Multiple wires on long run

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cal-gen

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I am having a tough time on deciphering what sub to choose, based on proposed work. We are building a 12,000 ft covered storage building (open sides), it is 500 feet from a detached garage and 550 ft from the main dwelling/main panel (200A single phase). The covered building is having a 50A sub panel put in for led lighting.
I want a new run from main panel to new building with 2/0 alum. All but one of the subs claim they can come from the garage panel with 2/0 alum and leave the #6awg feeder wire from the main panel in place.
The owners like that option because of a substantial savings in not having to trench through concrete patios, driveway and landscaping. I don't like it because of the feeder wire size to the garage sub panel, it is my belief that it the garage is used the feeder wires from main panel also need to be resized.
 
Sub panel at building is going to be 50A., calculated load for lighting and one 20A receptacle is 35A. It is an open "building" just roof and trusses with no side walls, led lighting is going to be used to supplement natural light 4-5 hrs in evenings during winter months.
 
At 35 amps there will be a significant drop (13%) but if it is led lighting that won't hurt but the amount of light might be a little less. It also depends on what is being used at the receptacle.
 
cal-gen said:
Sub panel at building is going to be 50A., calculated load for lighting and one 20A receptacle is 35A. It is an open "building" just roof and trusses with no side walls, led lighting is going to be used to supplement natural light 4-5 hrs in evenings during winter months.
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So one option is to run the new 50 amp feeder (35 amp calculated load) from the garage panel that is currently fed with #6 conductors?
 
Not specified is the length of the run from main to garage on the #6. If the run is short then it won't contribute much voltage drop.

Jon
 
synchro said:
Also not specified is how much load the garage is already placing on its #6 feeder from the main panel.
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Yes that the key missing component to find bith the voltage drop and whether or not the garage panel will be overloaded. For a difference of 50' I see no reason not to run this new feeder back to the 200 amp dwelling panel.
 
Trying to respond to all of the responses.... the garage sub panel is approx 70' from main panel. The subs wanting to feed the new building from the garage are trying to appease the owners and not have to come from the main panel at house which will involve cutting/demo/trenching through lots of hard and soft landscaping/flatwork surrounding the main house and to put it back to original condition- hard to do with concrete matching etc, ect.

The feeder from main panel to garage is #6 and is 70"
The run to the new building is 530' if done as a new stand alone feeder directly from main panel
The run to the new building if done from the garage sub panel is 480' (which is fed with 70" of #6 from main panel)
I am telling the owners that you cant just run the new building feeders from the garage panel because the garage sub panel is only fed by #6 copper, for a 550' run with a 50A panel with (35A calculated) you should be using at least 4/0 alum legs in the run.
The question I am worried about is I have stuck my neck out with the owners and insisted that if a new run from the main panel directly to the new building is not installed... then if the garage is used then the it makes no sense to run from the garage sub panel to the new building with 4/0 alum and not change the 70'of #6 feeder going back to main panel.

I am saying it is wrong for two reasons,
1. you cant just run new feeders from the sub panel in garage to the new building without changing the feeders supplying that sub panel (this is based on if you need a 50A panel at the new building - because of distance/voltage drop it requires min of 4/0 alum legs)
2. The loads on the garage also need to be involved and those calcs alone would require the original feeders from house main panel to garage to be changed.

If calcs show on a 550' foot run supplying 50A, legs should be 4/0, then why would it be ok to propose leaving 70' #6 copper in the first 70' of the run (main panel to garage sub panel)

Even though I believe these companies have employees that need to go back to training, I am starting to doubt my self because of the amount of subs that are claiming it is ok to do this - it is shocking... no pun intended!
 
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If you have 240V at the main panel, random voltage drop calculator tells me that 35A over 70' of #6 copper will drop 1.94V. Then the additional 480' of 4/0 Al will drop 2.6V at 35A, for a total drop of 4.5V. That's under 2% of 240V.

Unspecified is what the other load at the garage will be. If it's 30A, so that the #6 is being used at its full 75C ampacity of 65A, then the first leg drop will go up to 3.59V. So the total drop will go up to 5.2V, or 2.2%.

Cheers, Wayne
 
It may or may not be ok. Voltage drop is additive, so the 70' for #6 will have some voltage drop, and then the 480' of 4/0 will have some voltage drop, and you add them up.

70' of #6 _copper_ with 35A of load at 240V gives a voltage drop of 1%
480' of #4/0 aluminum with 35A of load at 240V gives a voltage drop of 1.9%

So the combination of the two will give you less than 3% which is probably ok.

If the existing feed to the garage is aluminium then that would push you up to more than 3% but less than 4%, probably still ok but less than ideal.

The real question is 'what is the calculated load at the garage? Can you add an additional 35A of load?

But perhaps given the existing feed to the garage, it might be possible to increase the conductors to the garage without so much ripping up of landscape. Change the garage to a 100A feeder using 4/0 and then continue the long feeder?

-Jon
 
wwhitney said:
random voltage drop calculator
Click to expand...
Random voltage drop calculator agrees with Southwire calculator for copper but not for aluminum. So perhaps my numbers are off.

Southwire calculator (for 1.0 power factor) gives 3.5V drop for the aluminum leg.

Cheers, Wayne
 
Well depending on the cost I can see doing what the contractors are suggesting. If you use VD=I*R

70 * 2 * 0.491 / 1000 * 35 = 2.4 volt dropped (main panel to existing sub panel #6 cu)

480 * 2 * 0.159 / 1000 * 35 = 5.34 volt dropped (existing sub panel to new sub panel using #2/0 al)

7.74 / 240 = 3.23% VD total

This will change if there is load added from the existing garage.
 
So grateful for everyone's input. I was getting nervous that I was possibly being unfair to the subs and/or backing the owners into an expense they didn't need. Having more input from neutral parties (that also know the trade) is priceless. Been an avid reader of the forum for years and just decided to join. Glad to be part of the group.

I will be directing the subs to provide proposals to pull new #2 copper to the garage sub panel The existing conduit from main panel to garage is 1.25 so that will be a huge savings for the owners. Then run 4/0 alum in a trench to the new building.... I get the wire sizes that I can sleep at night with and the owners do not have to pay for trenching thru all of the surrounding hardscaping at the dwelling.... a win win for everyone.

Cheers everyone.
 
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