NEC AC Services 250-24

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zabawire

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Please refer to the attached drawing as a reference to the following:

A customer with a residential premise has elected to electrically feed his/her electrical system in a manner as shown on the attached drawing. The electric utility will be providing a 240/480V, single-phase, 3w service up to the utility's meter. At that point, the customer will be utilizing a 480v, single-phase, 2w electrical system to feed a customer owned transformer where the voltage will be stepped down to a 120/240v single-phase, 3w system. As I understand, the customer owned transformer along with the conductors on the primary and secondary sides have to be grounded as per NEC articles 250.24 (A) and 250.24 (2). Am I interpreting the code appropriately? In addition, what article in the NEC governs the size of the primary and secondary CONDUCTORS FOR SUCH INSTALLATION? Please advice and thank you in advance.
 
First the conductors on the load side of the meter are not service conductors...they are feeders. You will need an EGC run with the single phase 480 volt feeder conductors between the two transformers. In my opinion, you need to install a service disconnect and OCPD between the meter and the first transformer. The wire size will be based on the load.
 
don_resqcapt19 said:
First the conductors on the load side of the meter are not service conductors...they are feeders. You will need an EGC run with the single phase 480 volt feeder conductors between the two transformers. In my opinion, you need to install a service disconnect and OCPD between the meter and the first transformer. The wire size will be based on the load.
Click to expand...

Why does an equipment grounding conductor is required?
 
zabawire said:
Why does an equipment grounding conductor is required?
Click to expand...

2008 NEC section 300.3(B) and all of 250.4 (and take a close look at 250.4(A)(5)
One reason you need to run the EGC from the meter to the transformer is, if a phase conductor on the high side was to touch the metal case of the transformer there would be no low impedance path back to the source. Now this would cause the transformer housing to be energized and anyone that touched it and was grounded would get shocked.
The scenarios go on and on, but "RETURN PATH IN A FAULT" is the reason.

Saving Baby's by safe grounding and bonding.:grin:
 
Ok I think there is a confusion,
This statement:
customer owned transformer along with the conductors on the primary and secondary sides have to be grounded
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makes it sound like the customer owns both the transformer feeding the meter and the transformer after the meter, but if I got it right your only talking about the transformer after the meter, right?

Is this a USA installation? because it is very odd that a power company would allow a residential dwelling to be fed with 480 volts, liability is just too high?

if it is, and is allowed, then yes you must have a service rated disconnect with OCPD's set between the meter and customers transformer, the system neutral conductor must continue to this point and be terminated in this disconnect, this will be your service disconnect, also all grounding electrodes will have to land in this disconnect, then a bonding conductor sized by table 250.66 must be brought to the transformer from the neutral in the disconnect along with the two 480 volt conductors, this is your fault current path, this will bond the transformer case and also be connected to any down stream panel on the secondary side of the transformer.

Very strange for this to be on a dwelling?:-?

Maybe others will chime in if I got anything wrong;)
 
don_resqcapt19 said:
First the conductors on the load side of the meter are not service conductors...they are feeders. You will need an EGC run with the single phase 480 volt feeder conductors between the two transformers. In my opinion, you need to install a service disconnect and OCPD between the meter and the first transformer. The wire size will be based on the load.
Click to expand...

Why are the conductors from the meter to the transformer feeders and not Service conductors? Where's the overcurrent device?

Can't the grounded conductor be used to ground things (either the disconnect or transformer)?

I agree that a disconnect between the meter and transformer would be a good idea. Then it is feeder from there.
 
suemarkp said:
Why are the conductors from the meter to the transformer feeders and not Service conductors? Where's the overcurrent device?

Can't the grounded conductor be used to ground things (either the disconnect or transformer)?

I agree that a disconnect between the meter and transformer would be a good idea. Then it is feeder from there.
Click to expand...

grounded conductor must be ran to the first disconnect means, 250.24(C) and be used for bonding and grounding.

Not sure what would be required if the transformer is outside, as a service disconnect would only apply to the conductors entering the building?
 
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suemarkp said:
Why are the conductors from the meter to the transformer feeders and not Service conductors? Where's the overcurrent device?

Can't the grounded conductor be used to ground things (either the disconnect or transformer)?

I agree that a disconnect between the meter and transformer would be a good idea. Then it is feeder from there.
Click to expand...

His drawing doesn't show the grounded conductor running out of the meter to the XFMR.
 
I got a feeling this is one of those installs where they have a problem of voltage drop, long run from the utility's transformer to house (customer doesn't want to give right a way to utility for primarys), so the utility (who posted) is allowing a 480 volt single phase feed all the way close to the house where the customer supply's a transformer to change over to a 240/120 volt system, then enters the house just like a regular service, the utility does not want the transformer ahead of the meter because of the wasted VARs, so they put the meter ahead of it out by the road, so the 480 doesn't even enter the house, Now home owner owns everything from the meter to the house so no right of way needs to be given.

now does the transformer need a disconnect? I wouldn't think so but if it is considered a structure, and the conductors do enter it, I say yes, does it need a neutral? same thing if considered a structure and requires a disconnect, then the system neutral has to land at the disconnect, along with a ground rod or two, the transformer needs to be bonded to the neutral in the disconnect also, now since this is a structure, then a 4-wire feeder will be required to the house and grounds and neutrals will have to be kept separate in the house panel.
Now the house will also be required to have a disconnect, and electrodes.

Now if this is an attempt to stop stray voltages? and keep the two systems isolated? if done by the NEC wont work, since the bonding conductor from the system neutral will still connect all the way through to the house grounding anyway?

Ok I think I covered everything possible, I think?:confused:
 
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The Utility owns the transformer on the line side of the meter. The customer has elected to go that route to mitigate stray voltages within his property. It appears their consultant has advice them to set their own transformer near the premise to convert the 480v single-phase, 2w to 120/240v single-phase 3w voltages.
 
zabawire said:
The Utility owns the transformer on the line side of the meter. The customer has elected to go that route to mitigate stray voltages within his property. It appears their consultant has advice them to set their own transformer near the premise to convert the 480v single-phase, 2w to 120/240v single-phase 3w voltages.
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I hate to say it, but if the stray voltage is comming from the MGN, this route is not the answer, because in a NEC compliant install they will still have a connection to the MGN at the pole, they have to have a fault current path for the transformer case from the neutral at the meter, then there would have to be a bonding conductor from the transformer XO to the house panel grounding, and if the house panel is existing all grounding and neutrals will have to be seperated, like a sub panel.

If this stray current was from the MGN a neutral blocker would have been the choice between the primary and secondary neutrals,

but if the stray voltage was from voltage drop on the service neutrals then this method will work.
 
hurk27 said:
...
now does the transformer need a disconnect? I wouldn't think so but if it is considered a structure, and the conductors do enter it, I say yes ...
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The transformer is customer owned and its installation will have to comply with Article 450. I don't see how you can comply with 450.3(B) without out a fusible disconnect or a breaker on the primary side of the transformer.
 
hurk27 said:
Ok I think there is a confusion,
This statement:
makes it sound like the customer owns both the transformer feeding the meter and the transformer after the meter, but if I got it right your only talking about the transformer after the meter, right?

Is this a USA installation? because it is very odd that a power company would allow a residential dwelling to be fed with 480 volts, liability is just too high?

if it is, and is allowed, then yes you must have a service rated disconnect with OCPD's set between the meter and customers transformer, the system neutral conductor must continue to this point and be terminated in this disconnect, this will be your service disconnect, also all grounding electrodes will have to land in this disconnect, then a bonding conductor sized by table 250.66 must be brought to the transformer from the neutral in the disconnect along with the two 480 volt conductors, this is your fault current path, this will bond the transformer case and also be connected to any down stream panel on the secondary side of the transformer.

Very strange for this to be on a dwelling?:-?

Maybe others will chime in if I got anything wrong;)
Click to expand...

Customer only owns transformer on load side of meter. A potential USA electrical installation? To be determined.....

I concur with rest of the statements and the transformer installation shall be in accordance with NEC article 450 as well. I am not sure how this is going to be accomplished. However, everyone may be able to share pros and cons for such installation. Thank you for all of your feedback
 
zabawire said:
Customer only owns transformer on load side of meter. A potential USA electrical installation? To be determined.....

I concur with rest of the statements and the transformer installation shall be in accordance with NEC article 450 as well. I am not sure how this is going to be accomplished. However, everyone may be able to share pros and cons for such installation. Thank you for all of your feedback
Click to expand...

That's why we are here, to share our oppinions, wanted or not.:grin:
 
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