# VD & Feeder Sizes

#### horsegoer

##### Senior Member
1) Yes over sizing the feeders for the future possible loads is the responsible way to design. The expense is minimal and the probability of the panel being used to its capacity is fairly probable.

2) Yes, those are correct steps but they are starting points only. You need to make sure those steps match your specific situation, like when you have a 600 foot run. I personally would have requested an additional utility transformer because a 600 foot feeder isn’t really the ideal case.

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Ok so when the engineer says the VD drop was calculated using 2% and 80% of the OCPD( 320A) in the case of 400A breaker and panel that would give you smaller wire when you want to go the other way..larger . It’s not clicking in my brain ..

well actually ultimately I’m trying to mKe sure the feeders are sized properly for the situation as I was told the electrician/estimator is responsible for verifying feeder sizes are correct or not. So to
Do that verification in the example I posted above . 400a ocpd, 400a panel, (1) 4” conduit w/4#600 thhn and keeping in mind VD has to be below 2% for the chosen feeder(I assume) How would you calculate that?

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#### electrofelon

##### Senior Member
Reality is this. The real load is likely to be 50% or so of the calculated load. So really the VD will be a lot less than you might think.
I was going to say the exact same thing, great minds think alike :. I generally figure half of the feeder rating for a VD calc. When you consider overly conservative NEC calcs, equipment nameplate values that aren't accurate, diversity, and the continuous load factor, you will usually be at 2-2.5 times the actual load.

#### infinity

##### Moderator
Staff member
400A breaker feeding a 400A MCB panel 600' away. The feeder they have designed is a (1) 4" conduit w/ 4#600 THHN.

Using the numbers you gave us we can input them and find the minimum conductor size:
Current: 320 amps (400*80%)
Voltage 480 volts
Phase: 3Ø
Length: 600'
Material: Copper
Max VD: 2%

Here is the result that I got from the app on my phone:
Voltage drop will be limited to 1.85% or less with 2 parallel(sets) of 400 KCMIL Copper conductor delivering a total of 320 amps on a 480 volt three phase system for a distance of 600'

#### romex jockey

##### Senior Member
Reality is this. The real load is likely to be 50% or so of the calculated load. So really the VD will be a lot less than you might think.

yup

it does seem odd that we can add up ocpd's in any given 200A panel to be 800-1000 amps , yet are required to address each piece of #12-2 out of it @ full amperage when pushing them through holes in wood frames, etc......~RJ~

#### horsegoer

##### Senior Member

Using the numbers you gave us we can input them and find the minimum conductor size:
Current: 320 amps (400*80%)
Voltage 480 volts
Phase: 3Ø
Length: 600'
Material: Copper
Max VD: 2%

Here is the result that I got from the app on my phone:
Ok I get it but my point is if you put in less current/amp(80%) your result is SMALLER wire. I thought you(or eng) should bewants wore larger so why not put on the full 400a at less than 2% VD??

#### infinity

##### Moderator
Staff member
Ok I get it but my point is if you put in less current/amp(80%) your result is SMALLER wire. I thought you(or eng) should bewants wore larger so why not put on the full 400a at less than 2% VD??
Because whoever designed this said to use only 80% resulting in smaller conductors.

#### infinity

##### Moderator
Staff member
Ok I get it but my point is if you put in less current/amp(80%) your result is SMALLER wire. I thought you(or eng) should bewants wore larger so why not put on the full 400a at less than 2% VD??
Look at it this way forget the voltage drop for a second.

You have a typical 400 amp feeder so you use 600 kcmil conductors (420 amps).

Now you say I want to compensate for voltage drop for 600' run and 80% of the feeder ampacity (400*80%=320 amps). Using the numbers from post #23, at 80% you need 2 sets of 400 kcmil and those parallel conductors are LARGER than the 600 kcmil conductors that you would use without VD compensation not smaller.
Using the numbers you gave us we can input them and find the minimum conductor size:
Current: 320 amps (400*80%)
Voltage 480 volts
Phase: 3Ø
Length: 600'
Material: Copper
Max VD: 2%

Here is the result that I got from the app on my phone:

Voltage drop will be limited to 1.85% or less with 2 parallel(sets) of 400 KCMIL Copper conductor delivering a total of 320 amps on a 480 volt three phase system for a distance of 600'

#### horsegoer

##### Senior Member

Using the numbers you gave us we can input them and find the minimum conductor size:
Current: 320 amps (400*80%)
Voltage 480 volts
Phase: 3Ø
Length: 600'
Material: Copper
Max VD: 2%

Here is the result that I got from the app on my phone:
Ok I get it but my point is if you put in less current/amp(80%) your result is SMALLER wire. I thought you(or eng) should bewants wore larger so why not put on the full 400a at less than 2% VD
Look at it this way forget the voltage drop for a second.

You have a typical 400 amp feeder so you use 600 kcmil conductors (420 amps).

Now you say I want to compensate for voltage drop for 600' run and 80% of the feeder ampacity (400*80%=320 amps). Using the numbers from post #23, at 80% you need 2 sets of 400 kcmil and those parallel conductors are LARGER than the 600 kcmil conductors that you would use without VD compensation not smaller.
ok I follow.
So essentially the engineer provided the wrong conductors to comply his own notes(VD 2% and 80% OCPD) ??!

#### infinity

##### Moderator
Staff member
Ok I get it but my point is if you put in less current/amp(80%) your result is SMALLER wire. I thought you(or eng) should bewants wore larger so why not put on the full 400a at less than 2% VD

If there will never be more than 80% load on the feeder sizing the conductors with VD compensation for 100% ampacity will be a waste of money.

##### Senior Member
In Florida 2% VD on feeders is code, it is one of only a couple modifications made to the NEC. The informational note in the NEC suggests 2% for feeders, 3% for branch so, 25 is actually the norm.

#### horsegoer

##### Senior Member
If there will never be more than 80% load on the feeder sizing the conductors with VD compensation for 100% ampacity will be a waste of money.
Yes but to be sure….the engineer incorrectly sized the 400a feeder if it is to follow the 2% VD and 80% of the OCPD???

#### infinity

##### Moderator
Staff member
Yes but to be sure….the engineer incorrectly sized the 400a feeder if it is to follow the 2% VD and 80% of the OCPD???
That's very possible. Are you required to verify his calculations?

#### horsegoer

##### Senior Member
That's very possible. Are you required to verify his calculations?
Yes, but page 50 uglys book VD calculator gives under 2% VD for 3P 480v 1 set #600 @320 amp @ 600'