Question about voltage drop and downsizing wire.

[LOAD]

Hi guys, I've had my 02 for about a year, most of my experience is with residential new construction so I still am learning a lot as I go.

Anyways, my initial plan was to do a 240v 70 amp outdoor subpanel at the other end of my parents property for an RV hookup and outdoor work plug. We oversized it just in case we wanted to ever add something else down there (Our pumphouse is down there - as well as some other sheds we maybe wanted power in in the future)

It is about a 230~ foot run which to account for voltage drop I ran 3 runs of 2 gauge copper thwn in pvc with a 6 gauge ground.

However, I have come across a bit of an issue when getting the wire from my crawlspace to my panel. I wanted to use SER and splice the run together in a jbox in the crawlspace, but now I'm having trouble finding SER that is the appropriate size. I could just bite the bullet and oversize the crap out of it, but that's a lot of $$$ and I feel is unnecessary. I tried to conduit up to my panel but I realized it would be a real pain to pull through due to the amount of bends I'd have to do. So, I've thought of one more plan but I wanted to get some additional input to see if it would be ok.

I was thinking maybe I can instead of doing 70amps, I would just do 50amps, forget the future power, and downsize the wire in the crawlspace to nm-b 6-3. But I wasn't sure if that would cause a problem with the voltage drop? If anyone could let me know that that would work, or if you have a better idea, that would be fantastic.

 

[augie47]

You can find a voltage drop calculator at mikes home page under free stuff. Use a #6 SE cable instead of NM on the interior run and a 70 amp breaker if your voltage drop calculations work our

 

[LOAD]

You can find a voltage drop calculator at mikes home page under free stuff. Use a #6 SE cable instead of NM on the interior run and a 70 amp breaker if your voltage drop calculations work our

I get about a 4% drop. If I'm remembering correctly it's recommended to be under 3%? Or is that 4% negligible?

I'm mainly just concerned about starting with the smaller interior run would effect the voltage drop on the whole run once it reaches the larger gauge wire... or does that matter? I really don't know lol! I've never done something where I've had to worry about voltage drop too much so I don't have much experience in that regard... I suppose I can't see it being too much of a problem since the smaller run is shorter... but I just want to make sure

 

[winnie]

Lots going on here.

1) 3 runs of #2...is there an EGC?
2) Just how many bends would be in the conduit run? There is a max limit, but you could add a pull point to make the pull easier.
3) SE or MC cable can generally be used at 75C, which gets you more ampacity in 6awg.

-Jon

 

[winnie]

Voltage drop adds in series. You calculate the drop in the short run of #6, and the drop in the longer run of #2, and then add them both together to get the total voltage drop.

Jon

 

[ggunn]

Hi guys, I've had my 02 for about a year, most of my experience is with residential new construction so I still am learning a lot as I go.

Anyways, my initial plan was to do a 240v 70 amp outdoor subpanel at the other end of my parents property for an RV hookup and outdoor work plug. We oversized it just in case we wanted to ever add something else down there (Our pumphouse is down there - as well as some other sheds we maybe wanted power in in the future)

It is about a 230~ foot run which to account for voltage drop I ran 3 runs of 2 gauge copper thwn in pvc with a 6 gauge ground.

However, I have come across a bit of an issue when getting the wire from my crawlspace to my panel. I wanted to use SER and splice the run together in a jbox in the crawlspace, but now I'm having trouble finding SER that is the appropriate size. I could just bite the bullet and oversize the crap out of it, but that's a lot of $$$ and I feel is unnecessary. I tried to conduit up to my panel but I realized it would be a real pain to pull through due to the amount of bends I'd have to do. So, I've thought of one more plan but I wanted to get some additional input to see if it would be ok.

I was thinking maybe I can instead of doing 70amps, I would just do 50amps, forget the future power, and downsize the wire in the crawlspace to nm-b 6-3. But I wasn't sure if that would cause a problem with the voltage drop? If anyone could let me know that that would work, or if you have a better idea, that would be fantastic.

In general, adding a short run of smaller wire to a long run of larger wire doesn't have much effect on the overall voltage drop as long as the ampacity of the smaller wire is sufficient.

 

[texie]

Hi guys, I've had my 02 for about a year, most of my experience is with residential new construction so I still am learning a lot as I go.

Anyways, my initial plan was to do a 240v 70 amp outdoor subpanel at the other end of my parents property for an RV hookup and outdoor work plug. We oversized it just in case we wanted to ever add something else down there (Our pumphouse is down there - as well as some other sheds we maybe wanted power in in the future)

It is about a 230~ foot run which to account for voltage drop I ran 3 runs of 2 gauge copper thwn in pvc with a 6 gauge ground.

However, I have come across a bit of an issue when getting the wire from my crawlspace to my panel. I wanted to use SER and splice the run together in a jbox in the crawlspace, but now I'm having trouble finding SER that is the appropriate size. I could just bite the bullet and oversize the crap out of it, but that's a lot of $$$ and I feel is unnecessary. I tried to conduit up to my panel but I realized it would be a real pain to pull through due to the amount of bends I'd have to do. So, I've thought of one more plan but I wanted to get some additional input to see if it would be ok.

I was thinking maybe I can instead of doing 70amps, I would just do 50amps, forget the future power, and downsize the wire in the crawlspace to nm-b 6-3. But I wasn't sure if that would cause a problem with the voltage drop? If anyone could let me know that that would work, or if you have a better idea, that would be fantastic.

The upsized conductors for voltage drop do not have to go the entire distance. I would just use whatever NM cable you need to match the breaker for the part in the house. Then convert to pipe and wire for the outside long run using upsized conductors for VD. And as winnie said you just then calculate appropriately for VD. I would calculate the drop based on 120 volt rather than 240 as you could have a fairly unbalance load at times.

 

[LOAD]

Lots going on here.

1) 3 runs of #2...is there an EGC?
2) Just how many bends would be in the conduit run? There is a max limit, but you could add a pull point to make the pull easier.
3) SE or MC cable can generally be used at 75C, which gets you more ampacity in 6awg.

-Jon

1 - Yes, I have a #6 ground (overkill, but i had it on hand already) ran in the raceway with them, and a ground rod pounded at the subpanel for my gec
2 - This is more just due to me being lazy and not wanting to build conduit in the crawlspace... it wouldn't be too bad but it's a bit more work than I wanna do myself... rather would just tack up some ser hahaha
3 - I was not aware of this, will look into that, thank you!

 

[LOAD]

In general, adding a short run of smaller wire to a long run of larger wire doesn't have much effect on the overall voltage drop as long as the ampacity of the smaller wire is sufficient.

Awesome! Great to know. Thank you all very much for your help. I have no further questions and everything I was looking for (and more) was answered. I really appreciate it guys!