voltage drop

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rcc

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me and my partner have a question regarding VD. our field engineer wants to run #6 thhn for a 40amp 208v circuit @ 320'. we did the calculation and it was way over the allowable %. our engineer says that he did the calculation at 20amps, because that will be the max that is used. this is for a UPS and the breaker feeding this circuit will be a 40amp double pole. will this drop affect the UPS? and do you not have to then use a twenty amp breaker?
 

charlie b

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The code rules are not going to answer this question for you. For starters, there is no limit on voltage drop. There are guidelines only. It's a good idea to stay within the guidelines, but the code will not force you to do so.

I agree that under the conditions you described, including a maximum load of 20 amps, a #6 will provide adequate voltage to the load. I calculated a VD of 3%.

You do not have to protect the circuit with a 20 amp breaker. But I am wondering why you have a 40 amp breaker? Is that what the manufacturer of the load equipment calls for? If so, then I next wonder what basis the field engineer is using to declare that the maximum load is 20 amps.

Welcome to the forum.
 

rcc

Member
yes the manufacturer states that it needs protection @40amps. our engineer and i checked the active load on the UPS it is 10.4AMPS at the present time.
 

brian john

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Leesburg, VA
If yu provide low voltage to the input you will deplete the battery in short order and destroy the battery in long before the predicted end of life.
 

markstg

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Big Easy
me and my partner have a question regarding VD. our field engineer wants to run #6 thhn for a 40amp 208v circuit @ 320'. we did the calculation and it was way over the allowable %. our engineer says that he did the calculation at 20amps, because that will be the max that is used. this is for a UPS and the breaker feeding this circuit will be a 40amp double pole. will this drop affect the UPS? and do you not have to then use a twenty amp breaker?

The voltage drop is to be calculated at the ratings of the UPS equipment. If the rated full load current of the UPS is 30A, than that is what the VD is to be calulated at. The VD is not calculated at the Circuit Breaker Rating. The VD is also not calculated at the currrent measure load of the UPS. Sizing all equipment based on the rating of the UPS allows the UPS to operate all the way to its Rating with no problems.
 
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charlie b

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The voltage drop is to be calculated at the ratings of the UPS equipment.
I agree that that is a better way to design the system. I would certainly do it that way myself. But are you suggesting that there is a code article that requires your approach?
 

markstg

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Big Easy

I agree that that is a better way to design the system. I would certainly do it that way myself. But are you suggesting that there is a code article that requires your approach?

No, I am not suggesting that the code requires it. I hope the user of this UPS knows not to load up the machine.
 
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brian john

Senior Member
Location
Leesburg, VA
No, I am not suggesting that the code requires it. I hope the user of this UPS knows not to load up the machine.

WHY, unless you are basing this on the VD they are suggesting. In my expierence unless closely monitored ALL ups end up at full load or more at some point.
 

markstg

Senior Member
Location
Big Easy
WHY, unless you are basing this on the VD they are suggesting. In my expierence unless closely monitored ALL ups end up at full load or more at some point.

My post #6 was for the Design of the feeder to the UPS. My subsequent post was in response to Charie B's comment that there was not a NEC violation to the OP using the the 20A VD calculation (vrs Rated Amp VD Calculation).

As you stated, most UPS's have Load added over time, therefore it is not good design practice to size service to the UPS at a present use Load.

Charlie...have I got this about right?
 

brian john

Senior Member
Location
Leesburg, VA
My post #6 was for the Design of the feeder to the UPS. My subsequent post was in response to Charie B's comment that there was not a NEC violation to the OP using the the 20A VD calculation (vrs Rated Amp VD Calculation).

As you stated, most UPS's have Load added over time, therefore it is not good design practice to size service to the UPS at a present use Load.

Charlie...have I got this about right?


That's what I thought.
 
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