Say you have a 20 amp circuit 500' long. Fifty foot from your panel you have a receptacle on the same circuit. At what point do you have to start your larger conductor? Does it start from the panel or can it start the box where you make joints to run to your first device?
voltage drop
- Thread starter Jon1980
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- Licensed Electrician
Either one.
JRW 70
Senior Member
- Location
- Eastern Central Missouri
- Occupation
- Testing and Engineer
Sections
Sections
If the smaller wire (which will have the greater VD) is
porportionally used with the larger wire, the sum total
of voltage drop will be fine. This may have to be an
iterative process to get the ratio correct, but your line
of thinking is in the right direction. If your calcs. go well
this will be fine. And the larger conductor can be in the
initial part of the circuit or the final segment.
You will be fine with this if the ratios are correct, and
hopefully the installation goes smoothly.
JR
Sections
If the smaller wire (which will have the greater VD) is
porportionally used with the larger wire, the sum total
of voltage drop will be fine. This may have to be an
iterative process to get the ratio correct, but your line
of thinking is in the right direction. If your calcs. go well
this will be fine. And the larger conductor can be in the
initial part of the circuit or the final segment.
You will be fine with this if the ratios are correct, and
hopefully the installation goes smoothly.
JR
Where a circuit has more than one size conductor, the total voltage drop is determined by the sum of segment voltage drops.
The hard part about calculating yours is the receptacle. A general-purpose receptacle has no "real" load until you plug in something, so you will have to assume a load to do a voltage drop calculation...
The hard part about calculating yours is the receptacle. A general-purpose receptacle has no "real" load until you plug in something, so you will have to assume a load to do a voltage drop calculation...
iceworm
Curmudgeon still using printed IEEE Color Books
- Location
- North of the 65 parallel
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- EE (Field - as little design as possible)
For an ok Vd at 500 ft, and a loaded 20A circuit you are looking at 3c - #4CU (Corwin theory adjustment). Is that what you had in mind?
Heaven forbid, if you put the first 50' in #12, you might have to up size the rest of the run to get the VD at the end down. Don't know - just wondering.
You probably want to run the VD calc for the intended load - then you will know.
ice
Perhaps #12 for the first 50', then ying-yang it with 480V for the remaining 450', also with #12.
JRW 70
Senior Member
- Location
- Eastern Central Missouri
- Occupation
- Testing and Engineer
Worst Case
Worst Case
Use the maximum posible load that could occur
in your circuit for the VD calcs. (or the iterative
process mentioned above of using varying sizes.)
Smart$'s idea of going 120/240 to 480 and back
down again using transformers is a good way to
minimize wire size. That is how utility distribution
works and definately saves on wire size but at
the cost of losses in the transformer.
Choices ..... Choices
JR
Worst Case
Use the maximum posible load that could occur
in your circuit for the VD calcs. (or the iterative
process mentioned above of using varying sizes.)
Smart$'s idea of going 120/240 to 480 and back
down again using transformers is a good way to
minimize wire size. That is how utility distribution
works and definately saves on wire size but at
the cost of losses in the transformer.
Choices ..... Choices
JR
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
If there is one load at the end of a run of mixed size wire, it is indeed only the total resistance that counts (assuming that each segment is at least large enough to carry the amps.)
But any time the loads are distributed along the way, or might be in the future as you expand, then the wire nearest the source will have a greater effect on the VD everywhere along the path.
- Occupation
- Licensed Electrician
Bigger wire near the source or far from, isn't VD dependant on loads?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
VD is dependent on loads. But if the wiring is in a straight line rather than a web, then all of the current from all loads passes through the wires nearest the source, making that the largest contributor (per foot) to the VD.
Let's say you have a 10A load at 500' and another 10A load at the 250' mark.
Assume for argument that the wire resistance ends up being .1 ohm per 250' for the "normal" size.
The voltage drop to midpoint will be 2V (20 x .1), the voltage drop to the far end will be 3V (20 x .1 plus 10 x .1).
If you replace the first segment with .05 ohm wire, the voltage drop to midpoint will be 1V (20 x .05) and to the far end will be 2V (20 x .05 plus 10 x .1).
If you instead replace the distant segment with .05 ohm wire, the drop to the midpoint will be 2V (20 x .1) and to the far end will be 2.5V (20 x .1 plus 10 x .05).
Which would you prefer?
- Occupation
- Licensed Electrician
Full disclosure I'm a wire pulling grunt that can barely do math and uses spell check every time I post.....
Am I on the right path?
Three less two is one.
Two less one is one.
2.5 less 2 is .5. That is less than 1.
Am I on the right path?
- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
I am still in the camp of wanting to know what the load is at each outlet. Say the first one which is at 1/10th of the circuit length only has maybe a half amp load and the further outlet has a load close to circuit capacity - I am likely running same size conductor the entire circuit. If a good chunk of the circuit load is at the first outlet - I will look harder at the point to point VD calculations before deciding what size conductors need to be.
jim.ward.37604
New member
- Location
- Dallas Texas
1 conductor(s) per phase utilizing a 1 AWG Copper conductor will limit the voltage drop to 2.47% or less when supplying 20.0 amps for 500 feet on a 120 volt 1 phase system.
This is from a voltage drop calculator from South Wire found in the App Store. I find it useful if you don't want to figure it long hand. It also displays the various formula inputs as well. Cool app.
Based on conductors in a conduit with a 3% drop 1 phase. Hope this helps some.
Sent from my iPhone using Tapatalk
This is from a voltage drop calculator from South Wire found in the App Store. I find it useful if you don't want to figure it long hand. It also displays the various formula inputs as well. Cool app.
Based on conductors in a conduit with a 3% drop 1 phase. Hope this helps some.
Sent from my iPhone using Tapatalk
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
OP has part of the total load of this circuit taken up at an outlet that is 50 feet from the beginning of the circuit though then a 450 foot run to next outlet. His drop at that last outlet is going to depend on two factors, 1 how much drop exists at the first outlet point, 2 how much drop is in the second segment of the circuit. Each of those factors will depend on the load at each outlet. If first outlet happens to serve a load in the milliamp range you can almost totally disregard it, but if it is a 12 amp load and the far outlet is only an amp or two you could have much smaller conductors in the entire run then if the large load were at the far end. < Add > and have an acceptable voltage drop at each outlet.
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