I know everyone will recite the NEC definition of service and SDS but I'm hoping for some different explanations. Imagine a industrial facility which owns the 12.47kV distribution loop around its site and the pad mount 12.47kV-P to 480/277V-S transformers. The secondary sides of each transformer feeds into MCC rooms. Previous engineers have called each of these transformers services but I refer to them as seperately derived systems. Are these considered services or seperately derived systems?
Service vs SDS
- Thread starter ckinney02
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buddhakii
Senior Member
- Location
- Littleton, CO
According to the defenition in the NEC it is an SDS.
Been all over that the last few weeks on a college campus.
I looked at it with a number of inspectors and it always ended up a SDS rather than a service.
Unfortunately, the reason is the NEC definitions.
The wires and electrons don't know the difference
, but the NEC definitions do.
It will be fun when you start looking at the grounding issues
I looked at it with a number of inspectors and it always ended up a SDS rather than a service.
Unfortunately, the reason is the NEC definitions.
The wires and electrons don't know the difference
, but the NEC definitions do.It will be fun when you start looking at the grounding issues
Let's then talk about grounding issues. Let's say the facility consists of multiple buildings. Each buildings has a MCC room. Each buildings' grounding electrode systems are tied together. Are these still SDS? This type of system doesn't meet the definition of a service because the facility owns the loop and it doesn't meet the requirement for a SDS because for a SDS there shall be no electrical connection, including the grounding electrode system.
kwired
Electron manager
- Location
- NE Nebraska
If you have a 480 volt distribution with SDS at each building you don't really do anything different as far as grounding goes - unless you use the neutral of the distribution system as a circuit conductor - then you will need an additional equipment grounding conductor run with all the feeders and you connect EGC to grounding electrodes instead of neutral.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
This is where you are getting it confused. The definition of an SDS excludes connection of the grounded circuit conductor (i.e., the neutral), not the equipment grounding conductor (EGC) and not the grounding electrode conductor (GEC). What you are describing is an SDS.
That said, I have a nearby client, a university with its own primary distribution loop, that has a design guideline document that tells us to treat each building's transformer as a service point.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
In fact the You usually do have a connection between the grounding electrode systems. Usually intentional, sometimes inadvertant. When you bring the GEC for a separately devrived system to building steel that IS part of the service's GES. When you bond to a water pipe that "may become energized" you are connecting the the service GES etc.
- Location
- Illinois
- Occupation
- retired electrician
You have to try very hard to install a code compliant SDS where there is not a solid electrical path between the primary and secondary grounding systems.
I ask more questions only to further learn.
Charlie B - Maybe I'm reading your sentence incorrect. You said "The definition of an SDS excludes connection of the grounded circuit
conductor (i.e., the neutral), not the equipment grounding conductor (EGC) and not the grounding electrode conductor (GEC)." It
looks like the definition excludes "connections through earth, metal enclosures, metallic raceways, and equipment grounding
conductors." It still appears to me that a requirement for a SDS is that the grounded circuit conductor cannot be connected to any
other system grounded circuit conductor or another system's grounding electrode. It seems the NEC definition would eliminate wye-wye transformers from being able to be SDS when the XO/HO is internally bonded.
Charlie B - Maybe I'm reading your sentence incorrect. You said "The definition of an SDS excludes connection of the grounded circuit
conductor (i.e., the neutral), not the equipment grounding conductor (EGC) and not the grounding electrode conductor (GEC)." It
looks like the definition excludes "connections through earth, metal enclosures, metallic raceways, and equipment grounding
conductors." It still appears to me that a requirement for a SDS is that the grounded circuit conductor cannot be connected to any
other system grounded circuit conductor or another system's grounding electrode. It seems the NEC definition would eliminate wye-wye transformers from being able to be SDS when the XO/HO is internally bonded.
meternerd
Senior Member
- Location
- Athol, ID
- Occupation
- retired water & electric utility electrician, meter/relay tech
Just curious. Where is the utility metering point. Is it primary metered before any of the customer owned 7,200/12,470 distribution? Code definition is pretty clear that a service is "from the serving utility to the wiring system of the premesis served", so SDS is the only option.
kwired
Electron manager
- Location
- NE Nebraska
SDS (for a grounded system) needs to have a grounded conductor on the secondary. If the primary also has a grounded conductor you can't avoid connecting them together. All the systems present need bonded together to prevent voltage potential between each system - this includes communications systems, grounded low voltage systems, metal piping systems, building steel, and other items.
The SDS is bonded to other systems, but its operation is not dependent on that bonding.
A non SDS would be an autotransformer - it has one conductor connected to primary as well as secondary and the conductor carries both primary as well as secondary current. The operation of the transformer will be effected if the primary-secondary connection is opened. A good example of such a transformer is a HID lighting ballast. Buck/boost transformers are another example. Neither one of those are a SDS.
Strathead
Senior Member
- Location
- Ocala, Florida, USA
- Occupation
- Electrician/Estimator/Project Manager/Superintendent
When and why would you interanlly ground a wye wye transformer? There are other code sections that would not allow this in most if not all buildings. Since it would create and alternate path to ground on the primary system through the bond at the said transformer.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
For starters, I have a copy of the 2011 at my desk, but I seldom open it, because my state has not (and will not) adopt it. Your comment was related to words added in the 2011 version. That brings in some confusion, because of my use of the word, "excludes." So let me put it another way.
In your post #4, you speak of ground rods and related stuff, and ask about whether the installation is still an SDS. My comment was that the definition of SDS tells us that whether a system is, or is not an SDS hinges on connections of the neutral wire. Putting it yet again another way, connecting parts of the grounding electrode system or connecting parts of the equipment grounding paths does not have anything to do with whether an install is an SDS. The revision in 2011 does not change this, but rather clarifies it.
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