# 480-120/240 transformer grounding



## Cirus97 (May 19, 2017)

Here's what I've got. I've built a service pole with a 100 amp 480 single phase service with a Square D enclosure and breaker. I'm going underground about 200 ft. to a metal building and feeding a 480 single phase panel. There are 2 feeds from the 480v panel. A 480 volt irrigation pivot and 480-120/240 15 kva transformer that will feed a 16 circuit panel for lights and plugs. I'm grounding the frame of the transformer from the 480v panel. Do I need a ground rod at the transformer from earth to neutral/xo ? Also is the 16 circuit panel considered a subpanel and should I send separate ground and neutral to it or just 3 wires and bond in the sub?I've driven a ground rod at the service pole and bonded there since it's my first means disconnect. I've pulled 4 wires into the 480v panel in the metal building even though there are no 277 loads just to cover my butt.


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## telsa (May 22, 2015)

My Poco won't even feed a 480 V single phase Service.

It'd have to be 100A 240/120.

You're introducing too much complexity for a 200 foot run.


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## RePhase277 (Feb 5, 2008)

You can either bond X0 to ground in the transformer and take 4 wires to the panel, or bond neutral to ground in the panel and just use 3 wires from the transformer. You need to bond the building steel as well. You don't need rods if the steel is a grounding electrode.

Also, if your 480 is single phase, you won't have 277 to neutral. It will be 480/240 if there is a neutral.


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## Cirus97 (May 19, 2017)

RePhase277 said:


> You can either bond X0 to ground in the transformer and take 4 wires to the panel, or bond neutral to ground in the panel and just use 3 wires from the transformer. You need to bond the building steel as well. You don't need rods if the steel is a grounding electrode.
> 
> Also, if your 480 is single phase, you won't have 277 to neutral. It will be 480/240 if there is a neutral.


Ok. Never dealt with sp 480 just 3p 480. The 2 panels are mounted to strut and red iron. Don't believe the steel will be a grounding electrode since it's concrete slab.


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## RePhase277 (Feb 5, 2008)

Cirus97 said:


> Ok. Never dealt with sp 480 just 3p 480. The 2 panels are mounted to strut and red iron. Don't believe the steel will be a grounding electrode since it's concrete slab.


Regardless, you should lug onto a main support and bond the steel.

To recap, you can either bond the neutral in the transformer or the panel but NOT both. And wherever you make that bond is where your grounding electrode(s) land.

Personally, I think I would run a #6 to a lay-in lug on a column and continue on to a couple of rods.


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## joebanana (Dec 21, 2010)

Sounds like what you've got is a separately derived system. So, ground it like a separately derived system. ECG to building steel.


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## Cirus97 (May 19, 2017)

So what about my 480 panel that feeds the pivot, xfmr and sub? I've got the can mounted to strut to red iron. Square D hasn't sent the interior yet. Bonding screw to can and no earth ground?


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## telsa (May 22, 2015)

In all my days, I've never heard of any Poco setting a 480 V single phase Service.

You've got a permit ?


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## Cirus97 (May 19, 2017)

This is a rural area. No permits.


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## Cirus97 (May 19, 2017)

telsa said:


> My Poco won't even feed a 480 V single phase Service.
> 
> It'd have to be 100A 240/120.
> 
> You're introducing too much complexity for a 200 foot run.


Thats nothing. The pivot is 1000' away from the mdp in the pump house..


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## Cirus97 (May 19, 2017)

A little info about our 2 local pocos. I did a job about 5 years ago that involved about 100kw of pole lighting. We wanted sp 480 for the lighting but they said the only way they would give us 480 was 3ph. No deal since it was about a mile way. This is a local coop on this job and they service a lot of farms.


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## RePhase277 (Feb 5, 2008)

I've seen a handful of single phase 480 services. I've also done a few irrigation pumps, but they were 3 phase 480, corner grounded delta.

If your service drop is at the pole 200 feet from the panel, you need to drive some rods and bond them to the neutral. Four wires to the sub at the building, and more rods and building steel bonded to the ground wire you brought in, not the neutral. These rods can be the same rods you drive for the transformer.


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## Cirus97 (May 19, 2017)

When you say 4 wires are you meaning from the service pole to the 480 mdp or the sub on the other side of the xfmr? I'm assuming the 120/240 sub.


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## RePhase277 (Feb 5, 2008)

Cirus97 said:


> When you say 4 wires are you meaning from the service pole to the 480 mdp or the sub on the other side of the xfmr? I'm assuming the 120/240 sub.


Yes, from the service drop to the MDP. There is a meter and diconnect at the drop, right? Then it runs 200 feet to the MDP? If there is a disconnect at the drop, you need a ground wire in the run. You could leave the neutral out if you're not using it.


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## Cirus97 (May 19, 2017)

Yes there is a dp 100 at the service pole but no meter. The poco is gonna primary meter it on there pole.


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## RePhase277 (Feb 5, 2008)

Cirus97 said:


> Yes there is a dp 100 at the service pole but no meter. The poco is gonna primary meter it on there pole.


Ok. That means you ground the neutral at that disconnect and carry a ground wire to your MDP. Neutral is optional. I made this quick and dirty diagram:


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## varmit (Apr 19, 2009)

Are you sure that the irrigation motor is 480 volt single phase? I have never seen such a critter. (or) Are you feeding a VFD with single phase 480 with a 3 phase output? (equally unusual)


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## varmit (Apr 19, 2009)

Where ever your first disconnect is located, is the service disconnect. In your case this is at the pole where the utility attachment (service drop) is located. The 480 panel in the shed, would be a sub panel requiring an EGC to the service disconnect. The 240/120 panel would be a separately derived system requiring grounding/bonding of: transformer XO, building steel, transformer enclosure, panel enclosure, and any grounding electrodes required in the jurisdiction.


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## Cirus97 (May 19, 2017)

Yep 480 single. There's no 3ph on the property. There is a 3ph pump that I also have to hook up that pumps water from the lake to the pivot. I've got a Ronk Add A Phase for that.


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## varmit (Apr 19, 2009)

varmit said:


> Are you sure that the irrigation motor is 480 volt single phase? I have never seen such a critter. (or) Are you feeding a VFD with single phase 480 with a 3 phase output? (equally unusual)


Ok, It was late and I was tired. I have seen a lot of fractional HP 480 motors on HVAC equipment. The large HP, 480 single phase motors, would make sense for irrigation, since a lot of these applications are in locations where
no 3 phase is available. A google search shows companies that make high HP, single phase motors, for this special application. 

I suppose that these motors are common in your part of the country? 
There is not much permanent irrigation in the part of the country where I live. Most of the temporary irrigation pumps around here are tractor PTO driven.


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## Cirus97 (May 19, 2017)

varmit said:


> Ok, It was late and I was tired. I have seen a lot of fractional HP 480 motors on HVAC equipment. The large HP, 480 single phase motors, would make sense for irrigation, since a lot of these applications are in locations where
> no 3 phase is available. A google search shows companies that make high HP, single phase motors, for this special application.
> 
> I suppose that these motors are common in your part of the country?
> There is not much permanent irrigation in the part of the country where I live. Most of the temporary irrigation pumps around here are tractor PTO driven.


There's not all that many pivots in my area of Texas. The man we're doing the job for has a game ranch and cattle. He's not a farmer. He has another pivot that's also fed from his 40 acre lake but it's 1ph 240. Right now he's got a huge diesel pump that feeds then existing pivot. This new pump is 480 3ph 20hp and will feed both. This is the first time I've worked with 1ph 480 from the poco. I thought all 480 was 277 phase to neutral until this job.


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## RePhase277 (Feb 5, 2008)

Cirus97 said:


> I thought all 480 was 277 phase to neutral until this job.


Think about it like single phase 240/120 where you have a single 240 volt winding that is tapped right in the middle to get 120 volts to either end. This is the same thing, except it's 480 across a single winding, which if split in half would give you 240 to either end.

277 comes around because in 3-phase, you divide the phase-to-phase voltage by 1.732 instead of 2.


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