There are two different ways (my understanding) of grounding a separately derived system, looking at atricle 250 the neutral and ground connection can either be made within the transformer enclosure or at the first means of disconnect. My question is what way is perfered by the contractor? Is there an advantage to one over the other? The question comes up because the office I currently work in has a grounding detail sheet that is issued with every Job new or exisitng (it is up to the engineer to make any modifications necessary as this is just a starting point) but for all derived systems it shows the neutral / ground connection at the transformer. Is there a distinct advantage to this? This would be more costly correct as the main bonding jumper would need to be sized per 250.66, our specs also call for 200% neutrals this increases the size of conduit also not to metion is there enough room within a transformer to terminate all these connections? One last thing by deriving your system at the transformer (neutral / ground bond) could there be a potential difference between neutral and ground as the neutral will cary the load inbalance and dependant on the distance voltage drop could occur witch would create a difference between ground and neutral (not equpitonteal) Just a question? Thanks in advance for your help.
Derived Systems
- Thread starter mike0w
- Start date
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- Location
- Logan, Utah
That would depend on the contractor. My personal opinion is that I like to install my system bonding jumper in the transformer. My feelings are that it is best to establish an equipment grounding conductor as soon as possible.
The system bonding jumper needs to be sized according to 250.66 regardless of where you establish your system bonding jumper. (see 250.30(A)(1)).
This is no different then the connection at the main disconnecting means.
Chris
- Location
- New Jersey
- Occupation
- Journeyman Electrician
From an installer perspective we prefer to make all of the bonding connections and the GEC connection within the transformer. From what I've seen out in the field this is how almost all transformers are done. But you are correct that this requires a larger bonding jumper when used as opposed to an EGC between the transformer and the disconnect. Not really too much of a factor since the transformer and the disconnect are usually very close together.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
True, but if the bonding jumper is within the transformer than another bonding jumper sized according to 250.66 would be used between the transformer and the disconnect.
- Location
- Illinois
- Occupation
- retired electrician
You need the same bonding jumper either way.infinity said:
Don
- Location
- New Jersey
- Occupation
- Journeyman Electrician
don_resqcapt19 said:
Couldn't the raceway serve as the bonding jumper?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
RUWired said:
But once I've made the bonding connection at the transformer, all of the neutrals and EGC's would be separate beyond that point. So do I need to use a conductor between the transformer and the disconnect or can the conduit serve as the EGC?
- Location
- Massachusetts
infinity said:
Trevor, what OCPD would you base the size of the EGC on?
It's really more of a bonding jumper than an EGC.
Check out 250.30(A)(5)
- Location
- New Jersey
- Occupation
- Journeyman Electrician
iwire said:
I agree, I should have called it a system bonding conductor as outlined in 250.30(A)(1). Which sends you to 250.28(A) which requires it to be a wire, bus, screw or similar suitable conductor. Disregard my other brain fart posts. I don't know what I was thinking about....
- Location
- Windsor, CO NEC: 2017
- Occupation
- Service Manager
I prefer mentally flatulent messages.infinity said:
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I say the latter, absolutely.infinity said:
- Location
- Massachusetts
LarryFine said:
Larry are you saying the conduit can serve as an EGC between a transformer and the first means of overcurrent protection?
I disagree, it is not an EGC it is a bonding jumper, it must meet the requirements of 250.28(A) through (D) which requires a conductor sized per 250.66 not an EMT or EGC sized per 250.122.
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Isn't the bonding jumper the connection between the neutral and the grounding system? A conduit between a transformer's secondary X-0/neutral bond and a panel's enclosure and EGC's does not meet this definition, as it does not serve to interconnect the system EGC with the system's neutral.iwire said:
- Location
- New Jersey
- Occupation
- Journeyman Electrician
That's what I though too Larry until I read this:
This directs you to:
Look like using the metallic conduit is out.
This directs you to:
Look like using the metallic conduit is out.
- Location
- Massachusetts
Imagine a 75 KVA transformer feeding a 225 amp panel with 4/0 copper.
We happen to run PVC between the transformer and the panel.
Obviously there needs to be a conductor run to bond the panel enclosure to XO of the transformer.
If we called that conductor an EGC how would we know what size it has to be?
Table 250.122 is based on the Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc.,
In this case the only OCPD ahead of the equipment is on the primary side of the transformer.
Now in this installation I described above one of the ungrounded conductors somehow shorts to the panel enclosure.
What OCPD is going to have to open to clear that short?
Is a conductor based on 250.122 going to be adequate?
If we could use it Table 250.122 would have us use a 4 AWG copper based on the 225 breaker.
In reality we need a 2 AWG copper based on 250.66 and the 4/0 phase conductors.
We happen to run PVC between the transformer and the panel.
Obviously there needs to be a conductor run to bond the panel enclosure to XO of the transformer.
If we called that conductor an EGC how would we know what size it has to be?
Table 250.122 is based on the Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc.,
In this case the only OCPD ahead of the equipment is on the primary side of the transformer.
Now in this installation I described above one of the ungrounded conductors somehow shorts to the panel enclosure.
What OCPD is going to have to open to clear that short?
Is a conductor based on 250.122 going to be adequate?
If we could use it Table 250.122 would have us use a 4 AWG copper based on the 225 breaker.
In reality we need a 2 AWG copper based on 250.66 and the 4/0 phase conductors.
Only metal raceways are allowed to be "egc's" as per 250.118. Which ever choice is made to make the grounding electrode conductor connection,the conductor between the xfrmr and first disconnect is either a system bonding jumper or an equipment bonding jumper, not an egc.
Rick
Rick
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
Looking at Trevor's quote of 250.30(A)(1), in part:
...and of 250.28:
I maintain that, just as the branch-circuit conduits are also permitted, and quite adequate, for use as EGC's, so is the feeder conduit. Ergo, the feeder conduit is not being used as a bonding jumper, but as part of the EGC system referred to in 250.30(A)(1), above.
The source end of the feeder conduit, properly fitted with a bonding bushing, can be connected to X-0 with, or to, a main bonding jumper as described in 250.28, as well as to any present and necessary GEC's, using an adeequate bus bar or other lug or strip.
I would consider the main bonding jumper to be entirely within the transformer enclosure. The conduit is merely the feeder's EGC. Now if, say, the GEC's are bonded at the source, but the neutral bonded at the panel, that would be using the conduit as a jumper.
Make sense?
infinity said:
...and of 250.28:
infinity said:
I maintain that, just as the branch-circuit conduits are also permitted, and quite adequate, for use as EGC's, so is the feeder conduit. Ergo, the feeder conduit is not being used as a bonding jumper, but as part of the EGC system referred to in 250.30(A)(1), above.
The source end of the feeder conduit, properly fitted with a bonding bushing, can be connected to X-0 with, or to, a main bonding jumper as described in 250.28, as well as to any present and necessary GEC's, using an adeequate bus bar or other lug or strip.
I would consider the main bonding jumper to be entirely within the transformer enclosure. The conduit is merely the feeder's EGC. Now if, say, the GEC's are bonded at the source, but the neutral bonded at the panel, that would be using the conduit as a jumper.
Make sense?
- Location
- Illinois
- Occupation
- retired electrician
Bob,
Don
Take a look at 250.30(A)(1) Exception #2.
Don
- Status