VD Calc

[Alwayslearningelec]

This MI run below is being converted to EMT w/ RHW 2 hr rated cable. The run is 405' and when I convert pipe and wire I want to make sure I'm accounting correctly for voltage drop. I know I need to determine the actual calculated load which not sure I have enough info. Is there a rating based on below pic I can use for the current for my calc and be safe e.g. the 90A disconnect switch?


Problem is when I put in the parameters into Southwire VD calc I get results below which is larger than the MI cable MI95B cable they designed 3#6. Guess the current at end of cable is important to know and must be less than the 90A I entered???

 

[ptonsparky]

Yes, the motor could only be a 22 amp load. YMMV.

 

[Alwayslearningelec]

Yes, the motor could only be a 22 amp load. YMMV.

Motor?

 

[Alwayslearningelec]

This is the panel schedule for LSLP-H-7S. Guess I could use 70A for current at end of cable? COuld it possibly bv=be even less I need to figure into my voltage drop calculation current?

On a side note how do I know weehther ot put 277v or 480v in my calculation?

 

[ptonsparky]

Doing a rough 3% I come up with #8 being adequate. No PF details & etc

 

[infinity]

So your sizing a feeder for the transformer that feds the panel. The feeder has a 90 amp OCPD at its origin. If you use 90 amps for you calculation the wire will never be too small.

 

[ptonsparky]

Motor?

I used motor load.
Your panel schedule is meaningless.

 

[Alwayslearningelec]

I used motor load.
Your panel schedule is meaningless.

What motor load?

 

[ptonsparky]

So your sizing a feeder for the transformer that feds the panel. The feeder has a 90 amp OCPD at its origin. If you use 90 amps for you calculation the wire will never be too small.

And most likely more than needed.
A design issue.

 

[Alwayslearningelec]

Seems like this is the table to use for MI which makes sense that they used #6. Good for 95A. Not sure they factored in the run is 405' so that may not be good id it were to stay MI cable.

 

[infinity]

The information given is not enough to make an accurate determination of voltage drop.

 

[ptonsparky]

What motor load?

I did not know what your loads were at the time of my post. We still don’t. Make assumptions for the highest load possible on the 90 amp breaker and you’re covered. Obviously the #6 were based on something else. Not VD.

 

[Alwayslearningelec]

 

[Alwayslearningelec]

The information given is not enough to make an accurate determination of voltage drop.

Lets' assume MI cable was staying and they sized the MI for 90A. # 6 would be ok. I don't think #6 would be ok for 405'.

 

[Strathead]

Someone correct me if I am wrong, but since the feeder in question ends in a transformer, I wouldn't worry about voltage drop. Adjust it with the transformer taps if necessary.

 

[Alwayslearningelec]

When doing a true voltage drop calculation

Someone correct me if I am wrong, but since the feeder in question ends in a transformer, I wouldn't worry about voltage drop. Adjust it with the transformer taps if necessary.

Transformer taps?? How does that take care of VD? Not familiar with that.

 

[Strathead]

When doing a true voltage drop calculation


Transformer taps?? How does that take care of VD? Not familiar with that.

Your transformer has taps on the primary that change the output voltage accordingly. For example:

 

[ptonsparky]

Someone correct me if I am wrong, but since the feeder in question ends in a transformer, I wouldn't worry about voltage drop. Adjust it with the transformer taps if necessary.

So you have high voltage when the current is low.
How do you adjust the taps from no load to full load?

 

[augie47]

Am I looking at it wrong ?
The load appears to be a 45 kva transformer.. Evan at full load that would be 67 amps.
Would that not be the maximum number you would use in a VD calculation (other than fire pumps)

 

[Strathead]

So you have high voltage when the current is low.
How do you adjust the taps from no load to full load?

How does the utility? Or any buidling?