...

The calculator spreadsheet provided by ProtectionEngineer is valid only for unity power factor.

 

[PHovnanian]

I'd be a bit careful with the FPNs. Some AHJs don't consider them to be a part of the code. Some do and will hold you to them. I don't consider them to be a replacement for good engineering judgment.

From the 2008 NEC (sorry, I don't have my electronic 2011 copy on my laptop yet):

210.19(A)(1) FPN 4: "Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation. See FPN No. 2 of 215.2(A)(3) for voltage drop on feeder conductors."

215.2(A)(3) FPN 2: "Conductors for feeders as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, will provide reasonable efficiency of operation."

Each FPN, one for branch circuits and one for feeders, suggests a 3 percent voltage drop budget. This leaves 2 percent for the other parts of the circuit not addressed by its paragraph. Clearly one cannot budget 3 percent for each and still keep to 5 percent maximum. So some engineering judgment must be made and should be documented in the design calculations.

 

[waross]

I would read that as 5% overall (5% budget) but neither feeders nor branch circuits may exceed 3%.
For example. 1% drop on feeders and 4% drop on branch circuits is within the 5% total budget but is not allowed.

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

 

[davidbeach]

So any engineer worth the paper his license is printed on would know to use the transformer taps to his advantage. Come out of every transformer at 5% high at no load and run your voltage drops from there. 1/0 on a 20A circuit is just cluelessness.

 

[jghrist]

The FPNs are not a code requirement. Note that they say that the voltage drop limits "will provide reasonable efficiency of operation." If you have high voltage drops, you also have high I²R losses. Increasing transformer taps may make the voltage OK at high load, but it will not make the system efficient. Also, make sure that you don't have excessively high voltage near the transformer at light load. The #1/0 for a 20A circuit is probably not the best way to limit the voltage drop. Increasing conductor size on heavily loaded cable nearer the source will probably be more efficient.

You have to look at the whole system before saying that the engineer needs to be replaced. Maybe you really need adequate voltage at the end of that 20A circuit to allow equipment to operate properly and you can't bump the taps up without causing high voltage at light load. If the rest of the system is already in place, it might be most economical to use #1/0 for the 20A circuit instead of costly changes to the rest of the system.

By the way, I'm not the engineer in question, so I have do dog in this hunt.