Increase wire size for every 75'

[Jeffbr]

I am wiring an office space. The plan calls for branch circuit conductors to be increased by one wire size for every 75' of length to meet the provisions of no more than 3% voltage drop on any branch circuit per the FBC.
Is that a requirement or just an architect covering his butt.
Would that mean measuring every conductor in every conduit and cable and making an increase for every 75'? Or would it be for each device over 75' from the panel?
Would I have to use over sized cable at switches and receptacles? Or just home runs?

 

[Dennis Alwon]

hat's an architects desire. I would call the architect to see exactly what he wants. Sounds like you may have to run #10 homeruns depending on the size of the space.

 

[Coppersmith]

The Florida Building Code (FBC) references the NEC. NEC 210.19(A)(1) info note 4 talks about a maximum voltage drop of 3% on branch circuits and 5% including feeders. So it's not just the architects desire. Whether 75' per size change gets you the correct voltage drop, I don't know. I'd run the VD calculations to get the exact wire size required. You certainly can run heavier wire from panel to a midpoint j-box and change to lighter wire for the final leg.

 

[augie47]

The Florida Building Code (FBC) references the NEC. NEC 210.19(A)(1) info note 4 talks about a maximum voltage drop of 3% on branch circuits and 5% including feeders. So it's not just the architects desire. Whether 75' per size change gets you the correct voltage drop, I don't know. I'd run the VD calculations to get the exact wire size required. You certainly can run heavier wire from panel to a midpoint j-box and change to lighter wire for the final leg.

The NEC does not make that mandatory. Does the FBC ?

 

[Strathead]

I am wiring an office space. The plan calls for branch circuit conductors to be increased by one wire size for every 75' of length to meet the provisions of no more than 3% voltage drop on any branch circuit per the FBC.
Is that a requirement or just an architect covering his butt.
Would that mean measuring every conductor in every conduit and cable and making an increase for every 75'? Or would it be for each device over 75' from the panel?
Would I have to use over sized cable at switches and receptacles? Or just home runs?

I am going to be careful here. I live in Florida. I see similar notes all the time. I almost never see it complied with to the letter. As stated it is a code in Florida that VD for branch circuits can not exceed 3%. So that would be what I consider etched in stone.

It is a rare circuit indeed that would exceed 3% at 75 or even 100 feet. If you bid a job or install a job ignoring a requirement like that from an Engineer then they would be within their rights and they would win, to force you to correct it, or provide a credit. If you bid your jobs with it fully accounted for, you will not likely win very many.

Example. One of the best, and I mean that I respect their ability and the thoroughness of their design, has a similar statement. On one job, they issued a change order to relocate the panels 25 feet further from its location. I issued a change request that accordingly increased the wire size of virtually every conductor. The price choked everyone, so of course, I was asked to justify it. I explained and the answer was, "our requirement is more stringent than it needs to be and an increase will not be necessary."

I know that when I do a job that they designed they expect 3% not to be exceeded. They don't worry too much about the rest.

Take all of that or leave it. If you work under someone else's direction I recommend you discuss it with them and follow what they tell you.

 

[Coppersmith]

The NEC does not make that mandatory. Does the FBC ?

I don't know however I was careful in my reply to not state it as a requirement, it's a recommendation (or desire if you will). So not just the architect's desire, the NEC's also.

However here's a quote from Mike Holt:

[FONT=&quot]"Fine Print Notes in the NEC are for informational purposes only and are not enforceable by the inspection authority [90-5(c)]. However, Section 110-3(b) requires equipment to be installed in accordance with the equipment instructions. Therefore, electrical equipment must be installed so that it operates within its voltage rating as specified by the manufacturer."

[/FONT]http://www.mikeholt.com/technnical-voltage-drop-calculations-part-one.php

 

[augie47]

Just out of curiosity, if you are supplying a 20 amp branch circuit feeding multiple convenience outlets how do you calculate the VD ? Be a big difference if the 1st or last outlet had the bigger load. (May be why the 75 ft rule was instigated)

 

[Coppersmith]

I might be wrong in my approach here, but I just calculate the VD to the furthest outlet without taking into account plug and cord connected equipment. If there is a fixed load, I do include that in the calc. Am I doing it wrong?

 

[Coppersmith]

The NEC does not make that mandatory. Does the FBC ?

Upon further research I believe the FBC makes the 2% feeder and 3% branch VD maximums a requirement.

http://www.floridabuilding.org/fbc/commission/FBC_1215/DEC_Statements/Staff_Analysis_DS2015-125.htm

 

[tkb]

This sounds like another case of lazy engineering putting the design on the contractors shoulders.

 

[peter d]

This sounds like another case of lazy engineering putting the design on the contractors shoulders.

:thumbsup::happyyes:

 

[gadfly56]

This sounds like another case of lazy engineering putting the design on the contractors shoulders.

He could have required you to run the calculation for every single wire run instead.

 

[brantmacga]

This sounds like another case of lazy engineering putting the design on the contractors shoulders.

I've seen a lot of guys just putting tables in now for what size wire to use based on the length of the feeder or branch circuit.

Some of them just make a note to upsize every 100'.


Sent from my iPhone using Tapatalk

 

[infinity]

Upon further research I believe the FBC makes the 2% feeder and 3% branch VD maximums a requirement.

http://www.floridabuilding.org/fbc/commission/FBC_1215/DEC_Statements/Staff_Analysis_DS2015-125.htm

What does that even mean, it says "at design load"? Would a receptacle at the end of a 20 amp circuit need to carry 20 amps with the stated VD?

 

[Carultch]

What does that even mean, it says "at design load"? Would a receptacle at the end of a 20 amp circuit need to carry 20 amps with the stated VD?

I could see that you might assume any receptacle within the circuit would carry 20A, while the others are assumed unloaded, and then restrict the circuit to 2% VD.

The chance that all receptacles in the branch circuit will carry the full 20A is not just rare, but impossible, as its 20A breaker will shut off before that happens.

 

[petersonra]

I am wiring an office space. The plan calls for branch circuit conductors to be increased by one wire size for every 75' of length to meet the provisions of no more than 3% voltage drop on any branch circuit per the FBC.
Is that a requirement or just an architect covering his butt.
Would that mean measuring every conductor in every conduit and cable and making an increase for every 75'? Or would it be for each device over 75' from the panel?
Would I have to use over sized cable at switches and receptacles? Or just home runs?

It may turn out that as a rule of thumb this works out so the architect is using it so the EC does not install wiring that is undersized to meet the architect's desired voltage drop.

 

[kwired]

Long run with only a 1 amp load may not have enough voltage drop to justify cost of increasing conductor size.

Intermediate length run with 13 amp load however may need increased two sizes to achieve 3% max voltage drop.

 

[cadpoint]

Job contract spec's around here use this stipulation a lot. Campus's and large Corps's use this all the time. They even go farther in that they will spec. larger wire to first point of branch circuit use. I really don't see what the problem is.

 

[kwired]

Job contract spec's around here use this stipulation a lot. Campus's and large Corps's use this all the time. They even go farther in that they will spec. larger wire to first point of branch circuit use. I really don't see what the problem is.

It is wasteful as a general rule for everything. Like I said in earlier post, if you have a particular circuit that has a fixed low level load, upsizing the conductor may not be necessary, voltage drop might be fine on even a 14 AWG conductor.

Say a dedicated circuit to an individual load, long run but the load is only an amp or two, compared to a lesser length, but still long enough general use circuit that maybe sees significant voltage drop if a 14 amp load should be plugged into it.

On top of that voltage drop is less of an issue if you are supplying say 277 volt lighting vs same lighting at 120 volts.

 

[Carultch]

On top of that voltage drop is less of an issue if you are supplying say 277 volt lighting vs same lighting at 120 volts.

Voltage drop itself is still an issue, and the same percentages are still just as valid a guideline. It is just that you have a greater nominal voltage for comparison with 277V loads, when you express voltage drop as a percentage. For the same power output and same circuit conductors, twice as much nominal voltage means a quarter the voltage drop.