More about voltage drop

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kevinware

Senior Member
Location
Louisville, KY
Hey everyone,
I need some opinions. I am installing two dedicated 120V 20A single phase circuits that will terminate at two quadplex outlets in an information technology environment that will power some Networking equipment. My concern is this. The two outlets will be located 200 feet away from the panel I have to come out of and I am worried about voltage drop. I understand that the NEC only recommends a 3% voltage drop limit on branch circuits. I do not know the exact load that will be on these circuits because these circuits will be providing power for a Networking test area so the load will vary from day to day. Using the VD formula
2 x K x I x D divided by CM, I use the following
numbers.
K = 12.9

I = 16A ( I am using this figure because I don't know what the load will be and 210.21(B) (2) tells me that "where connected to a branch circuit supplying two or more receptacles or outlets, a receptacle shall not supply a total cord-and-plug-connected load in excess of the maximum specified in Table 210.21 (B) (2) )

D= 200 feet

CM= 6530 (From table 8)

Plugging these numbers into the above formula I get a voltage drop of 12.64V, that is 10%. When should I start being concerned about voltage drop?

Thanks for your comments,

Kevin
 
Re: More about voltage drop

Use the formula:

2 x K x D x I / max VD desired

So, if you want to limit to 3%:

2 x 12.9 x 200 x 16 / 3.6 = 22,933 cm = #6 awg.


In my opinion, 16A is probably excessive. I think you could also extend the voltage drop to 5% without too much of an issue. If I were running the circuit as you describe, I would probably just increase my conductors by one size to accommodate the length. I don't think load will be an issue for general purpose receptacles.
 
Re: More about voltage drop

You might want to check what the actual voltage is at the panel to begin with. Many times I have found that remote panels already have a few volts drop at the panel. I've seen voltages as low as 110V coming out of lighting panels. if you drop that down another 5 volts going out to the receptacle, you might be asking for trouble with whatever is being plugged in.

Then I would check with the user and find out what the maximum load is that he might actually plug into the outlet.

These pieces of information would help you make a more informed decision.
 
Re: More about voltage drop

Yes I agree #6 does seem big and that is why I posted my question. "When does Voltage Drop become important??????"

Keep the opinions coming
 
Re: More about voltage drop

If you have at least 120V at the panel, I would not be concerned about loss up to 5%. If you can connect the two circuits as a multiwire circuit that will reduce voltage drop by up to 50%.
 
Re: More about voltage drop

Being involved with data center design myself over the years, I have collected tons of data and wrestled with this subject many times. Most of it is not much concern anymore due to we use PDU?s located close to where the power is utilized, so voltage drop is not an issue. But here are my thought and facts.

1. There is no NEC requirement to control voltage drop.

2. The value 12.9 for K is too liberal and is a worst case scenario, which will not likely to ever be experienced, in a controlled environment. Use a conservative value of 10.8-to-11.1 for K.

3. Using 80% of the breaker size for a branch circuit again is too liberal, which will not likely to ever be experienced. I use 50% or less, so choose 8 or 10-amps. Since these are dedicated circuits, it is not likely that the circuits will ever exceed 50% capacity.

That being said, IMHO 10 AWG is more than enough to CYA provided the panel is at or near 120.
 
Re: More about voltage drop

I tend to agree that if you are starting out with close to 120V. & you use # 10, in most cases you should be okay with up to a 10 or 12A load. Do not be surprised if the typical data center user tells you that it is a 15A load or 20A load.

Yes, using a multiwire ckt.- (2) opposite hots sharing a neutral- will help. Use high quality spec grade/industrial grade receptacles & they will easily accept the #10. As always, try to use continuous runs of wire without splices.
 
Re: More about voltage drop

Just out of curiosity, when you use that VD formula, you are not considering power factor at all? or are we assumming .85 PF or just ignoring it?
 
Re: More about voltage drop

Originally posted by petersonra:
Since we are looking at VA in the calcs, or current draw for the VD calcs, its already considered.
Click to expand...
What does that mean? In this formula, VD will be the same regardless of PF.
 
Re: More about voltage drop

What's to stop you from calculating a power factor if there was one. In this case the load is unknown.
 
Re: More about voltage drop

Hey I just wanted to think everyone for their valuable opinion. I called my supplier to order my wire and receptacles and he asked me if I was going to use that #10 wire on those receptacles? I said yes and I told him I was worried about voltage drop and he kind of laughed at me and said I was over killing. But I believe I am right to be concerned and with everyone here having about the same opinion I am ready to get to work.

Thanks again for all of your opinions,

Kevin
 
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