ohm's law sucks.

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Fulthrotl

~~Please excuse the mess. Sig under construction~~
missed a 120 volt receptacle on a bid.

30' up in the air, feeding a wifi repeater.
7 amp load.

490' from the nearest panel.
#6 copper gives 2.81% voltage drop.

anyone got any magic math, or did i just
eat 1,000' of #6 thhn for one outlet?
 

GUNNING

Senior Member
PROOF

PROOF

7 AMPS?
Take 2 500 foot rolls of #10 and see what the actual voltage drop is. Is it more than 5%?:roll:
Is it functional voltage? ie over 115 from 120?
Might need a change order for a transformer that wasn't on/in the plans?
Don't give up yet, make a play for their wallet.
 

jumper

Senior Member
Is voltage drop a part of the specs? If not, I would care about the actual voltage at the end, not the drop percentage.

I used #10 @ 8 amps and got a tad over 110V at 500' starting with 120V at the breaker. That would work IMO.

If the supply voltage was running high, using 490', and 7A load you could possibly fudge numbers to make #12 work, I am just conservative.
 

jusme123

Senior Member
Location
NY
Occupation
JW
..if the NEC is mentioned in the specs, that pretty much covers voltage drop. Looks like you have to eat the cost of the mistake.
 

growler

Senior Member
Location
Atlanta,GA
missed a 120 volt receptacle on a bid.

30' up in the air, feeding a wifi repeater.
7 amp load.

490' from the nearest panel.
#6 copper gives 2.81% voltage drop.

It's hard to believe that this was missed if it was shown correctly on the drawings/plans.

What's actually called for on the plans? I would think a dedicated circuit would be needed and not just shown as a receptacle.

If all they are showing on the plans is a receptacle I wouldn't worry about how far to the panel I would connect to the nearest receptacle circuit ( In theory) and ask for a change order to run a dedicated circuit (what's needed ).
 

Cow

Senior Member
Location
Eastern Oregon
missed a 120 volt receptacle on a bid.

30' up in the air, feeding a wifi repeater.
7 amp load.

490' from the nearest panel.
#6 copper gives 2.81% voltage drop.

anyone got any magic math, or did i just
eat 1,000' of #6 thhn for one outlet?
My magic math says 3 wires multiplied by 490' is more than 1000'.......:)

In all seriousness, most electronics can handle a pretty wide voltage tolerance. Since this a dedicated circuit just for the repeater I wouldn't have a problem using #8 with a 5.4% drop.
 

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
missed a 120 volt receptacle on a bid.

30' up in the air, feeding a wifi repeater.
7 amp load.

490' from the nearest panel.
#6 copper gives 2.81% voltage drop.

anyone got any magic math, or did i just
eat 1,000' of #6 thhn for one outlet?
What is the actual delivered voltage at the panel? 126 volts will be a #8 with a 4.6 volt drop... Still 121.4 volts under load
# 10 at 126 volts will be 119 volts under load... Is this load constant?
 

jumper

Senior Member
Is voltage drop a part of the specs? If not, I would care about the actual voltage at the end, not the drop percentage.

I used #10 @ 8 amps and got a tad over 110V at 500' starting with 120V at the breaker. That would work IMO.

If the supply voltage was running high, using 490', and 7A load you could possibly fudge numbers to make #12 work, I am just conservative.
Actually using this chart, depending on the supply voltage, #12 might work.

 

Fulthrotl

~~Please excuse the mess. Sig under construction~~
Randy is on a big EMT job right now, he is working all weekend, probably using EMT as EGC.
no, there are actually 3 wires. two hots and a neutral, and a #12 ground.

and i screwed up when i took off the bid. this repeater is a critical component, and if it
doesn't function properly, the pick program in this warehouse won't relay info to the
displays on the forklifts, and that isn't happening on my watch.

it's not that big a deal, and it's going in fast. i was just whiny when i realized what i was
gonna have to do... :rant:
 

kwired

Electron manager
Location
NE Nebraska
This is a MWBC? Lemme think a sec. Can this be calculated like a feeder then at 208V?
If you run a 120/208 branch circuit out there and end up using only 1 ungrounded and the grounded conductor you still only effectively have 120 volts to base your voltage drop from. Now if you run a 208 volt circuit out there and transform to 120 near the load you will have less voltage drop to deal with, but will have added cost with the transformer and associated accessories and labor to put it together - may cost the same or more than just pulling larger conductors in the first place.
 

satcom

Senior Member
If you run a 120/208 branch circuit out there and end up using only 1 ungrounded and the grounded conductor you still only effectively have 120 volts to base your voltage drop from. Now if you run a 208 volt circuit out there and transform to 120 near the load you will have less voltage drop to deal with, but will have added cost with the transformer and associated accessories and labor to put it together - may cost the same or more than just pulling larger conductors in the first place.
That is how we do it, and it avoids future issues, plus it provides for additional branches in the area
 

Strife

Senior Member
missed a 120 volt receptacle on a bid.

30' up in the air, feeding a wifi repeater.
7 amp load.

490' from the nearest panel.
#6 copper gives 2.81% voltage drop.

anyone got any magic math, or did i just
eat 1,000' of #6 thhn for one outlet?
1:does the repeater NEED to be that far away? At that amperage I assume that repeater has a PRETTY LARGE range. 150-200 feet closer would solve your VD problem.
2: Do you have any other loads in that area that might require oversized wires to allow for VD. Might be worth to run a 100A subpanel 250-300 from the main.
3: Might want to consider a buck/boost transformer. You should be able to send #12 208 or 240 and play the taps a little. Probably just as expensive as running #6.
4: Last, based on my calculations at 120V you'll have a VD of 8.51V at 490' length. Which gives me 11.5V at load. Considering the POCO over pumps a little their voltage(it's more like 124-125), I assume you'll end up with LEAST 114V at the load. I seriously doubt the repeater will not work properly at 114V. Even at 111V I'm pretty sure it'll work just fine.
 

kwired

Electron manager
Location
NE Nebraska
1:does the repeater NEED to be that far away? At that amperage I assume that repeater has a PRETTY LARGE range. 150-200 feet closer would solve your VD problem.
2: Do you have any other loads in that area that might require oversized wires to allow for VD. Might be worth to run a 100A subpanel 250-300 from the main.
3: Might want to consider a buck/boost transformer. You should be able to send #12 208 or 240 and play the taps a little. Probably just as expensive as running #6.
4: Last, based on my calculations at 120V you'll have a VD of 8.51V at 490' length. Which gives me 11.5V at load. Considering the POCO over pumps a little their voltage(it's more like 124-125), I assume you'll end up with LEAST 114V at the load. I seriously doubt the repeater will not work properly at 114V. Even at 111V I'm pretty sure it'll work just fine.
I still find it hard to believe the repeater uses 7 amps @ 120 volts. There is likely more than just the repeater being supplied - maybe an anti condensation heater in a cabinet is the majority of the load? That kind of distance is not your typical wifi range either.
 

Strife

Senior Member
I still find it hard to believe the repeater uses 7 amps @ 120 volts. There is likely more than just the repeater being supplied - maybe an anti condensation heater in a cabinet is the majority of the load? That kind of distance is not your typical wifi range either.
Agreed, I have a wifi modem in my office with a small 100W transformer. I get a pretty good 300 feet range out of it. HECK, I get about 70 feet range out of my hotspot mobile phone running on a 7W battery and it last me 2-3 hrs. So 800W plus on a repeater has to be either a VERY LARGE range, or something else attached to it.
 

satcom

Senior Member
Agreed, I have a wifi modem in my office with a small 100W transformer. I get a pretty good 300 feet range out of it. HECK, I get about 70 feet range out of my hotspot mobile phone running on a 7W battery and it last me 2-3 hrs. So 800W plus on a repeater has to be either a VERY LARGE range, or something else attached to it.
The commercial wifi equipment has better output, nothing like your home wi f. And 7 amp sounds about right most of them carry both wi f and cell signals
 
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Other than for fire pumps and maybe a couple of other things the NEC does not have voltage drop limitations.
210.19 Conductors ? Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
FPN No. 4: Conductors for branch circuits as defined inArticle 100, sized to prevent a voltage drop exceeding3 percent at the farthest outlet of power, heating, and lightingloads, or combinations of such loads, and where themaximum total voltage drop on both feeders and branchcircuits to the farthest outlet does not exceed 5 percent,provide reasonable efficiency of operation. See FPN No. 2of 215.2(A)(3) for voltage drop on feeder conductors.

NEC has specified percentages of voltage drop!
Transforming it should be cheaper.
 
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