Article 250.20(B)

[E=IR]

This is something I have never seen.

Can anyone tell me if there are any circumstances where Article 250.(B)(2) supercedes the requirement of Article 250.24(A) and 250.24(C) when none of the the conditions of Article 250.21 are met and/or where none of the exceptions of Article 250.22 apply?

I recently visited a site that has two 800A services, 277/480v, 3?, two 4" PVC conduits per service with three 500 MCM conductors only, in each one. Using a clamp on DGM, the impedance of the Utility Company grounding/grounding electrode conductor was 81.5 ohms. I spoke with the AHJ and they do not have jurisdiction at this location; it is all being handled by the owner and GC.

The main bonding jumper consists of a 3/0 bare cu conductor, one-hole mechanical lugs, and a split bolt, none of which appeared to be torqued correctly. The bond begins on the left rear of "Service 2" enclosure, loops to the right rear of of "Service 1" enclosure, then a short jumper from the left rear of "Service 1" enclosure to the Ultility Company "ground" from the three transformers mounted above the services. The impedance of this last 3/0 jumper was 18.3 ohms proving the NEC required main bonding jumper

My opinion is this is a dangerous situation where touch potentials are quite likely; there has already been one instance where lightng took out most of the process computers and the utility company lost one of its' transformers.

Many thanks to Mike for creating this network.

Regards,

 

[augie47]

Please start with a clarification for me. We have no grounded conductor installed with your phase conductors on a 480Y/277 system, correct ?

 

[charlie]

. . . 250.(B)(2) supersedes the requirement of 250.24(A) and 250.24(C) when none of the the conditions of Article 250.21 are met and/or where none of the exceptions of Article 250.22 apply?

Sorry, there are no conflicts in these sections of the Code.

I recently visited a site that has two 800A services, 277/480v, 3?, two 4" PVC conduits per service with three 500 MCM conductors only, in each one.

Since this is (I assume) the serving electric utility's service lateral, the serving electric utility is in violation of:
Rule 092 Point of Connection of Grounded Conductor
B Alternating Current Systems That Are to Be Grounded
1 750 V and below
The point of the grounding connection on a wye-connected three-phase four-wire system . . . shall be the neutral conductor. . . .

The point of grounding on a three-phase three-wire system . . . may be any of the circuit conductors, or it may be a separately derived neutral.

The grounding connections shall be made at the source, and at the line side of all service equipment.

This rule is essentially the same as 250.24(A)

. . . the impedance of the Utility Company grounding/grounding electrode conductor was 81.5 ohms. . .

That is of no consequence since the NESC does not have a 25Ω requirement. The MGN (multi-grounded neutral) has an extremely low impedance because of all the grounding electrodes that are installed by the electric utility.

I spoke with the AHJ and they do not have jurisdiction at this location; it is all being handled by the owner and GC.

You may have to go to the Engineering Manager with the serving electric utility to get this resolved if you can't get anyone to listen to you.

. . . The bond begins on the left rear of "Service 2" enclosure, loops . . .

I do not understand this part of your description.

My opinion is this is a dangerous situation . . .

I certainly agree that this is completely wrong unless this is actually an ungrounded 480Y/277 volt transformer where the neutral point is not brought out and is not grounded. :-?

 

[charlie]

By the way, if this is actually an ungrounded 480Y/277 volt transformer where the neutral point is not brought out and is not grounded, the serving electric utility will not have a grounded conductor and they should only have the three phase wires for the service lateral. All of the grounding electrode system should be done exactly the same as if it were an ungrounded 480 volt delta system.

 

[E=IR]

Article 250.20(B)

Please start with a clarification for me. We have no grounded conductor installed with your phase conductors on a 480Y/277 system, correct ?

Gus, that is correct, only three conductors per conduit riser. If you take voltage measurements between any phase and ground, it is nominal 277v.

 

[E=IR]

Sorry, there are no conflicts in these sections of the Code. I asked the question in that manner because the electrical contractor is claiming he does not have to provide a neutral conductor because "there are not, and never will be, any 277 volt loads".


Since this is (I assume) the serving electric utility's service lateral, the serving electric utility is in violation of:
Rule 092 Point of Connection of Grounded Conductor
B Alternating Current Systems That Are to Be Grounded
1 750 V and below
The point of the grounding connection on a wye-connected three-phase four-wire system . . . shall be the neutral conductor. . . .The Utility Company (UC) is not in violation, their system is grounded via their neutral.

The point of grounding on a three-phase three-wire system . . . may be any of the circuit conductors, or it may be a separately derived neutral.

The grounding connections shall be made at the source, and at the line side of all service equipment.

This rule is essentially the same as 250.24(A)

Correct.

That is of no consequence since the NESC does not have a 25Ω requirement. The MGN (multi-grounded neutral) has an extremely low impedance because of all the grounding electrodes that are installed by the electric utility. I agree about the MGN. The two impedance values were given to illustrate the potentially (no pun) dangerous condition between the premises electrical system and the UC.

You may have to go to the Engineering Manager with the serving electric utility to get this resolved if you can't get anyone to listen to you.




I do not understand this part of your description. I'll try to give a better picture.

The electrical service is outdoors. The UC set two poles, built a platform and set three single phase transformers on it. All of the primary and secondary grounding and bonding of the UC equipment is made to 3/0 copper grounding conductors that run along the platform and down each pole to (I assume) an electrode at the base of each pole. This is what I referred to as the Grounding/Grounding Electrode Conductor (Grounding Electrode Conductor because of the electrical contractors bond to it from Service 1.


The premises electrical service consists of two NEMA 3R enclosures each having an 800A service disconnect (breaker) and other feeder breakers. They are installed on unistrut which is secured to the UC's two poles directly below the transformers. Each service has two (2) PVC conduit risers with three service conductors each (no neutral/grounded conductor) extend up to the service drop conductors (which run horizontally along the platform).

Looking at the front of the service(s), Service 2 is on the left and Service 1 is on the right.

The UC pole on the right is the one where I took the impedance readings with an AEMC model 3710 Digital Ground Resistance Meter (DGRM).


What I called the main bonding jumper is a 3/0 bare copper conductor installed on the back of Service 2 and runs to the back of Service 1; on the opposite side of Service 1, is the 3/0 bare copper conductor I called the Grounding Electrode Conductor; bonded to the UC grounding conductor with a split bolt.


​

I certainly agree that this is completely wrong unless this is actually an ungrounded 480Y/277 volt transformer where the neutral point is not brought out and is not grounded. :-?

See above, it is grounded.

 

[E=IR]

Nec: 250.20(b)

Nec: 250.20(b)

Charlie, I thought I posted a reply to the questions you asked yesterday but do not see it. Did you see it today?

Thanks,

 

[charlie]

. . . I thought I posted a reply to the questions you asked yesterday but do not see it . . .

Joe, until you have a few posts, the moderators have to approve each post. This is to help prevent spam and DIY people from gaining general access to this site. I don't recall how many posts it takes but there will be a lag until you have made that many posts. Sorry.

 

[charlie]

See above, it is grounded.

Therein lies the problem and it is installed wrong by the serving electric utility. Please read this again from the NESC:
Rule 092 Point of Connection of Grounded Conductor
(B) Alternating Current Systems That Are to Be Grounded
(1) 750 V and below
The point of the grounding connection on a wye-connected three-phase four-wire system . . . shall be the neutral conductor. . . .

The point of grounding on a three-phase three-wire system . . . may be any of the circuit conductors, or it may be a separately derived neutral.

The grounding connections shall be made at the source, and at the line side of all service equipment.

This rule is essentially the same as 250.24(A) and I have underlined the part that points out the problem. Either the serving electric utility has to un-ground the neutral point of the secondary of the transformer bank (float the neutral) or they have to take the neutral to the customer. By not taking the grounded neutral to the customer, a dangerous situation has been set up as you have already noticed. :smile: