Voltage Drop

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horses

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Bradenton, Florida
I have a situation I would like some feed back on. I'm doing an independant living facility & we did some value engineering to reduce the cost & one of the deducts was to use aluminum feeders in lieu of copper. The engineer is requesting one thing & I keep coming up with something different. Below is the calculation formula that I used. This formula is to determine the CM on any run to avoid vd. This formula is out of Mikes book. Let me know what you think. If this is wrong please explain. This is a 800A 3P panel that is 265' away from the MDP. The load on the panel schedule is less than 300A but the engineer says the connected demand load is 640A which is 80% of the rating of the ocpd.

Thanks,


Three Phase Voltage Drop Calculations (2) scenario's





(3P) (AL) (AF) (LOAD) (D)

1- Cm= 1.732 x (21.2 x .054) x 640A X 265? =6,563,722.10

208V X 3% = 6.24 = 1,051,878.5

(3) Runs = 350,626.16 CM

2- Cm= 1.732 x 21.2 x 640A X 265? = 6,227,440.60

208V X 3% = 6.24V = 997,987.27

(3) Runs = 332,662.42 CM
 
Do you really have a 640 amp load 265' from the source? If so you may want to change the design or voltage drop may always be a problem. Alss, I am nots sure where the .054 factor comes from.
 
This is a 800A 3P panel that is 265' away from the MDP. The load on the panel schedule is less than 300A but the engineer says the connected demand load is 640A which is 80% of the rating of the ocpd.
Click to expand...
Using my program the voltage drop using you info is:
400 kcm AL
3.11%
6.48 Volts Line-to-Line
3.74 Volts Line-to-Neutral
500 kcm AL
2.63%
5.47 Volts Line-to-Line
3.16 Volts Line-to-Neutral
It is not necessary to use the 80% value of the breaker. Use the caculated
figure and add some for future additions.

Using the 300 amps and 2 #350 kcm AL the voltage drop is:
2.40%
4.98 Volts Line-to-Line
2.88 Volts Line-to-Neutral
 
The formula for 3 phase voltage drop is 1.732xKxLxA/VD

the number for K=12.9 for copper and 21 for Al

the voltage drop allowed VD=voltagex3%

L =length
A =amperage
 
Voltage Drop

The engineer stated that the energy code requires him to use 2% voltage drop calculations on feeders. We had already install (3) runs of 3" conduit & we were going to use 400AL to feed this dist. panel. He wants (3) runs of 600AL. The NEC does not have any rules on voltage drop they just make a recommendation of no more than 5% on feeder/branch circuits. I used 3% vd in my calculation.
 
The most important thing would be creating an accurate determination of the expected load so that an accurate determination of voltage drop may be calculated. Even then, it will be a best guess. How is a regulation like that ever going to be enforced?
 
horses said:
The engineer stated that the energy code requires him to use 2% voltage drop calculations on feeders. We had already install (3) runs of 3" conduit & we were going to use 400AL to feed this dist. panel. He wants (3) runs of 600AL. The NEC does not have any rules on voltage drop they just make a recommendation of no more than 5% on feeder/branch circuits. I used 3% vd in my calculation.
Click to expand...
I used 500 cu and 300 amps and got the following results:
% VD 2.75%
5.71 Volts Line-to-Line
3.30 Volts Line-to-Neutral

Using 3 runs of 400 AL and 300 amps results:
1.46%
3.03 Volts Line-to-Line
1.75 Volts Line-to-Neutral

Would 1 run of 500 Cu be cheaper than multi runs of AL?
 
The Florida Building Code has an "Energy" section where one of the items is voltage drop.....2% for feeders, 3% for branch circuits. Not sure about other States.
 
Using aluminum conductors
104?F ambient temperature
75?C terminal temperature
metal conduit

with Volts Electrical Design Software and IEEE Exact Formulae for voltage drop computations...

2 sets of 900 kcmils - 1.57%
3 sets of 500 kcmils - 1.70%
4 sets of 300 kcmils - 2.03% (close enough)
 
The Energy Code is probably based on some version of ASHRAE 90.1. Here in Michigan it's the 1999 version.

8. POWER
8.2 Mandatory Provisions
8.2.1 Voltage Drop
8.2.1.1 Feeders. Feeder conductors shall be designed for a maximum voltage drop of 2% at full connected load.
8.2.1.2 Branch Circuits. Branch circuit conductors shall be designed for a maximum voltage drop of 3% at full connected load.

Now I would argue to the Engineer that the Energy Code states "full connected load" and not "connected demand load". I would use the load on the panelboard schedule. An 800A panel for a 300A load seems overkill. If the engineer knows of a specific future load then add that also. But assuming maximum potential future demand is beyond the Energy Code.
 
Full disclosure: I'm an engineer.

The Engineer of Record specifies a particular feeder configuration and you want to change it? Why would you do that? The liability could be huge.

It looks to me like there's something missing. Why is the panel 800A if the load is under 300A? This looks like a design that anticipates a substantial future load increase. What happens in a couple of years when the owner wants to add load and finds out that the contractor didn't put in the specified feeder and that there's no capacity for added load? I wouldn't sign off on a change that reduces the feeder capacity (the engineer would have to sign off on this where I live) unless I had a letter from the owner, approved by my lawyer, in which the owner acknowledged that the capacity for future loads was being eliminated and that he understood that any future additions would require that the panel feeder be replaced, which could require ripping out concrete, extensive downtime, etc.

Martin
 
This panel has a connected load of 300A with only (4) 1P 20A spare spaces.The 800A panel no matter what could only have a load of 640A max to be within code. So to do a worst case scenario I used the 640A in my voltage calc. I also used 3% vd not 2 %. Why the engineer put an 800A dist. panel I dont know. My problem is that I wanted to run (3) runs of 400AL & he wants me to install 600's.
 
horses said:
This panel has a connected load of 300A with only (4) 1P 20A spare spaces.The 800A panel no matter what could only have a load of 640A max to be within code. So to do a worst case scenario I used the 640A in my voltage calc. I also used 3% vd not 2 %. Why the engineer put an 800A dist. panel I dont know. My problem is that I wanted to run (3) runs of 400AL & he wants me to install 600's.
Click to expand...

Did the plans not call for 600 kcm AL? Did you not bid on the 600 kcm?
Why not install what is call for. One problem I see is that a 3"C will only carry
3 600 kcm conductors. What about the neutral? Is that the reason for the push for 400 kcm?
 
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