3 Way Switches and Voltage Drop

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taking the EM lights out of the equation and dealing with 2 groups of 10 t5's, 10 amps per circuit ,Every online VD calculator i've used when I put in 120 o123 volts with a length of 170 and a 10 amp load calls for#10 to be within the 3% drop not #12.

What am I figuring wrong?
 
jap said:
taking the EM lights out of the equation and dealing with 2 groups of 10 t5's, 10 amps per circuit ,Every online VD calculator i've used when I put in 120 o123 volts with a length of 170 and a 10 amp load calls for#10 to be within the 3% drop not #12.

What am I figuring wrong?
Click to expand...

The problem comes from being stuck on 3% voltage drop. If you assume 120 volts, then 3% amounts to 3.6 volts

120 - 3.6 =116.4 volts (I think I wrongly wrote 116.8 earlier)

But the bottom line is that 3% drop from 120 volts means you have a target minimum voltage of 116.4

Now here's where it start to sound crazy. Suppose you actually have 123 volts to start

123 - 116.4 = 6.6

6.6 ? 123 = .053

You can drop 5% from 123 volts, and you still have more than 116.4.

Try your online voltage drop calculator with 123 volts and 5% drop, and you will see it suggests a #12 circuit.

Also, several people have noted that electronic ballasts will operate at lower than 116 volts, too.

Anything larger than #12 is a waste of money, imho.
 
jap said:
taking the EM lights out of the equation and dealing with 2 groups of 10 t5's, 10 amps per circuit ,Every online VD calculator i've used when I put in 120 o123 volts with a length of 170 and a 10 amp load calls for#10 to be within the 3% drop not #12.

What am I figuring wrong?
Click to expand...
What is the reason for keeping voltage drop under 3%?

5% is generally accepted for design purposes on branch circuits, most any equipment can tolerate 10% with no loss of performance or damage.
 
ActionDave said:
What is the reason for keeping voltage drop under 3%?

5% is generally accepted for design purposes on branch circuits, most any equipment can tolerate 10% with no loss of performance or damage.
Click to expand...

Seriously this, wire it up and move on. The lights will work.
 
jap said:
taking the EM lights out of the equation and dealing with 2 groups of 10 t5's, 10 amps per circuit ,Every online VD calculator i've used when I put in 120 o123 volts with a length of 170 and a 10 amp load calls for#10 to be within the 3% drop not #12.

What am I figuring wrong?
Click to expand...
Is your 170 foot length the one way run or does it include the length of the return path?
Does voltage drop calculator you are using double your distance to account for the return path? Many do.
 
I was taught to use the wire length both ways in the VD calculation.

Also I'm pretty sure that the code says you can only have a 3% VD total.
I don't have the books in front of me now.
Thanks
MIke
 
Except for fire pumps and maybe MV, the only references in the NEC are suggestions in informational notes, and therefore doubly unenforceable.
:)
 
210.19(a) Chicago code book, but I think it's the same in the NEC.
Say
Conductors for branch circuits as defined in article 100, shall be sized to prevent a voltage drop not to exceed 3 percent at the farthest outlet of power, heating and lighting loads, or combination of such loads.

I took a class for passing the electrical license test and the instructor gave us a paper. The paper showed how, just up sizing the the conductor one size was not enough some times. The example was parking lot lights a 1000 feet to the furthest one. You had to use 2000 in the calculation because of the distance for both ungrounded and grounded conductors. This was an example where using #12 on a 15 amp circuit was not enough. You had to up size to #10.

I will see if I can find the papers for this.
Thanks
Mike
 
mike1061 said:
210.19(a) Chicago code book, but I think it's the same in the NEC.
Say
Conductors for branch circuits as defined in article 100, shall be sized to prevent a voltage drop not to exceed 3 percent at the farthest outlet of power, heating and lighting loads, or combination of such loads.....
Click to expand...
210.19 is about the ampacity of the conductors, nothing about voltage drop. The part about three percent is in the fine print note at the bottom, is not an enforceable part of the code, and doesn't even hint at any kind of requirement, only that if sized for 3% you get an efficient circuit.
 
GoldDigger said:
Except for fire pumps and maybe MV, the only references in the NEC are suggestions in informational notes, and therefore doubly unenforceable.
:)
Click to expand...

:happyno:

647.4
(D) Voltage Drop. The voltage drop on any branch circuit
shall not exceed 1.5 percent. The combined voltage drop of
feeder and branch-circuit conductors shall not exceed
2.5 percent.
(1) Fixed Equipment. The voltage drop on branch circuits
supplying equipment connected using wiring methods in
Chapter 3 shall not exceed 1.5 percent. The combined voltage
drop of feeder and branch-circuit conductors shall not
exceed 2.5 percent.
(2) Cord-Connected Equipment. The voltage drop on
branch circuits supplying receptacles shall not exceed
1 percent. For the purposes of making this calculation, the
load connected to the receptacle outlet shall be considered
to be 50 percent of the branch-circuit rating. The combined
voltage drop of feeder and branch-circuit conductors shall
not exceed 2.0 percent.
 
ActionDave said:
210.19 is about the ampacity of the conductors, nothing about voltage drop. The part about three percent is in the fine print note at the bottom, is not an enforceable part of the code, and doesn't even hint at any kind of requirement, only that if sized for 3% you get an efficient circuit.
Click to expand...

In Chicago, the voltage drop for feeders and branch circuits is adopted as mandatory, not a FPN or informational note.
 
ActionDave said:
210.19 is about the ampacity of the conductors, nothing about voltage drop. The part about three percent is in the fine print note at the bottom, is not an enforceable part of the code, and doesn't even hint at any kind of requirement, only that if sized for 3% you get an efficient circuit.
Click to expand...

I don't understand that. It looks to me like the FPN notes are after the part I quoted.
 

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mike1061 said:
I was taught to use the wire length both ways in the VD calculation.

Also I'm pretty sure that the code says you can only have a 3% VD total.
I don't have the books in front of me now.
Thanks
MIke
Click to expand...

Correct, the total circuit length is from the source to the load and back to the source again, so double the one way lenght is what you should be using. For three phase circuits though you have three conductors/three phases and what comes out on one conductor returns at a factor of 1.73 on the other two conductors, so for three phase circuits instead of doubling the one way length you use 1.73.


Add: That explanation maybe not quite right, but the phase angle still makes it necessary to use 1.73 instead of double the one way length for three phase circuits.

Others have already explained the fine print note so I will not elaborate on that.
 
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So in my situation, exactly how far would that be? considering a 60' wide 100' long building (5) Bays 20' wide, with a 3 way on each end of the 100',and 20 fixtures (4) in each of the 5 bays?
 
jap said:
So in my situation, exactly how far would that be? considering a 60' wide 100' long building (5) Bays 20' wide, with a 3 way on each end of the 100',and 20 fixtures (4) in each of the 5 bays?
Click to expand...
Can't say. Distance is the length of wire. Your 3-ways could be in opposing corners and the panel could be in a third. Changes in elevation also count. You could end up with a 250' circuit in a 60' x 100' building.
 
And as for the effect of loads distributed along the length of the circuit, see the actual VD calculations given in some earlier comments.
 
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