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Author Topic: Two Electrical Services in one Building  (Read 12413 times)

David Buckley

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Re: Two Electrical Services in one Building
« Reply #20 on: February 05, 2016, 08:12:07 PM »

However, interconnecting the grounds between two different services will create HUGE ground loop currents.
Reminds me of an occasion back in the 80s I was tying in a bunch of dimmers to an (isolated!) panel and got my screwdriver between neutral and the metal enclosure, which produced an impressive spark and a warmed up screwdriver.  It was only a few volts different, but with a hell of a current drive capability, caused by voltage drop across the neutral in the building.


And I've even heard of a few cases where the neutral opened up on a service and that neutral current returned through all the ground/shield connections.
A lot of plumbers get caught by this; neutral fails, and the current return path is throughput the pipework.   Undo the pipework to replace the water meter and plumber is in the return path.  I hear that plumbers now put a car starter jumper cable across the water meter plumbing before undoing to pipework.
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Stephen Swaffer

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Re: Two Electrical Services in one Building
« Reply #21 on: February 05, 2016, 09:55:03 PM »


A lot of plumbers get caught by this; neutral fails, and the current return path is throughput the pipework.   Undo the pipework to replace the water meter and plumber is in the return path.  I hear that plumbers now put a car starter jumper cable across the water meter plumbing before undoing to pipework.

State and local inspectors in my area require and verify that any metallic plumbing be bonded and if the supply line into the building is metallic a bonding jumper must be installed around the meter to prevent this from happening.

Mike,

I understand your concern.  However, code requires any object likely to become energized to be bonded to the service.  So which service?  Code also requires all grounding electrodes in a building to be bonded together.  I don't see how you can not bond the grounds together.

I understand the issue of a ground taking neutral current.  Last summer I dealt with a situation where a piece of romex charred a floor joist for 10 feet.  It was on a proper breaker that appeared to be fine.  In the process of repairing the service (required by the inspector because no one could understand the cause) we discovered that the POCO's neutral was bad up the street.  The wire that was melted went to the gas furnace.  I really think in this case that the #14 romex provided a neutral path through the gas piping up to the next house.  A scary proposition because this was dangerous in both/several homes.

That said, best practice would be to run ONLY HVAC from one service (it would seem to be fully or even overloaded anyway) and run everything else from one service.
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Steve Swaffer

Mike Sokol

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Re: Two Electrical Services in one Building
« Reply #22 on: February 06, 2016, 12:42:33 AM »

Mike,

I understand your concern.  However, code requires any object likely to become energized to be bonded to the service.  So which service?  Code also requires all grounding electrodes in a building to be bonded together.  I don't see how you can not bond the grounds together.

I would agree that you DO need to bond to two incoming service grounds together. However, you probably don't want to have your XLR cable shields be connected between these two systems. There's another thread about pulling power from two different houses to run a single outdoor sound system. That's another reason why you don't want to do it.

My experiments show that even 1/10 of a volt Ground Loop Differential Voltage (GLVD?) on the XLR shield can create ground loop hum in many powered speakers with the pin-1 problem.

Tom Bourke

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Re: Two Electrical Services in one Building
« Reply #23 on: February 06, 2016, 12:55:02 AM »

Two services to one building is a code violation, but as you noted, the AHJ has overridden that.
I really want to know why!!!
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David Buckley

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Re: Two Electrical Services in one Building
« Reply #24 on: February 06, 2016, 06:06:15 AM »

I really want to know why!!!
You really want to know why code mandates one supply to a building (230.2, unless exceptions apply), or why the AHJ overrode the NEC?
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Stephen Swaffer

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Re: Two Electrical Services in one Building
« Reply #25 on: February 06, 2016, 09:52:42 AM »

The reason for only one service is for emergency responders-people may have never been in the building before, but need to get power off NOW.  Best if they don't have to figure how many places they have to go to de-energize the building.

There are a hole list of exceptions-one being "different uses"-it was reasonably common years ago to see 2 services on a home to allow the POCO to meter electricity used for heat and applied a different rate code to that.  So, you might be OK with code IF HVAC is on one service and the POCO charges differently for that.

The bottom line is there must be a technical/practical reason-not just "I wanna".  It would be a first responders nightmare if every home needed an upgrade opted to just add another service instead.  In any case, code requires a plaque/directory indicating all services whenever one of the exceptions is invoked.
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Tom Bourke

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Re: Two Electrical Services in one Building
« Reply #26 on: February 06, 2016, 10:13:37 AM »

You really want to know why code mandates one supply to a building (230.2, unless exceptions apply), or why the AHJ overrode the NEC?
Why the AHJ is overriding the NEC in such a way to create a more dangerous situation.  In my experience upgrading your power service is not a problem unless the capacity is simply not available. 
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Frank DeWitt

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Re: Two Electrical Services in one Building
« Reply #27 on: February 06, 2016, 11:07:43 AM »

Why the AHJ is overriding the NEC in such a way to create a more dangerous situation.  In my experience upgrading your power service is not a problem unless the capacity is simply not available.

I don't understand. You seem to be attempting to find logic, or reasoning in a AHJ.  It just isn't in there.  I'm not even sure it would be permitted. The words "authority" and  "jurisdiction" tend to preclude logic or reasoning.  Rules are rules because I say so
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Tom Bourke

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Re: Two Electrical Services in one Building
« Reply #28 on: February 06, 2016, 11:55:45 PM »

I don't understand. You seem to be attempting to find logic, or reasoning in a AHJ.  It just isn't in there.  I'm not even sure it would be permitted. The words "authority" and  "jurisdiction" tend to preclude logic or reasoning.  Rules are rules because I say so
You are correct on so many levels.  One of our standard replies at work "There you go applying logic to the situation."  As AV techs we are now required to wear bright blue flame retardant clothing to plug in anything over 120V.  Our space has LOTS of L21-30, L21-20, and L6-30.
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Jonathan Johnson

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Re: Two Electrical Services in one Building
« Reply #29 on: February 07, 2016, 11:38:17 PM »

I don't understand. You seem to be attempting to find logic, or reasoning in a AHJ.  It just isn't in there.  I'm not even sure it would be permitted. The words "authority" and  "jurisdiction" tend to preclude logic or reasoning.  Rules are rules because I say so

I'll repeat what the OP said:
Quote
The new HVAC system draws a hypothetical 180A max, and the city was not about to let us upgrade our service (small town politics), but was more than willing to allow us to add a second 200A service.

So the question is, WHY wasn't the city (I presume it is a city-owned electric utility) willing to let them upgrade the service? We can only guess.

I have a few guesses:
  • Over 200A requires demand metering or changes the rate structure, and someone in authority didn't want the organization subjected to that.
  • The AHJ's relative is the electrician doing the job, and said electrician isn't qualified to install larger service, requiring the hiring of some out-of-town contractor, OR the contractor qualified to do the upgrade has lost favor by the AHJ
  • Utility (thinks they) would have to install 3-phase power, and it would require installing a more expensive transformer bank or line extension
  • The city taxes electrical services; two services is double the tax money
  • The city charges exorbitant "impact fees" for new service, but not for upgrades

There is no logical reason for two services if the customer is willing to upgrade the existing service.

If this were my situation, I would probably go for three-phase 120/208V 400A (or greater) service to supply the HVAC system, and a 208V to 120/240V 200A dry transformer off of that panel to supply the existing panel. Or better yet, a service disconnect panel with two breakers, one for the 120/208V HVAC panel, and one for the transformer supplying the existing 120/240V single-phase panel.
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Stop confusing the issue with facts and logic!

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Re: Two Electrical Services in one Building
« Reply #29 on: February 07, 2016, 11:38:17 PM »


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