VD & Feeder Sizes

horsegoer

Senior Member
This seems like overkill. VD usually want to keep under 3% and why 80% of the OCPD?

xptpcrewx

Power System Engineer
This seems like overkill. VD usually want to keep under 3% and why 80% of the OCPD?
Its only overkill if you assume the service and voltage regulation are ideal. 80% load is typical for load flow analysis, but without any context its hard to say.

Npstewart

Senior Member
The feeder for a building is usually permanent whereas the tenants or occupants in a building change. Your voltage drop should ideally take into account future loads.

For example, if you were doing a shell retail building there wouldn’t be any loads initially but then when a tenant moves in the load would be added to which would increase the voltage drop. You would want to make sure the feeder is sized for the future load so 80% would likely be a fairly accurate but conservative estimate.

The 80% isn’t a code requirement, it’s good practice.

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infinity

Moderator
Staff member
The 80% is to allow for possible future expansion otherwise you could just use the calculated load which would be the present load.

Let's say that you 800 feeder has a calculated load of 400 amps, if you use 400 amps in you VD calc then when adding a few hundred amps in the future the VD will increase. 80% is just providing a reasonable buffer for the future unknown.

horsegoer

Senior Member
The feeder for a building is usually permanent whereas the tenants or occupants in a building change. Your voltage drop should ideally take into account future loads.

For example, if you were doing a shell retail building there wouldn’t be any loads initially but then when a tenant moves in the load would be added to which would increase the voltage drop. You would want to make sure the feeder is sized for the future load so 80% would likely be a fairly accurate but conservative estimate.

The 80% isn’t a code requirement, it’s good practice.

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makes complete sense. But then this same engineer has a 400A breaker feeding a 400A panel 600' away with (1) 4" PVC & 4#600's which is wrong

horsegoer

Senior Member
The 80% is to allow for possible future expansion otherwise you could just use the calculated load which would be the present load.

Let's say that you 800 feeder has a calculated load of 400 amps, if you use 400 amps in you VD calc then when adding a few hundred amps in the future the VD will increase. 80% is just providing a reasonable buffer for the future unknown.
Thank so basically your saying if I have 400A breaker feeding 400A panel @ 480v attached VD calculator would be the input parameters.
So when the calculator says enter current at end of run it would be 80% of OCPD as I did?

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petersonra

Senior Member
makes complete sense. But then this same engineer has a 400A breaker feeding a 400A panel 600' away with (1) 4" PVC & 4#600's which is wrong
what is wrong with it?

Npstewart

Senior Member
what is wrong with it?

It’s unlikely #600s could feed a 400A panel 600 feet away. VD would probably be extreme.

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Npstewart

Senior Member
It’s unlikely #600s could feed a 400A panel 600 feet away. VD would probably be extreme.

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Actually if it’s 480 then it’s probably fine. I get about 3.4% VD fully loaded.

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infinity

Moderator
Staff member
Thank so basically your saying if I have 400A breaker feeding 400A panel @ 480v attached VD calculator would be the input parameters.
So when the calculator says enter current at end of run it would be 80% of OCPD as I did?
Basically they want you to calculate with a load of 80% of the feeder size and use conductors large enough to get down to 2% or less VD. For a 1000 amp feeder you would use 800 amps in your calculation.

horsegoer

Senior Member
Actually if it’s 480 then it’s probably fine. I get about 3.4% VD fully loaded.

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Right but engineer note said VD to be under 2%.

horsegoer

Senior Member
Basically they want you to calculate with a load of 80% of the feeder size and use conductors large enough to get down to 2% or less VD. For a 1000 amp feeder you would use 800 amps in your calculation.
I don't 100% follow. . Soto check the feeder sizes already given by the engineer and comply with his note what would you input( or would of he) for a 400A breaker feeding a 400A panel at 480v.?

infinity

Moderator
Staff member
I don't 100% follow. . Soto check the feeder sizes already given by the engineer and comply with his note what would you input( or would of he) for a 400A breaker feeding a 400A panel at 480v.?
400*80%=320 amps

petersonra

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.

horsegoer

Senior Member
400*80%=320 amps
No I understand that. For the VD calculation the calculator said to put the current at the end of the cable run which would be the 80% of the OCPD... so not the load??

infinity

Moderator
Staff member
No I understand that. For the VD calculation the calculator said to put the current at the end of the cable run which would be the 80% of the OCPD... so not the load??
Yes they told you specifically to use 80% not the calculated load.

horsegoer

Senior Member
OK Guess I’m a little lost. If you take 80% of the overcurrent protection device that will result in smaller wire Being needed.

So in my scenario above you would need two sets of 400 MCM?

infinity

Moderator
Staff member
OK Guess I’m a little lost. If you take 80% of the overcurrent protection device that will result in smaller wire Being needed.
Yes, we're talking about calculating voltage drop which requires that you input into the VD formula the amount of current that will be on the feeder. For a 400 amp feeder they're telling you to use (400*80%) =320 amps when you input the current into the VD calculator. Does that make sense? Why don't you give us all of the numbers (length, voltage, feeder ampacity, Cu or Al) and we can run some examples.

horsegoer

Senior Member
Yes, we're talking about calculating voltage drop which requires that you input into the VD formula the amount of current that will be on the feeder. For a 400 amp feeder they're telling you to use (400*80%) =320 amps when you input the current into the VD calculator. Does that make sense? Why don't you give us all of the numbers (length, voltage, feeder ampacity, Cu or Al) and we can run some examples.
400A breaker feeding a 400A MCB panel 600' away. The feeder they have designed is a (1) 4" conduit w/ 4#600 THHN.

1. My point is if your designing with future expansion in mind( good practice but I guess pricey) wouldn't you want to oversize the wire a bit?

2. When designing and electrical system, simple scenario for feeding a panel that will feed loads the steps in my mind, in correct order would be!!??
A. Calculate totals loads panel will feed.
B. Size panel accordingly.
C. Size OCPD that will feed panel according to panel rating.
D. Size wire according to OCPD size.

Npstewart

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

1. My point is if your designing with future expansion in mind( good practice but I guess pricey) wouldn't you want to oversize the wire a bit?

2. When designing and electrical system, simple scenario for feeding a panel that will feed loads the steps in my mind, in correct order would be!!??
A. Calculate totals loads panel will feed.
B. Size panel accordingly.
C. Size OCPD that will feed panel according to panel rating.
D. Size wire according to OCPD size.

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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