GROUNDING SEPARATELY DERIVED SYSTEMS PER NEC HANDBOOK EXHIBITS 250.14 AND 250.15

TanMike,

Myself, an electrical engineer since 1986 and my 70 year old electrical engineer veteran colleague have the following question about NEC Handbook Exhibits 250.14 and 250.15 for separately derived systems:

1. The two Exhibits for separately derived systems described the two available grounding and bonding methods depending on whether the grounding electrode connection is made at the source (the transformer) or the first disconnect. Do these figures only apply to a service entrance? What about downstream transformer and subpanels downstream of the main distribution panel?

2. It is assumed that NEC Exhibit 250.14 would be used in very rare situations because when is a service transformer ever going to be located right nest to building steel?

3. In the course of every electrical engineering design we have ever seen, neither Exhibit is ever used. Grounding to the electrode is done at the transformer and at the main distribution panel with the main disconnect. The ground wire is not connected between the transformer and the main distribution panel to keep unbalanced currents off of it that kill people. Electrical Engineers still submit their packages with this ground wire included because they still have not figured this out. This ground wire is not connected between the transformer and the main distribution panel I don't care if you got a delta wye separately derived transformer service, a wye-wye non separately derived transformer service, a center tapped or corner grounded Delta separately derived transformer service. You agree with this paragraph?

Thank You,
Tom from San Diego.
 

augie47

Moderator
Staff member
Location
Tennessee
Sounds as if there is some confusion between service transformers and SDS systems. I don't have my Handbook handy but it sounds as if the figures you reference are SDS installs where indeed the NEC allows the grounding electrode to connect to either but not both.
Most of the installs I inspect have the bonding jumper installed in the transformer and the electrode connected there also {required to be at the same location per 250.30(A)(1)}. (These connections are allowed to be at the 1st disconnect but not at both)
A supply side bonding jumper is required between the transformer and 1st disconnecting means
(it can be a 250.118 wiring method or conductor).

If, as I believe you describe, you were to connect a grounding electrode conductor at both locations I believe it would establish a secondary current path which the Code tries to eliminate.

With a service there is no equipment grounding conductor or supply side bonding jumper between the power co. transformer and the service disconnect..
 
Last edited:
ONE MORE QUESTION

ONE MORE QUESTION

My colleague also suspects that two primary utility feeds are intended for these two Exhibits 250.14 and 250.15 as
the scenario that is being presented in the Exhibits entitled "separately derived".

EC&M Electrical Engineering magazine recently wrote an article where OSHA had them investigate Electrical Engineers and Electricians and found 39 Electrical Engineering violations and electrician termination and grounding/bonding location violations indicating there is still mass confusion concerning grounding and bonding. Grounding and bonding is the most important issue for those involved in electricity for safety of human life and machine.

Tom
San Diego
 

augie47

Moderator
Staff member
Location
Tennessee
This may be the same as the Figures you reference but does portray the methods commonly used on SDS grounding.
transformer.jpg
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
My colleague also suspects that two primary utility feeds are intended for these two Exhibits 250.14 and 250.15 as
the scenario that is being presented in the Exhibits entitled "separately derived".



Tom
San Diego
Your colleague is mistaken, SDS's and Services are two completely different animals. The problem with Services is that the NEC has no control over what happens before the service equipment and due to the utilities MGN and the NEC's 250.24, earthing will take place at both, the utilites transformer and the premise service.

Roger
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
I am looking at the handbook now. It seems that these exhibits were numbered 250-13 and -14 in the 2011 version, numbered 250-14 and -15 in the 2014 version, and numbered 250-15 and -16 in the 2017 version. That's inflation for you. :lol:

I will only say now that this pertains to separately derived systems, not to services. I need to read a while before I offer any more comments.
 
BACK TO THE DRAWING BOARD

BACK TO THE DRAWING BOARD

Thank You,

Myself and my colleague both know the utility does not have to follow the NEC. We speak of that numerous times.

My colleague does not understand SDS nor do I.

I have been trying to put my arms around what is SDS for years. We know how it applies to a Generator and a ATS, but for the
life of us can't understand it as it would apply to a transformer feeding a disconnect (no Generator involved).

We will go back to the drawing board, but will not give up.

Tom from San Diego.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Two quick comments (I am still reading, and am not ready to offer more yet):

  1. The exhibits are “essentially” identical for all three NECH editions. There are slight changes in the use of a terminal bar internal to the transformer, as opposed to making connections directly to the case. But they are identical in terms of how the electrical connections work.
  2. There is no need to read too much into the phrase “Separately Derived System.” The most common application of an SDS starts when the utility provides 480 volts to a building. That system powers such things as large mechanical loads and some lights. Then we install step-down transformers that give us 120 volts for use in powering receptacles and smaller mechanical loads. The 120 volts was “derived” from the 480V system, and is completely “separate” from it.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Do these figures only apply to a service entrance?
No. Not at all. These apply to transformers inside the building that are fed from the system that the utility provides.
What about downstream transformer and subpanels downstream of the main distribution panel?
That is the only place to which these figures apply.
In the course of every electrical engineering design we have ever seen, neither Exhibit is ever used. Grounding to the electrode is done at the transformer and at the main distribution panel with the main disconnect.
I have always used the version in which the N-G bond takes place at the panel downstream of the step-down transformer. But here again, we are not talking about the main service panel that gets its power from the utility-owned transformer located outside the building. Neither exhibit applies to that situation. I still put the N-G bond and the connection to the building’s grounding electrode conductor at the main service panel, but that is not related to your question.
 
APPRECIATE YOUR INFORMATION YOU HAVE PROVIDED AND ANY FURTHER RESEARCH

APPRECIATE YOUR INFORMATION YOU HAVE PROVIDED AND ANY FURTHER RESEARCH

Charlie,

I appreciate your information about the 480V utility service example feeding large motor loads. We have that for sewage lift station pumps and water boost station pumps.

I am beginning to get a feel for separately derived and will look at the disconnect, bonding and grounding and see how this fits your example.

If you provide more information in your reading, we will not refuse it.

With much great appreciation.

Tom,
San Diego


Two quick comments (I am still reading, and am not ready to offer more yet):

  1. The exhibits are “essentially” identical for all three NECH editions. There are slight changes in the use of a terminal bar internal to the transformer, as opposed to making connections directly to the case. But they are identical in terms of how the electrical connections work.
  2. There is no need to read too much into the phrase “Separately Derived System.” The most common application of an SDS starts when the utility provides 480 volts to a building. That system powers such things as large mechanical loads and some lights. Then we install step-down transformers that give us 120 volts for use in powering receptacles and smaller mechanical loads. The 120 volts was “derived” from the 480V system, and is completely “separate” from it.
 
Rabbit Hole

Rabbit Hole

Your Article 100 definition graphic makes it as simple as it gets.

Article 100 could have said separately derived system does not
apply to the service entrance, then me and my buddy would
have not gone down a rabbit hole for years and years (maybe decades?).

Thank You,

Tom,
San Diego
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Article 100 could have said separately derived system does not apply to the service entrance, then me and my buddy would have not gone down a rabbit hole for years and years (maybe decades?).
It says exactly that! :happyyes: The definition starts with, "An electrical source, other than a service. . . ."

 

don_resqcapt19

Moderator
Staff member
Location
Illinois
The language "other than a service" first appeared in the 2005 version of the definition, but was not at the beginning of the definition until the 2014 code.
Prior to 2005, there was no mention of "service" in the definition of Separately Derived System.
 
Part II System Grounding Vs. Part III EGC and Gounding Electrode Conductor

Part II System Grounding Vs. Part III EGC and Gounding Electrode Conductor

I notice after watching some Mike Holt videos, God Bless him, my question is in the System Grounding section Part II of Section 250 (Grounding), not the EGC and GEC grounding section Part III of Section 250 NEC. This System Grounding section discusses SDSs and these SDSs can apply to a wye source or delta source as part of the derivation for large motors, such as sewage lift stations, water pump stations, etc. System grounding is to account for over voltages created from 7 different sources to correct for the original damage due to overvoltage created by ungrounded systems in the past such as the non return to zero effect when a circuit is opened and the over voltages created by that in the transient decay of the arc.

Myself and my colleague will research all of the SDS issues given all the help you have provided.

Thank You,

Tom,
San Diego
 
Top