Derating Upsized Voltage Drop Wires?

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KD4315

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Afternoon Everyone,
I'm bidding on a hotel where the the electrical rooms are located on one end of the building. The rooms on the other end of the hotel are 350 feet away. Per the specs and plans home runs that far must be #6 for 20A circuits. Each room has 2 power circuits and 1 lighting circuit. In some home run conduits there are 16 #6 plus ground. Do I have to derate these in addition to the voltage drop upsize? Are there NEC tables for larger wire size derating?
 
KD4315 said:
Afternoon Everyone,
I'm bidding on a hotel where the the electrical rooms are located on one end of the building. The rooms on the other end of the hotel are 350 feet away. Per the specs and plans home runs that far must be #6 for 20A circuits. Each room has 2 power circuits and 1 lighting circuit. In some home run conduits there are 16 #6 plus ground. Do I have to derate these in addition to the voltage drop upsize? Are there NEC tables for larger wire size derating?
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Neither the NEC nor good practice do require you to derate or otherwise adjust the voltage drop in a conductor. A higher ambient temperature around a wire or a higher wire density in a raceway will not significantly affect the voltage drop across that wire.
If ampacity derating (from the ampacity of the larger wire size) would cause the ampacity to be lower than the actual load then the "increased" wire size would have there for ampacity purposes rather than just to limit voltage drop!
 
Voltage drop compensation is not required by the NEC but if you put 16 CCC's in a raceway you must derate. With #6 and 16 CCC's in one raceway you're well within the 20 amp ampacity. {75*50% = 37.5 amps adjusted ampacity.} I would calculate for the voltage drop and then see if you comply with the derating requirements.
 
That's incredibly poor planning on an architect's fault to not have centralized electrical rooms on each floor. I know space is at a premium there but really running all that up size conduit and conductors is a enormous waste of money.

I am surprised you don't have four or more home runs per room... One for lighting, one for the bathroom, one for the microwave, one for the room receptacles, and one for the PTAC.

16 current carrying conductors in a Raceway will require 50% derating. Also, if you can use it, a multiwire branch circuit will reduce voltage drop, and if you have it available, 277 volts to the PTACs will reduce amperage over 208 or 240, and require smaller wire.

If 350 ft is your maximum, the rooms that are closer to the electrical panel can be wired with smaller wire as well, which will save on labor and materials cost
 
JFletcher said:
That's incredibly poor planning on an architect's fault to not have centralized electrical rooms on each floor. I know space is at a premium there but really running all that up size conduit and conductors is a enormous waste of money.

I am surprised you don't have four or more home runs per room... One for lighting, one for the bathroom, one for the microwave, one for the room receptacles, and one for the PTAC.

16 current carrying conductors in a Raceway will require 50% derating. Also, if you can use it, a multiwire branch circuit will reduce voltage drop, and if you have it available, 277 volts to the PTACs will reduce amperage over 208 or 240, and require smaller wire.

If 350 ft is your maximum, the rooms that are closer to the electrical panel can be wired with smaller wire as well, which will save on labor and materials cost
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It really is poor planning. There is a closet in the middle of the floor that we tried to get enlarged and turned into an electrical closet but it can only be a telcom closet due to the 300' LV max length. This is the first hotel we've done where this has been an issue. We've always been able to convince the design team to either have 2 electrical rooms on each side with smaller panels or one centralized electrical room.
 
infinity said:
Voltage drop compensation is not required by the NEC but if you put 16 CCC's in a raceway you must derate. With #6 and 16 CCC's in one raceway you're well within the 20 amp ampacity. {75*50% = 37.5 amps adjusted ampacity.} I would calculate for the voltage drop and then see if you comply with the derating requirements.
Click to expand...

Thank you. This is very helpful. I will do this
 
GoldDigger said:
Neither the NEC nor good practice do require you to derate or otherwise adjust the voltage drop in a conductor. A higher ambient temperature around a wire or a higher wire density in a raceway will not significantly affect the voltage drop across that wire.
If ampacity derating (from the ampacity of the larger wire size) would cause the ampacity to be lower than the actual load then the "increased" wire size would have there for ampacity purposes rather than just to limit voltage drop!
Click to expand...

While this is true, if the plans and specs specifically state that all 20A home runs over 300' shall be #6 THHN then that's what we're going to install.
 
KD4315 said:
I'm bidding on a hotel.. Per the specs and plans..
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1) CA Energy code requires high-efficiency fixed lighting in those rooms. Why #6 to LED's & fan drawing 1 to 4 amps max.
The ability to change the plans, and re-calculate accordingly, is what separates the wheat from the chaff.

2) What if your walk-thru finds room for sub-panels, in maintenance closets, on roof, or adjacent floor?
Contractors can submit bids with code-compliant design changes, if capable of recognizing stupid when they see it.
 
How big are the closets in the middle of the floor, and how many rooms per floor? I used to do data cabling for hotels and I can tell you that the gear takes up very very little space... If I had an IDF on each floor, I could run 25 to 30 rooms worth of coax, internet, Cat6 for TV, and Cat5e for VOIP telephone, all home runs, plus any security cameras or access points into a space on the wall about 9 square feet, or less if I had a rack for the switches and patch panels.
 
Here's a picture of the very first MDF I ever did by myself from start to finish. there are approximately 450 cables in the picture... the cable company mounted their Taps below and to the left of the 110 blocks, and the rack, which I believe was an 84U, was turned 90 degrees then bolted to the floor.

Https://www.flickr.com/photos/32712057@N06/3391641622/in/dateposted-public/

If you backup two pictures, you can see more or less the finished product of the hardwiring of the building. all of that cable fit on less than one half of the short wall in the electrical room
 
KD4315 said:
While this is true, if the plans and specs specifically state that all 20A home runs over 300' shall be #6 THHN then that's what we're going to install.
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I never suggested that you not follow the contractual requirements. I am just saying that if the only reason that you install #6 is to meet that requirement, there is no need to go larger than #6 just because of raceway fill derating. As long as the derated ampacity of the #6 is high enough, there is no reason to go larger than #6.
 
ramsy said:
1) CA Energy code requires high-efficiency fixed lighting in those rooms. Why #6 to LED's & fan drawing 1 to 4 amps max.
The ability to change the plans, and re-calculate accordingly, is what separates the wheat from the chaff.

2) What if your walk-thru finds room for sub-panels, in maintenance closets, on roof, or adjacent floor?
Contractors can submit bids with code-compliant design changes, if capable of recognizing stupid when they see it.
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1' deep by about 6' wide. It's a shallow double door phone closet. There are 8 panels serving each floor. I don't think we'll be able to fit more than 2 in there at most.
 
KD4315 said:
1' deep by about 6' wide. It's a shallow double door phone closet. There are 8 panels serving each floor. I don't think we'll be able to fit more than 2 in there at most.
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Feeding sub-panels in each room seems more efficient, than 8 panels full of #6 hitting 4-11/16" box fill & wire transition to devices.

Main-Breaker subs, in closet or rooms, share even larger feeders between several rooms, so switch-gear can serve each floor with no de-rating.
 
ramsy said:
Feeding sub-panels in each room seems more efficient, than 8 panels full of #6 hitting 4-11/16" box fill & wire transition to devices.

Main-Breaker subs, in closet or rooms, share even larger feeders between several rooms, so switch-gear can serve each floor with no de-rating.
Click to expand...

All but the far end rooms on the floor should be under 300 feet. Dunno how many floors or rooms the original poster has to wire with #8, but I can't imagine it would be that many... A good 70% or more of the rooms will be under the 300-foot/ #8 specification, and if there is a similar rule for 15 amp circuits, it's probably #10.
 
Anytime wire is upsized for voltage drop on circuits 15 thru 30 amps it turns the job into a rats nest unless its for parking lot pole lights or something like that.

As far as for termination purposes, I agree with the others that feel a Subpanel installed closer to the load should be installed.

The OP probably doesn't have the luxury of bidding it that way at this time though. A subpanel closer to the load may be some's suggestion but not what the engineer drew up.

Always cleaner to upsize a feeder for voltage drop to a subpanel and install standard size conductors for the branch circuits.

Bad design in the office always ends up being a bad install out in the field.

JAP>
 
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