Outdoor NON-service transformer

[Grouch1980]

Hi,
I put 'non' in caps since most of the previous posts i went through had to do with transformers feeding a building. In my case below, this is a transformer being fed BY a building...

In a particular project i am working on, we are designing for a new feeder to originate from an existing 480 volt switchboard inside a building. The feeder will exit the building, connect to a step-down delta-wye transformer for 120/208 volt secondary, and feed a 120/208 volt switchboard next to it. both the transformer and switchboard are outdoors. I am connecting the GEC from a ground rod to the neutral inside the transformer, and providing a system bonding jumper from the transformer neutral terminal to the grounding terminal at the main switch on the load side of the transformer.

My questions are: (simple questions, but can still get you in trouble)
1. is the 8 foot ground rod all I need as a grounding electrode system for this application?
2. How far away can the ground rod be from the transformer? can it be directly underneath, to the side, 50 feet away, or it doesn't matter?

Thanks!

 

[Carultch]

The term is "customer-owned transformer" for what you are describing. That is, in contrast from a utility-owned transformer which is the case for most service transformers.

The answers:
1. It depends on if you can get 25 ohms or less. The NEC allows one 8 ft ground rod, if the contact resistance to the ground is 25 ohms or less, which will depend on the properties of the soil. Alternatively, you can drive two 8 ft ground rods, a minimum of 6 ft apart, and not even think about Ohms. This is the "two and done rule".

You might think that if one rod gets you 60 Ohms, that driving a second rod would only bring it down to 30 Ohms, and you would be right. But the NEC still calls two rods "good enough", and doesn't require you to continue driving rods to bring it down to 25 Ohms or less.

2. It is best if it is as close to the transformer as possible. Right below it, as part of the transformer's foundation is best. The NEC doesn't provide any guidance on how to determine the upper limit of a GEC's length, and switching to a larger size doesn't really help increase your range. You want to make the GEC as short as possible, with as few bends as possible.

If the transformer is supplying another building, the electrode should be in the building with the transformer, rather than all the way in the destination building. The destination building will have its own grounding electrode system, which will connect with the EGC that supplies it.

 

[Grouch1980]

Thanks! I may just do 3 rods as a tripod then, each 6 feet apart from each other. And I followed everything, except for your last sentence... could you maybe rephrase it? "the electrode should be in the building with the transformer"... which building? source or destination?

 

[bwat]

Thanks! I may just do 3 rods as a tripod then, each 6 feet apart from each other. And I followed everything, except for your last sentence... could you maybe rephrase it? "the electrode should be in the building with the transformer"... which building? source or destination?

You can certainly do 3 if you’d like, but you’ll be hard pressed to come up with a good reason for why you’re doing it if ever asked. The 3 rod triad has become somewhat of the snake oil of the industry.

(Full disclosure.. my company also tends to go beyond the grounding electrode requirements because it makes people feel good for whatever reason)

 

[retirede]

(Full disclosure.. my company also tends to go beyond the grounding electrode requirements because it makes people feel good for whatever reason)

My first job out of school was at a large manufacturing plant. They were in the process of installing a new CNC machine tool and I noticed a ground rod was being driven next to the machine foundation. I asked the old plant engineer why. All he could say was “Oh, these new machines with electronics have to have a good ground”.

Being a new kid, I politely replied “OK”.

 

[electrofelon]

I am connecting the GEC from a ground rod to the neutral inside the transformer, and providing a system bonding jumper from the transformer neutral terminal to the grounding terminal at the main switch on the load side of the transformer.

This is incorrect or you are using the wrong terminology. The SBJ should be at the same point as the GEC connection, in your case the transformer.

 

[Grouch1980]

This is incorrect or you are using the wrong terminology. The SBJ should be at the same point as the GEC connection, in your case the transformer.

Maybe i described it incorrectly... yes, the SBJ and the GEC connection occur at the transformer. The other end of the SBJ I have connected to the grounding terminal of the load side switch on the secondary of the transformer.

 

[Grouch1980]

The answers:
1. It depends on if you can get 25 ohms or less. The NEC allows one 8 ft ground rod, if the contact resistance to the ground is 25 ohms or less, which will depend on the properties of the soil. Alternatively, you can drive two 8 ft ground rods, a minimum of 6 ft apart, and not even think about Ohms. This is the "two and done rule".

You might think that if one rod gets you 60 Ohms, that driving a second rod would only bring it down to 30 Ohms, and you would be right. But the NEC still calls two rods "good enough", and doesn't require you to continue driving rods to bring it down to 25 Ohms or less.

Awesome... thanks!

 

[electrofelon]

Maybe i described it incorrectly... yes, the SBJ and the GEC connection occur at the transformer. The other end of the SBJ I have connected to the grounding terminal of the load side switch on the secondary of the transformer.

The conductor from the tranny to the Disco is s SSBJ. The SBJ connects the egc bus to the neutral/grounded conductor/XO inside the transformer

Redirect Notice

 

[hillbilly1]

Let me guess, EV chargers? Or festival power?

 

[Carultch]

Could you maybe rephrase it? "the electrode should be in the building with the transformer"... which building? source or destination?

Suppose the transformer is in Building A, and all the loads supplied by the transformer are in Building B. The grounding electrode of the transformer secondary's separately derived system should therefore be in Building A.

 

[Carultch]

You can certainly do 3 if you’d like, but you’ll be hard pressed to come up with a good reason for why you’re doing it if ever asked.

I was playing devil's advocate on the "two and done" rule. I don't know what a realistic ohm measurement for a ground rod generally ends up being, and I would expect that it is a rare case that it exceeds 50 ohms, such that 2 rods is typically enough. I made up 60 ohms as a hypothetical ground rod resistance to demonstrate the point that the NEC would still accept just two ground rods despite not being 25 ohms or less.

 

[Grouch1980]

Suppose the transformer is in Building A, and all the loads supplied by the transformer are in Building B. The grounding electrode of the transformer secondary's separately derived system should therefore be in Building A.

Understood. we're looking at placing the transformer at the building B site, this way the feeder leaving building A is 480 volts and much less wiring and conduit will be needed... the run is a few hundred feet. At building B we'll step it down to 208 volts.

 

[Grouch1980]

Let me guess, EV chargers? Or festival power?

It's a building, but more like a temp building... it's a 2 story facility that doesn't sit on grade, but rather elevated about a foot from grade. The building will be there not forever, but for a long while... a few years maybe. From article 225.30 (NEC 2008 cycle), I'm interpreting this to be acceptable, to feed this 'building' from another building... it's all on the same property and under one management. any feedback would be great.

 

[Grouch1980]

The conductor from the tranny to the Disco is s SSBJ. The SBJ connects the egc bus to the neutral/grounded conductor/XO inside the transformer

Redirect Notice

Interesting... I'm on the 2008 NEC code cycle, I'm trying to get a colleague to send me a picture of the wiring that the 2008 handbook shows. I could've sworn that wire from the neutral in the transformer to the grounding terminal in the load side switch was the SBJ. Maybe they changed the terminology after and called it the SSBJ? I'm on the lookout for his email and picture.

 

[Grouch1980]

The conductor from the tranny to the Disco is s SSBJ. The SBJ connects the egc bus to the neutral/grounded conductor/XO inside the transformer

Redirect Notice

ok, I got the handbook diagrams. this is from the 2008 NEC handbook, which is what NYC follows... so i'm following Exhibit 250.13... and in NYC it looks like it's called the SBJ. I think they changed in the later code versions...

 

[kwired]

Hi,
I put 'non' in caps since most of the previous posts i went through had to do with transformers feeding a building. In my case below, this is a transformer being fed BY a building...

In a particular project i am working on, we are designing for a new feeder to originate from an existing 480 volt switchboard inside a building. The feeder will exit the building, connect to a step-down delta-wye transformer for 120/208 volt secondary, and feed a 120/208 volt switchboard next to it. both the transformer and switchboard are outdoors. I am connecting the GEC from a ground rod to the neutral inside the transformer, and providing a system bonding jumper from the transformer neutral terminal to the grounding terminal at the main switch on the load side of the transformer.

My questions are: (simple questions, but can still get you in trouble)
1. is the 8 foot ground rod all I need as a grounding electrode system for this application?
2. How far away can the ground rod be from the transformer? can it be directly underneath, to the side, 50 feet away, or it doesn't matter?

Thanks!

If it doesn't supply loads inside the building where supply comes from then I'd say it doesn't necessarily need to connect to the GEC of that building.

If I understand correctly you are supplying a second building and later mentioned you now were planning to run the primary all the way to that building. That could change things a little, depending on if this transformer is still a stand alone thing or if it is in/on the second building.

 

[Grouch1980]

If it doesn't supply loads inside the building where supply comes from then I'd say it doesn't necessarily need to connect to the GEC of that building.

If I understand correctly you are supplying a second building and later mentioned you now were planning to run the primary all the way to that building. That could change things a little, depending on if this transformer is still a stand alone thing or if it is in/on the second building.

All correct. And the transformer and most of the distribution is located outside that second building, right up against it; the 8 ft. ground rod would be right next to the transformer. And there are some regular breaker panels inside that second building. What would be this change you mention?

 

[electrofelon]

ok, I got the handbook diagrams. this is from the 2008 NEC handbook, which is what NYC follows... so i'm following Exhibit 250.13... and in NYC it looks like it's called the SBJ. I think they changed in the later code versions...

View attachment 2553198

Yeah IIRC, SSBJ is a somewhat new term. But anyway, in practice nothing has changed so it seems like you are on top of it.

 

[Grouch1980]

another related question... I'll need a disconnect switch prior to the transformer at this second building, according to section 225.31 (2008 NEC). This will be my disconnecting means. Section 225.36 requires this switch to be suitable for service equipment, meaning i have to bond the neutral and grounding terminals (which seems useless from what i've been reading)... BUT, since i'm running the primary feeder to the delta-wye transformer, my primary feeder won't have a neutral conductor since the delta terminals of the transformer don't require a neutral... only 3 phase conductors and an EGC. how do i call for this disconnect switch, located on the line side of the transformer, to be service rated if i have no neutral going through it?