ECN Forum
Posted By: Specialty Voltage Drop on Neutral - 02/06/06 07:54 PM
We are supplying (120v) 2000VA of parking lot lights 130' away. The foreman ordered #10 pulled. The boss questioned the #10 and recomended adjusting ungrounded conductor to #8 for voltage drop (the foreman agreed). There is sufficient ampacity in the 10's, so does the #10 neutral also need to be replaced with #8, or just the hot since we don't care about voltage drop going to ground?
Posted By: gfretwell Re: Voltage Drop on Neutral - 02/06/06 08:43 PM
Voltage drop doesn't care if it is coming or going, they add. You should make the neutral the same size as the ungrounded conductor. It might be argued that in a multiwire the neutral would not need to be any bigger than the 240.4(D) requirement since if the load was balanced it would not carry any current at all. If this is just a single circuit it carries the same as the ungrounded wire.
Posted By: jfwayer Re: Voltage Drop on Neutral - 02/06/06 08:55 PM
Electricity needs a complete path, from source to load, through load, from load back to source. This involves both the hot and the neutral or grounded conductor.

The current (I) is 2000VA/120V or 16.67A

The resistance of #10 is 1.21/kFT and #8, .764/kFT (NEC table 8)

120FT of #10 is 120 * 1.21 / 1000 = .146ohms
120FT of #8 is 120 * .764 / 1000 = .092ohms
The voltage drop comes from the resistance of the hot wire and the neutral.

The voltage drop for 120FT #10 is 16.67 * .146 = 2.43V
The voltage drop for #8 is 1.53V

#10 neutral and hot is 2.43 + 2.43 = 4.86V
#8 neutral and hot is 1.53 + 1.53 = 3.06V
one #10 and one #8 is 2.43 + 1.53 = 3.96V

3% of 120V is 3.6V, so both #8 is ok, one #8 and one #10 is marginal, and both #10 is poor.


If the load is distributed (or the circuit is 120/240V) the analysis is different.
Posted By: John Crighton Re: Voltage Drop on Neutral - 02/06/06 09:42 PM
Another issue: Is an EGC being run with the circuit? If you oversize the conductors for voltage drop, you need to oversize the EGC by the same amount.
Posted By: iwire Re: Voltage Drop on Neutral - 02/06/06 10:09 PM
As John C. brought up the EGC is required to be increased in size if you increase the circuit conductors.

In this case we can assume this is a 15, 20 or 30 amp circuit all of which require a full size EGC.

If you run 8 AWG for the 'hots and neutral' you will have to run a 8 AWG EGC to comply with 250.122(B).

In my area inspectors are sharp on this and will catch this if you ignore it.
Posted By: Specialty Re: Voltage Drop on Neutral - 02/06/06 10:39 PM
You guys are great, thanks for your help in confirming what we suspected. The poles are required to be grounded directly so the EGC is not at issue.

Thanks Again
Posted By: iwire Re: Voltage Drop on Neutral - 02/06/06 10:43 PM
Quote
The poles are required to be grounded directly so the EGC is not at issue.

Please explain what you mean.

Some sort of EGC absolutely must be run with the circuit conductors back to the panel.

It could be a metal raceway or a conductor but one or both must be used.
Posted By: HotLine1 Re: Voltage Drop on Neutral - 02/06/06 10:46 PM
Specialty:

What do you mean.."You guys are great, thanks for your help in confirming what we suspected. The poles are required to be grounded directly so the EGC is not at issue"

Some specs require a driven grd rod at each pole...this however is in addition to the EGC that you run with the circuit.

I'll second or third what the other esteemed members stated regarding the EGC upsizing, and increasing the neutral conductor.

John
Posted By: walrus Re: Voltage Drop on Neutral - 02/06/06 11:13 PM
CAn anyone explain the use of a ground rod at each pole?? Lightning protection?
Posted By: Roger Re: Voltage Drop on Neutral - 02/06/06 11:43 PM
Walrus, lightning protection and/or a HV event is the only reason for the rod.

Specialty, Welcome to the forum.

Now, the individual pole grounding will not protect the pole, light, or people from a fault, and infact without a EGC it can add to the dangers in the way of dangerous voltage gradients around the rod in a fault condition.

You MUST install an EGC for people safety. This is non negotiable.

Roger
Posted By: Specialty Re: Voltage Drop on Neutral - 02/07/06 02:34 AM
The grounding is through the 1 rmc raceway w/ a grounding bushing at both ends. The poles also have a ground rod with the rod bonded to the pole. Perhaps the term "direct" was misleading or is there a reason the raceway cannot be used?
Posted By: mxslick Re: Voltage Drop on Neutral - 02/07/06 02:50 AM
One word: corrosion!

RMC directly buried will eventually corrode to the point of radically increasing it's resistance for the path as an EGC. You would then have high risk of creating voltage gradients as well. It would not be wise for either safety or economy to omit the grounding conductor.

Relying on raceway as a grounding path is poor practice anyway.

And welcome to ECN!!

edited typos

[This message has been edited by mxslick (edited 02-06-2006).]
Posted By: dereckbc Re: Voltage Drop on Neutral - 02/07/06 04:09 AM
John & iwire, IMO I do not think you would have to increase the EGC size if you only increase the size of the grounded circuit conductor. You have not change the size of the line conductor
Posted By: iwire Re: Voltage Drop on Neutral - 02/07/06 09:34 AM
Dereck I agree with you but I do not think that is what is happening here.

Check out my response.
Quote
If you run 8 AWG for the 'hots and neutral' you will have to run a 8 AWG EGC to comply with 250.122(B).

If the neutral alone was increased as is the case with much of the 'super neutral' MC we run the EGC can remain the size of the ungrounded conductor.


Specialty RMC can be used as the EGC per the NEC.

That aside I do agree with mxslick that for underground or outside runs it is better to run a 'wire' EGC.

I also would be running PVC in the ground but that is another personal choice. [Linked Image]

Bob
Posted By: Roger Re: Voltage Drop on Neutral - 02/07/06 10:58 AM
Specialty, thanks for clearing that up (that there will be an EGC) but I agree with Mxslick, at some time in the future the EGC will be lost due to corrosion.

I would do as Iwire and use PVC (my preference too) with a wire conductor for the EGC.

Roger
Posted By: renosteinke Re: Voltage Drop on Neutral - 02/07/06 02:41 PM
Please note that the original poster said "parking lot lights." I suspect that these are HID type lights, and some folks will over-size the neutral out of a concern for "harmonics."

Of course, lots of times folks use terms like "harmonics" without having a clue as to what they mean; but making the wire larger is generally not a problem.
Posted By: HotLine1 Re: Voltage Drop on Neutral - 02/07/06 10:45 PM
Specialty:
Yes, welcome to ECN, and I have to say 'scuse me for omitting that above.

Again, PVC is my choice, but IF RGC, I would still install an EGC within the RGC.

I've seen quite a few specs recently for PL lighting with 3/4 x 10' rods (2) at each 'base', sonotube or pre-cast, and 'Cadweld" termination. The EE's say lightning protection, and a "redundant grounding system". I just assure that it is all bonded together, and 'to spec' in addition to NEC compliance.

John
Posted By: dereckbc Re: Voltage Drop on Neutral - 02/08/06 04:16 AM
iwire, sorry bout that. I should read more closely. Wished I never took that speed reading course. :lol:
© ECN Electrical Forums