Voltage Drop Calculation

[BTE]

Good Morning,
I would like to know when using your free "voltage drop calculator" and
calculating for a single phase 240/120 volt residential electrical service do I
use 240 volts in the "select voltage" cell?
Also, the length of underground run is 600 feet, with a calculated house load of 135 amps, using 300 kcmil stranded Aluminum, with a 5% voltage drop. Is this a safe voltage drop percentage or should I use 3% and up size my conductors to 500 kcmil?

Thanks for your input.
BTE

 

[bob]

The calculation uses 240 volts assuming you can balance the load. What kind of load do you have? A/C size? There is nothing magic about 5% or 3%. You have to make that decision. With 300 kcm I get about 12 volts drop using the 135 amps which leaves you 228 volts. That seems low to me. Is the 135 amps the calculated load? If so, that actual would be less. Parallel 4/0's would leave you with 238 volts. Would that be cheaper that the 500 kcm?

 

[broadgage]

Although code does not specify what voltage drop is allowable, I believe that 5% is excessive for a feeder or service to a house.
Remembering that 5% is only the loss in this cable, and that ANOTHER perhaps 3% could well be dropped in branch circuits.

The starting of an A/C compressor could well draw more than 135 amps, very briefly, and would result in most displeasing lamp flicker.
Even the cycling on/off of a domestic size fridge could produce noticeable lamp flicker.

 

[Smart $]

Although code does not specify what voltage drop is allowable, I believe that 5% is excessive for a feeder or service to a house.
Remembering that 5% is only the loss in this cable, and that ANOTHER perhaps 3% could well be dropped in branch circuits.

...

Exactly. As I recall, the recommeded Vd is 5% maximum from service point to the load: 2% to branch circuit ocp, another 3% to the load. This can be "stretched" as it is not a requirement. Knowing the service point voltage reduces the guesswork.

 

[BTE]

Voltage Drop

Voltage Drop

The calculation uses 240 volts assuming you can balance the load. What kind of load do you have? A/C size? There is nothing magic about 5% or 3%. You have to make that decision. With 300 kcm I get about 12 volts drop using the 135 amps which leaves you 228 volts. That seems low to me. Is the 135 amps the calculated load? If so, that actual would be less. Parallel 4/0's would leave you with 238 volts. Would that be cheaper that the 500 kcm?

Thanks for the call in:
The 135 amps is the calculated load when using Mike Holts "Residential Standard Calculation" sheet. At 3% I get 233 volts when installing 500 MCM AL conductors. Do you think that works better?

 

[Smart $]

Thanks for the call in:
The 135 amps is the calculated load when using Mike Holts "Residential Standard Calculation" sheet. At 3% I get 233 volts when installing 500 MCM AL conductors. Do you think that works better?

Didn't check your cal', but IMO, yes. Part of the stretch I referred to earlier is knowing that the service is typically not fully loaded most of the time. So most of the time, the actual Vd will be less. Just can't overlook those rare instances when the service is maxed.

 

[saratoga1]

Is there any way to get the power company xfmer closer to the house, here, we dig the ditch, install the pvc conduit and the power co. will pull the wire. I did this for my buddies house and put a 400 amp service on it. I pulled the wire from the xfmer to meterbase. You might need to install a small pvc vault half way, here I think they told me no longer than 350' without any relief, I barely made it under their specs to add one. Call the pwoer co. and see what they say, it could be cheaper!! Good Luck

 

[Smart $]

Is there any way to get the power company xfmer closer to the house... Call the pwoer co. and see what they say, it could be cheaper!! ...

I agree. Explore this option before committing.

 

[BTE]

Is there any way to get the power company xfmer closer to the house, here, we dig the ditch, install the pvc conduit and the power co. will pull the wire. I did this for my buddies house and put a 400 amp service on it. I pulled the wire from the xfmer to meterbase. You might need to install a small pvc vault half way, here I think they told me no longer than 350' without any relief, I barely made it under their specs to add one. Call the pwoer co. and see what they say, it could be cheaper!! Good Luck

Already looked into that with the Utility. Owners do not want a meter away from the house they want it on the house. Thanks for your thoughts.

 

[BTE]

Didn't check your cal', but IMO, yes. Part of the stretch I referred to earlier is knowing that the service is typically not fully loaded most of the time. So most of the time, the actual Vd will be less. Just can't overlook those rare instances when the service is maxed.

I have thought about that. I am going to calculate the VD using 180 amps, which is 90% of a 200 amp service. That should give me enough flexibility going forward.

 

[Smart $]

Already looked into that with the Utility. Owners do not want a meter away from the house they want it on the house. Thanks for your thoughts.

What saratoga1 suggests has nothing to do with where the meter is located. IF the poco would "comply", the only difference would be the transformer closer to the house. The xfmr primary operates at a higher voltage. Power in equals power out (roughly). That means the xfmr primary conductors can be substantially smaller. Meter can still be located on the house. The biggest question, if poco "complies", is where to locate the xfmr and whether HO is agreeable to that location.

 

[weressl]

Good Morning,
I would like to know when using your free "voltage drop calculator" and
calculating for a single phase 240/120 volt residential electrical service do I
use 240 volts in the "select voltage" cell?
Also, the length of underground run is 600 feet, with a calculated house load of 135 amps, using 300 kcmil stranded Aluminum, with a 5% voltage drop. Is this a safe voltage drop percentage or should I use 3% and up size my conductors to 500 kcmil?

Thanks for your input.
BTE

Not a direct answer to your question, but some things to think about.

I usually do voltage drop calculations to the average expected current draw, not the full current cababilty of the conductors. If the expected load has great max/min variation, I calculate it for that too. In some cases the source voltage at the transformer can be adjusted with the taps, so I may bump the source voltage up. In areas where the electricity unit cost is high, you may also calculate the cost of the allowed voltage drop and see if it has any impact in making the decision.