Pad mounted transformer

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Jpflex

Jpflex

Electrician big leagues
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Victorville
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Electrician commercial and residential
Me and another guy at work will be attempting to properly cabinet ground (not system ground and not derive a neutral by owner’s choice) a delta concrete pad/slab 3 phase 4800/480 volt? transformer

There is the typical rebar 20 feet connected within the concrete lab connected to a typical copper rod with a stub up bend to be connected to transformer metal BUT NOT AT SYSTEM SECONDARY NEUTRAL POINT XO since this will be an ungrounded SYSTEM

There is a ground bolt connected to a wide thin ground copper looking strap with a hole inside it which looks to connect to a ground rod or grounding conductor somehow or some type of connector needed?

I have not checked if this connects to system XO / neutral point yet but if it does I cannot connect here but need to go to cabinet and bond somehow

Lastly the transformer will be powering all 3 phase motors and control circuits for motor starters but no line to neutral loads without another step down transformer downstream - another situation which may impede ungrounded system permissibility

From what I know the system must be installed with ground detectors in transformer secondary since this will be considered an ungrounded system

Ultimately how do we connect this ground rod to this pad mounted transformer? Thanks
 
What kind of system is the 4800 and what kind of conductors are used?

Basically an ungrounded system is the same as grounded in that you have egc's/SSBJ's with both primary and secondary conductors and a GEC which all go together and to the transformer case. Only thing you wouldn't have is a system bonding jumper that connects the neutral / grounded conductor / XO to all the ground/Bond wires.
 
The only difference between grounded and ungrounded system as far as bonding and grounding goes, is that the ungrounded system does not have a main bonding jumper if it is a service or does not have a system bonding jumper if it is a separately derived system. Everything else is the same.
 
don_resqcapt19 said:
The only difference between grounded and ungrounded system as far as bonding and grounding goes, is that the ungrounded system does not have a main bonding jumper if it is a service or does not have a system bonding jumper if it is a separately derived system. Everything else is the same.
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Utility is feeding transformer primary. Secondary would not have a derived neutral nor system bonding jumper as you stated.
 
Jpflex said:
Utility is feeding transformer primary. Secondary would not have a derived neutral nor system bonding jumper as you stated.
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Why are you choosing a 480V delta ungrounded secondary? Why not go with a Wye secondary and use an impedance ground?

My experience has been most electricians less than 50 years of age are not skilled in maintaining or troubleshooting ungrounded systems, and those that are will likely be retiring relatively soon.
 
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I am very surprised the utility supplies you with a 480 delta. All of the pad transformers on our mnay POCOs are wye.
Did you specifically request delta ?
 
augie47 said:
I am very surprised the utility supplies you with a 480 delta. All of the pad transformers on our mnay POCOs are wye.
Did you specifically request delta ?
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The owner makes these decisions even if us electricians advise him otherwise.

The delta transformer will work for the 3 phase motors but there will be less line to neutral load options plus the delta high leg

All I remember is our engineer saying primary utility was 4800v being stepped down to 480v secondary by this delta pad mounted transformer
 
electrofelon said:
So this is a utility transformer then also...why are you taking it upon yourself to ground their transformer?
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Power poles are mine site and a utility substation is upstream so I do not know who owns poles downstream from substation. The transformer mentioned is purchased and chosen by company.

Owner does not likely understand system bonding jumpers or deriving neutrals. He just wants 3 phase wires coming out
 
augie47 said:
I am very surprised the utility supplies you with a 480 delta. All of the pad transformers on our mnay POCOs are wye.
Did you specifically request delta ?
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Company owner chose delta at this mine site not covered by NEC

I follow NEC and am trying to get my hours for licensing
 
Jpflex said:
Power poles are mine site and a utility substation is upstream so I do not know who owns poles downstream from substation. The transformer mentioned is purchased and chosen by company.

Owner does not likely understand system bonding jumpers or deriving neutrals. He just wants 3 phase wires coming out
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Can you clarify where the service point and service disconnect is?
 
Jpflex said:
The owner makes these decisions even if us electricians advise him otherwise.

The delta transformer will work for the 3 phase motors but there will be less line to neutral load options plus the delta high leg
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You will not have any neutral at all. There will not be any high leg.

The padmount transformer will have an equipment grounding mounting pad/plate on it for you bonding connection.

Mining locations regularly have ungrounded distribution systems, so their people are usually quite familiar with them. You might want to look into MSHA requirements and not just the NEC.
 
jim dungar said:
You will not have any neutral at all. There will not be any high leg.

The padmount transformer will have an equipment grounding mounting pad/plate on it for you bonding connection.

Mining locations regularly have ungrounded distribution systems, so their people are usually quite familiar with them. You might want to look into MSHA requirements and not just the NEC.
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I only mentioned delta high leg because another worker chose to phase tape one leg with orange tape. I told him to place this on b phase in case in the future a neutral is added

Although there is no neutral derived, voltage from b phase to the middle of one phase winding will still be higher than the rest and is something to consider for future alterations

I don’t want to study MSHA because my ultimate goal has always been to become NEC journeyman licensed. I need a couple more years for my 8k hours prior to test qualification
 
Jpflex said:
I only mentioned delta high leg because another worker chose to phase tape one leg with orange tape. I told him to place this on b phase in case in the future a neutral is added

Although there is no neutral derived, voltage from b phase to the middle of one phase winding will still be higher than the rest and is something to consider for future alterations
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The only why you can have a leg with a higher voltage to ground is if one of the secondary windings of the delta transformer has a center tap and that center tap is connected to earth. While a high leg is possible with a 480 volt transformer, they are rare. Two of the line to neutral voltages would be 240 volt, and the third would be 416. Also if this is a single pad mount transformer you will be typically be limited to neutral current that does not exceed 5% of the transformer full load current. If this is a transformer bank with two or three single phase transformers that limit would not apply.
Jpflex said:
I don’t want to study MSHA because my ultimate goal has always been to become NEC journeyman licensed. I need a couple more years for my 8k hours prior to test qualification
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But the MSHA requirements rule the electrical installation at a mine, not the NEC.
 
The only why you can have a leg with a higher voltage to ground is if one of the secondary windings of the delta transformer has a center tap and that center tap is connected to earth. While a high leg is possible with a 480 volt transformer, they are rare. Two of the line to neutral voltages would be 240 volt, and the third would be 416. Also if this is a single pad mount transformer you will be typically be limited to neutral current that does not exceed 5% of the transformer full load current. If this is a transformer bank with two or three single phase transformers that limit would not apply.

But the MSHA requirements rule the electrical installation at a mine, not the NEC.
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How do you determine transformer full load current or kva, is it full amperes available from each system secondary winding x 3 windings for total transformer KVA?
 
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