Voltage Drop and EGC

Status
Not open for further replies.
Shockedby277v

Shockedby277v

Senior Member
Location
Michigan
ok. Let's say :

I have a 300' lighting run, that's 277v 20a. I would need to run a #8 Cu THHN. With a 2.9% VD. If I ran a #12 the maximum distance I could run while staying within the 3% would be 121'. Question 1 : For a 300' run can i change to #12 from #8 at 179' and still have a 2.9%??

Question 2 :
The EGC. T250.122 lists 20A to be protected by a #12. With voltage drop coming into play and needing to upsize to #8, 250.122 (B) says I need to also increase the size of the EGC. If I used my conduit system (EMT) as my EGC, I bond to my junction box where I make my transition. Is there a size limitation for using my conduit system as my EGC relating to my ungrounded conductors???
 
Question #1 No.

Question #2 The EGC, if used, must be a # 8. You could not use one since the EMT qualifies as an EGC.
 
That's how I read it. I've been to 2 of these stores almost identical with the exception of it being just flipped around. The first store was already up and running so I didn't get to dig into the panels and see what they ran but it looked like they ran 3/4" for their lighting home runs. The second store had #12's for just about everything and everything was installed very poorly. The width of the store is 330' alone, not including exterior lighting. It is spec'd that VD must be taken into consideration. I don't want to do it this way but I know I will be told to. I will find out Monday when I talk to my project manager.
 
Is my voltage calculator screwed up?
Volts:------277-----End Volts:-------269.39
Amps:-------20----Total Vd:---------2.75%

Section---Distance----Wire Size---CU/AL-----Vd-----
1-----------179---------8----------CU------2.74
2-----------121--------12----------CU------4.67
------------300
Click to expand...
I get 2.75% for a 179' run of #8 and a 121' run of #12.

That's assuming a full 20A, which this circuit will likely not see, correct?
 
Last edited:
Shocked I doubt this will surprise you but what the prints and specs say is not always what happens out in the field.

Another issue is it appears you are figuring your voltage drop based on the breaker size.....don't do it that way.

Look at the panel schedule on the prints and you should find the calculated load for each circuit. Lighting circuits in particular are not going be loaded to 100%, 80% max and good design will have it more in the 50 to 60% range.

This difference in load will greatly effect the VD calcs.

It is likely the majority of the circuits home runs should be 10 AWG.

Increasing from 12 to 10 is not to expensive, increasing to 8 or larger really starts adding up.

IMO 1" or 3/4" EMT with 10 AWG is about as efficient as your going to get. If you start running 1.25", 1.5" or 2" home runs derating will kill you....unless you ignore it like so many do. I don't, Peter D can back that up, he has worked for me in large retail....miles of 3/4" and 1" EMT.

By the way....is EMT required....MC is a nice product if acceptable to the customer or their engineer.
 
Ahhh!!! That clears things up greatly. The prints I have are very vague and doesn't give me the draw. The home runs will be piped in EMT then change over to MC conceled in the deck ribs and drop out at the lights. Exposed conduit is very limited, another issue I'll have to deal with down the road, especially with solid precast walls. Ugh, I'm in for a journey.
 
georgestolz said:
Is my voltage calculator screwed up?

I get 2.75% for a 179' run of #8 and a 121' run of #12.

That's assuming a full 20A, which this circuit will likely not see, correct?
Click to expand...


George you may be correct. I didn't run the numbers so my response was based on the OP. Seems that if you had a VD of 2.9% with #8 AWG conductors for the entire run than changing the circuit by replacing a portion of the #8 with #12 AWG conductors for should increase the VD.
 
Shockedby277v said:
ok. Let's say :

I have a 300' lighting run, that's 277v 20a. I would need to run a #8 Cu THHN. With a 2.9% VD. If I ran a #12 the maximum distance I could run while staying within the 3% would be 121'. Question 1 : For a 300' run can i change to #12 from #8 at 179' and still have a 2.9%??

Question 2 :
The EGC. T250.122 lists 20A to be protected by a #12. With voltage drop coming into play and needing to upsize to #8, 250.122 (B) says I need to also increase the size of the EGC. If I used my conduit system (EMT) as my EGC, I bond to my junction box where I make my transition. Is there a size limitation for using my conduit system as my EGC relating to my ungrounded conductors???
Click to expand...

#8 300 ft VD = 3.4%

#8 179 Ft VD = 2.0%
#12 121 FT VD = 3.5%
Total VD = 5.5%
 
georgestolz said:
Bob, which formula do you use?
Click to expand...
Its a computer program. I believe its VD = I x Z of the conductor.
I checked it on several programs and the answer was the same.
 
Last edited:
Status
Not open for further replies.
Top