I am needing to calculate wire size for a 350' run 120/208 3phase to a 125 amp sub panel from a 1200a mdp. I do not know the actual load on the sub. Customer wants me to go with"SAFE". Should i calc at 125a? I came up with paralel 3/0 for my feeders that way. Does that sound right?
Calculating VD for 350' to 125 amp subpanel?
- Thread starter Huntxtrm
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
I don't know where you got parallel 3/0 from. I get 3/0 no parallel with a 125 amp load. That was copper. For aluminum it would be 350kcm
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
Save yourself trouble and use this. Click here
http://www.nooutage.com/vdrop.htm This is the one I use. It allows for temp. I was trying to refresh my brain skills a little. never hurts to be able to manualy do a calc, when needed. Trying to get my brain back to par on some calcs.
weressl
Esteemed Member
3 phase, 208,120V, 350', 125FLA, 0.86pf,
3/0 cable(single) will result in 5.79V or 2.78%.
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
You don't derate for temperature in a voltage drop calculation. Table 8 resistance data is for 75 degrees C/167 degrees F
kingpb
Senior Member
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- SE USA as far as you can go
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- Engineer, Registered
Except the 75deg temp in Table 8 is the conductor temp, not related to ambient temp.
You would however limit the current because of ambient temp per 310.16 which keeps the max temp limited in Table 8. Technically, you would need to use the formula in Note 2 of Table 8 if you use 90deg C cable, because resistance does change with temperature.
You would however limit the current because of ambient temp per 310.16 which keeps the max temp limited in Table 8. Technically, you would need to use the formula in Note 2 of Table 8 if you use 90deg C cable, because resistance does change with temperature.
weressl
Esteemed Member
Temperature derating @10*C delta is negligible for VD calculations, current carrying capability is a different kettle of fish.
http://www.electrician2.com/calculators/wireocpd_ver_1_parallel.htm
Ok, I played the Temp numbers back and forth on this calculator. Seemed to not make any difference on VD. But still, what is the formula? That is what i want to know? The long of it, Not just the basic perfect world formula. But don't get to engineery on me, i am but an electrician. LOL Thanks fellas
Ok, I played the Temp numbers back and forth on this calculator. Seemed to not make any difference on VD. But still, what is the formula? That is what i want to know? The long of it, Not just the basic perfect world formula. But don't get to engineery on me, i am but an electrician. LOL Thanks fellas
marti smith
Senior Member
There's a tidbit on voltage drop:
http://www.mikeholt.com/technical.php?id=technicalvoltagedrop1
and note that sometimes D for distance is substituted for L: length. Here, they do get engineery but are very insightful!
http://www.mikeholt.com/technical.php?id=technicalvoltagedrop1
and note that sometimes D for distance is substituted for L: length. Here, they do get engineery but are very insightful!
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