Corner grounded delta service

[patc]

I'd like to go over what I understand to be the requirements for wiring a 240v. cnr. grounded delta service.
The service may be installed using a 3 pole cicuit breaker, never fuses, grounding and bonding the incoming grounded phase just as you would a neutral in a 120/240, 3w service. The three pole circuit breakers are not slash rated so dual rating won't be a problem.
I have been told that it is okay to install this service in a single phase panel with a 2 pole main and just land the grounded phase on the panel supplied neutral bar. While landing the grounded phase conductor is not a problem, it seems as though the main rating (slash) will be.
If a 3 pole main breaker panel is installed, can a 3 phase circuit be run from this panel, using a rated 2 pole breaker(?) and then taking the grounded phase off of the grounded bar?
I have also had someone tell me that I need to use a "grounded B phase" breaker. I can't find where that is required. Thanks in advance

 

[kiloamp7]

Re: Corner grounded delta service

I am familiar with corner-grounded 3-phase systems & patc, you have some good, thought-provoking questions.
I am not going to try & answer, I'm sure others will.
I just wanted to clarify that this is a service & not a feeder ? You make it sound like this is a new installation. Do Power Co.'s still install this type service in new applications?

 

[jim dungar]

Re: Corner grounded delta service

Yes you can use some (maybe not all but for sure Square D's) single phase panels by landing the grounded conductor on what you are calling the "neutral" bar. Of course, this is not possible if the main breaker is slash rated.

Many (maybe most) normal 2 pole 240V breakers are not rated for more than 5KAIC, this is why you need to have special breakers.

Motor circuits and disconnect must always have three poles even if the main/service is only two poles. Yes this means you would be fusing a grounded conductor if you use fuse instead of breakers..

 

[hurk27]

Re: Corner grounded delta service

Motor circuits and disconnect must always have three poles even if the main/service is only two poles. Yes this means you would be fusing a grounded conductor if you use fuse instead of breakers..

Jim do you have a code for that. I have never heard that a motor needs OCPD's in all three phases? and I have worked on many that even the manufacture installed only a two pole motor control relay. some were three pole with a jumper bar across the middle leg where a heater would be with a "Y" service or a ungrounded delta. What code requires the "B" phase protection? I always though that it is the marking on the breaker determins if you have to use a 3-pole if it only has a 240 or 480 volt rating? But if it has 1?-3? rating it is allowed to be a two pole breaker. If I remember right it is in 240.85

 

[hurk27]

Re: Corner grounded delta service

I just read 430.36 and yes it does require fuses in all three phases. and with the requirments of 430.37 using table 430.37 it requires overload units to be in all three phases but they only have to interupt two of the three legs with what it say's in 430.38.
What confuses me is in most installations the breaker at the feed end of the circuit in most cases is only for short circuit/ ground fault protection. Why would it be nessasary to protect the grounded phase when there are overloads installed that will protect the conductors from overloads? This would be like using a fuse to protect a neutral in a single phase circuit.

 

[patc]

Re: Corner grounded delta service

Yes this is a new installation feeding some motor loads. Is anyone familiar with the "grounded B phase" breakers. I actually found them on a sq. D website but can't find any requirement for them in the code.

 

[jtester]

Re: Corner grounded delta service

quote from hurk27
I just read 430.36 and yes it does require fuses in all three phases.
I believe that 430.36 applies to fuses used for overload protection. If you are looking at short circuit protection and GFP, 430.56 seems to be the reference, it refers to 240.20 which omits OCPD in the grounded phase. 430.111, for disconnects, reinforces the idea that only the ungrounded conductors are required to be opened.
I don't believe that opening the grounded conductor is forbidden, if done properly, but it is not required.
Jim T

 

[petersonra]

Re: Corner grounded delta service

Just how common is corner grounded delta? I can't recall running across it at all, but maybe I would not have noticed.

 

[dereckbc]

Re: Corner grounded delta service

Originally posted by petersonra:
Just how common is corner grounded delta?

It is used a lot in lift and pumping stations.

 

[petersonra]

Re: Corner grounded delta service

I can't recall being in a lift or pumping station recently. if that is where it is commonly used I can understand not having run across it.

 

[tom baker]

Re: Corner grounded delta service

Our POCO won't install a corner grounded delta for a new service. We have one 480V corner grounded and its handy to have to confuse new electricians. They are more dangerous than a 480/277Y as the voltage to ground is 480V and puts higher stress on the insulation.
Our POCO has required a disconnect ahead of the meter on a 480 V service for 25 years. The 2005 NEC just now recognizes this practice.

 

[rbb]

Re: Corner grounded delta service

Two pole circuit breakers used to control three phase loads are required to be marked 1Ph-3Ph per 240.85. (you can find these at the supply house right next to chicken lips).

 

[rbb]

Re: Corner grounded delta service

PATC,

Just kidding about the chiken lips.

A single phase switch is prohibited from being used on a corner grounded delta where required to be horsepower rated. The UL white book states "the switch is not intended for use with motors on circuits having voltages or number of phases different from that shown on the marking".

You can use a three-pole switch with a slug in the grounded phase (usually the b-phase in the metering equipment and c-phase at the switch).

You probably come out better using three-pole circuit breakers with a straight voltage rating. I have never priced 1ph-3ph breakers but I would imagine they are expensive.

[ February 11, 2005, 04:08 PM: Message edited by: rbb ]

 

[neal]

Re: Corner grounded delta service

There is two ways that you can wire a corner grounded delta.check out these codes.250.26(4) 200.6 230.90(b) 250.24(a)(5) 240.22 240.22(1) Where the grounded conductor is disconnected by a switch or breaker it is important that a bonding connection be made on line side of the disconnect or breaker.Soares book on grounding page 63 and 64 will help you.Electrical Grounding 6th edition by Ronald P O'Riley page 90 and 91 . 240.85 250.24 b