Voltage Loss Calculation

[scotttt99]

I am installing a subpanel 220v residential service. I used the spreadsheet provided on this website. However, it seems to use a different formula than I have seen before....(2*K*L*I)/D.... The spreadsheet says that L is already 2* the conduit length.... Why is 2* included in the formula. Also, I have used a K of 10.09 for copper before. What is the right formula?

What size wire do I need for a 165 feet conduit? What is the maximum voltage loss allowed by the Code? I would like to have 60 amps of continuous power. Thanks

 

[ron]

Re: Voltage Loss Calculation

#4 copper will get you below 3%.
There is no code maximum voltage drop.

 

[dereckbc]

Re: Voltage Loss Calculation

Originally posted by scotttt99:
Why is 2* included in the formula. Also, I have used a K of 10.09 for copper before. What is the right formula?

2 is to get your loop distance (Phase + Neutral). L is only the one-way distance, so you multiply by 2 to get loop distance.

K is a constant for copper and is dependant on temperature. If ranges 10.8 to 12.9. I personally use 11.1 and I simplify the formula to:

CM = (22.2 * I * D)/VD

Where:

CM = Circular Mills
I = Max Load Current In Amps
D = One-Way Distance (Not Loop)
VD = Voltage Drop

There is no right value for K as long as it is between 10.8 and 12.9. The larger you make K, the larger the cable will be.

The formula provided by Mike's Calculator uses the impedance list in Chapter 9 I believe rather than the standard K format which produces more accurate results.

All design work is measure with a micrometer, mark it with chaulk, cut it with an axe. 310.16 only requires you to use a # 6 AWG minimum if you are using a 60-amp breraker. You are free to use something larger, not smaller

[ March 17, 2004, 09:52 AM: Message edited by: dereckbc ]

 

[scotttt99]

Re: Voltage Loss Calculation

ron & derekbc:

Thanks for all your help. Your quick responses were greatly appreciated. I was alittle concerned that I might not be factoring everything I needed to.

To minimize the voltage drop (and for conservatism) I am going to run #4 to the subpanel. Since #4 can handle it, I am going to use an 80 amp / 2 pole breaker in the main panel although I don't think I will ever use more than the 60 amp originally anticipated.

What size ground do I need to run with the 2 hots and the neutral.... Can that be #6 or #8??

Also, what is the minimum size PVC conduit to fit the 4 cables?

Thanks Again

 

[spsnyder]

Re: Voltage Loss Calculation

You can put 3 No. 4, 1 No. 8 CU GND in a 1-1/4" Sched 80 PVC Conduit if you are using THWN etc. Use No. 6 GND if you are using Al. Hope this helps.