DISTRIBUTION UTILITY AMPACITY REFERENCE FOR GROUNDED CONDUCTOR

bobby ocampo

Senior Member
What ampacity table are Distribution Utility using in sizing the grounded service conductor?

Is there a code other than the NEC that DU is using for the ampacity of their service conductors?
 

iwire

Moderator
Staff member
Location
Massachusetts
What ampacity table are Distribution Utility using in sizing the grounded service conductor?

Is there a code other than the NEC that DU is using for the ampacity of their service conductors?
First off we need to understand the utility does not go by our load calculations or breaker sizes at all.

If we install a 600 amp service per the NEC the power company is going to look at their years of history and decide what that building will really draw. They may say that 600 amp service is never going to draw more than 300 amps.

So right of the bat they will be using smaller wire than we would.

On top of that they don't mind overloading the wire or transformers for short periods of time.


That said, I don't know what table they use, I suspect it is close to our single conductor free air table.
 

Iron_Ben

Senior Member
Location
Lancaster, PA
What ampacity table are Distribution Utility using in sizing the grounded service conductor?

Is there a code other than the NEC that DU is using for the ampacity of their service conductors?
For our service conductors, overhead and underground both, we used triplex and quadruplex with a neutral two sizes smaller than the phase conductors. 3/0 had a 1/0 neutral, 4/0 had a 2/0 neutral, etc. We didn't "size" neutrals as such. We chose the appropriate phase conductor size, and the neutral just came along as part of the assembly, two sizes smaller.

As far as what we used to determine conductor ampacities in various conditions of service, it was - as iwire alluded to - a mish mash of NEC Tables, manufacturers' listings, our Standards departments best guesses, and probably most often, the engineer's best guesses based on years of experience. Despite the lack of one all-encompassing chart or table, the system worked pretty well, striking the right balance among safety, economics, and customer satisfaction.
 

mbrooke

Senior Member
Location
United States
First off we need to understand the utility does not go by our load calculations or breaker sizes at all.

If we install a 600 amp service per the NEC the power company is going to look at their years of history and decide what that building will really draw. They may say that 600 amp service is never going to draw more than 300 amps.

So right of the bat they will be using smaller wire than we would.

On top of that they don't mind overloading the wire or transformers for short periods of time.
Not only that but wire in free air can dissipate heat without any problem not requiring the same reduction as indoor wiring is subjected to. Second if building has a winter load peak the wire size can be further reduced since this will take place with the triplex surrounded by low temps.

In any case it can also be economical for the customer to reduce the service drop size to knock down fault current. A 22kaic supply at the vault or pole can become a 10kaic supply when enough length and size reduction is involved.



That said, I don't know what table they use, I suspect it is close to our single conductor free air table.
This may help:

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet34

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet35

http://www.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet269



Table 310.15(B)(20) may also be a good guide (though unsure how applicable it is code wire) I know Ive used that table before in messenger triplex sizing.
 

mbrooke

Senior Member
Location
United States
For our service conductors, overhead and underground both, we used triplex and quadruplex with a neutral two sizes smaller than the phase conductors. 3/0 had a 1/0 neutral, 4/0 had a 2/0 neutral, etc. We didn't "size" neutrals as such. We chose the appropriate phase conductor size, and the neutral just came along as part of the assembly, two sizes smaller.

As far as what we used to determine conductor ampacities in various conditions of service, it was - as iwire alluded to - a mish mash of NEC Tables, manufacturers' listings, our Standards departments best guesses, and probably most often, the engineer's best guesses based on years of experience. Despite the lack of one all-encompassing chart or table, the system worked pretty well, striking the right balance among safety, economics, and customer satisfaction.

Is Table 310.15(B)(20) appropriate? Ive used it many times as a guide to selecting triplex sizing, but never thought about its true applicability in depth.
 

mivey

Senior Member
Is Table 310.15(B)(20) appropriate? Ive used it many times as a guide to selecting triplex sizing, but never thought about its true applicability in depth.
Not for utility wire but for THHN, etc they can be appropriate, given the same assumptions.

Manufacturer tables, IEEE tables, or industry tables are most appropriate for utility type cable. Or you could make your own calcs but we usually use tables. You will find that, given the same assumptions, different tables and calcs are close in agreement.

NEC loads are a different story and rarely will they agree with utility calcs.
 

Iron_Ben

Senior Member
Location
Lancaster, PA
Is Table 310.15(B)(20) appropriate? Ive used it many times as a guide to selecting triplex sizing, but never thought about its true applicability in depth.
310.15(B)(20) is a little conservative. As an example, we used 336 quad for our largest overhead conductor for three phase service. (Even with it being aluminum, it's *very* heavy and needs a lot of support.) Interpolating, it looks like the table would suggest 354 amps or so for 336 ACSR at 90 deg C rated. I've seen us push 450+ amps through that continuously on a super hot south Louisiana day, 95 degrees F with a blazing sun and no cloud cover. On the particular day I'm remembering you could not touch the main disconnect on the building wall without gloves on - too hot.
 

bobby ocampo

Senior Member
My concern mainly is if there is no regulation on the size of neutral conductor. Utility may use a conductor that has a high resistance for the grounded service conductor.

Using GROUNDED SERVICE CONDUCTOR that is of high resistance is an advantageous to the Distribution Utility. The DU meter will measure line to ground leak without tripping the OCPD. If the GROUNDED SERVICE CONDUCTOR (wire connected in the neutral of the transformer) is of HIGH RESISTANCE. the DU meter will read consumption on a single line to ground fault without tripping the OCPD.
:?
 

iwire

Moderator
Staff member
Location
Massachusetts
My concern mainly is if there is no regulation on the size of neutral conductor. Utility may use a conductor that has a high resistance for the grounded service conductor.

Using GROUNDED SERVICE CONDUCTOR that is of high resistance is an advantageous to the Distribution Utility. The DU meter will measure line to ground leak without tripping the OCPD. If the GROUNDED SERVICE CONDUCTOR (wire connected in the neutral of the transformer) is of HIGH RESISTANCE. the DU meter will read consumption on a single line to ground fault without tripping the OCPD.
:?
You asked about service conductors. There is no OCPD to trip, they are unprotected conductors.
 

bobby ocampo

Senior Member
What standard is used by the Authority Having Jurisdiction on the size of Neutral or grounded service conductor that the Distribution Utility can use?
 

GoldDigger

Moderator
Staff member
The Authority Having Jurisdiction with respect to the NEC generally does not have any jurisdiction over the utility.
In particular utility wiring is not subject to the NEC.
Utilities are subject to other authorities including utility regulators and OSHA.
 
Last edited:

iceworm

Curmudgeon still using printed IEEE Color Books
Location
North of the 65 parallel
Occupation
EE (Field - as little design as possible)
... Using GROUNDED SERVICE CONDUCTOR that is of high resistance is an advantageous to the Distribution Utility. The DU meter will measure line to ground leak without tripping the OCPD. If the GROUNDED SERVICE CONDUCTOR (wire connected in the neutral of the transformer) is of HIGH RESISTANCE. the DU meter will read consumption on a single line to ground fault without tripping the OCPD ...
Bobby O
Good to hear from you. Your trolls are always interesting. You always keep the subject sufficently nebulous, one can't always tell the context. For example, I can't tell if you are discussing residential of industrial - and yes it matters.

Starting from there, give some thought to the physics. The grounded service conductor resistance doesn't matter - high or low indifferent.

Your premisis is line to ground (earth) leakage after the meter is a problem (safety or money - can't tell) because the utility intentionally installs a higher than needed resistance service neutral. This is either cheating the customer, or unsafe because the leakage won't trip the unspecified OCP - I can't tell which concerns you.

Basic Physics:
The resisitance of a line to earth leakage connection will be much higher than any service neutral conductor. Other than gfci, the system is not designed to trip on earth leakage.

And you know this. You have a copy of IEEE green book. So, where are you going?

Possibly:
The utility is undersizing neutrals in a effort to cheat the customer on metering?
The utility is, with forethought and malice, undersizing neutral conductors to prevent customer CBs from tripping on earth faults?

Either is pretty ludicrus. Because: Other than gfci, the system is not designed to trip on earth leakage.

Sorry to hit and run, but I got to get to work. Got a customer that wants to pay.


ice
 

mbrooke

Senior Member
Location
United States
My concern mainly is if there is no regulation on the size of neutral conductor. Utility may use a conductor that has a high resistance for the grounded service conductor.

Using GROUNDED SERVICE CONDUCTOR that is of high resistance is an advantageous to the Distribution Utility. The DU meter will measure line to ground leak without tripping the OCPD. If the GROUNDED SERVICE CONDUCTOR (wire connected in the neutral of the transformer) is of HIGH RESISTANCE. the DU meter will read consumption on a single line to ground fault without tripping the OCPD.
:?
None of that is correct. POCOs do not care about neutral current. It does not change metering.
 

bobby ocampo

Senior Member
You asked about service conductors. There is no OCPD to trip, they are unprotected conductors.
If the Grounded Service Conductor or the Neutral is of High Resistance or poor conductor, there is a big possibility that the OCPD in the Service Equipment will not trip. There should be a regulation on the size of Service Conductors(grounded and ungrounded conductor)
 

mbrooke

Senior Member
Location
United States
If the Grounded Service Conductor or the Neutral is of High Resistance or poor conductor, there is a big possibility that the OCPD in the Service Equipment will not trip. There should be a regulation on the size of Service Conductors(grounded and ungrounded conductor)
Correct, if the conductor breaks. Such is possible with any size.
 
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