480 volt grounded B phase ??????

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sdbob

Senior Member
Not tricky at all. Do same thing you would do for any other three phase system, and line to line loads - there is no line to neutral loads in this system.

If you have multiple single phase loads of same rating put 1/3 of them from A-B, 1/3, B-C and 1/3, C-A. Doesn't matter if grounded phase is run through breakers or not, that is how loads need connected to balance the load. If you tie every load to grounded phase it will have more load on it than the other two phases should be about 1.73 times more load - it is not a neutral and doesn't carry imbalanced current like a neutral in a three wire single phase source.

Would it make a difference if the system was derived from two single phase transformers as an open delta?
 

jim dungar

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Would it make a difference if the system was derived from two single phase transformers as an open delta?
No.

Balancing is balancing.

The only real difference between an open delta and a closed delta is the full load capacity of the bank.
 

mivey

Senior Member
No.

Balancing is balancing.

The only real difference between an open delta and a closed delta is the full load capacity of the bank.
As Jim noted, as far as legitimate supply voltages there is no real difference.

However, voltage unbalance becomes an issue when the transformer has a heavy load because the voltage regulation is not as good on the open delta. Even a balanced load on an open delta will have a voltage unbalance. This can be mitigated to certain levels by oversizing the transformer bank but you also have to consider the source impedance, up-line voltage regulation, and load starting start configuration/tolerances.
 

kwired

Electron manager
Location
NE Nebraska
Open delta (whether grounded phase or high leg output) is most often used when there is either limited load or in the high leg delta case where there is quite a bit of 120 volt loads and limited amounts of three phase loads.

If the service capacity is not relatively small and/or is relatively balanced with higher capacity loads the transformer bank is much more likely to be full delta.

Or in the absence of all three primary phases being present you will need larger transformers than would normally be used for full delta if phases are to be relatively close to balanced.
 

don_resqcapt19

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.... I've seen both 3-wire and 4-wire drops on the exact same system. I've seen where A, B, and C are delivered by the POCO and B is bonded at the service. I've also seen 4-wire drops, where A,B,C and a grounding conductor are delivered. The grounding conductor is nothing more than a 2nd B phase. My assumption is the utility believes providing this 4th wire ensures B gets bonded at the customer's service, since this configuration is so rare and often misunderstood.
I have never seen that extra conductor.
If it's a 3-wire service and the bonding jumper between B and the customer's GEC is either missing or resistive than shock hazards abound as current tries to flow from the customer's GEC through the earth to get back to the grounded phase on the utility's transformer, yes, like any other grounded conductor, with the exception that this grounded conductor carries a MUCH higher voltage than grounded conductors in a wye or center tapped system.
Even without a customer side grounding connection, the current would not be flowing through the earth unless there is a ground fault. The grounded conductor is "grounded" and has little voltage to "earth" unless there is a problem with the grounded conductor. Yes, an open grounded conductor will have a voltage that is equal to the phase to phase voltage, but if the grounded phase system is 240, then that voltage would be less than when you have an open grounded conductor on a 480/277Y system.
... But while we're on the subject, what is the right[/] way to wire single phase loads from a corner grounded system? Install a single phase panel and connect the grounded phase to an insulated buss labled "B Phase"? Install a 3-phase panel and connect the white wires to a breaker? If you use a 3-phase panel; what issues arise from imbalance assuming it's an open delta? I would think that you'd only want to connect the single phase loads to the legs that have windings, and not to the open leg. Which would mean using a single phase panel to supply the single phase loads.

.Anyone?...Anyone?...Bueller?

Assuming the panel in question is the panel that contains the service disconnect, there is no need for the grounded conductor to be on an isolated or insulated bus. If it is not the service panel then the grounded phase needs to be landed on an isolated bus just like any other system that has a grounded conductor. Single phase loads on this system are really line to line loads and any two phases can be used. If the grounded phase is one of the phases, then you would only need a single pole breaker. If the single phase circuit is not using the grounded phase, then you would need a two pole breaker.
Note that not all single phase equipment is listed for use on grounded phase systems and the breakers would have to be fully rated breakers, and not "slash" rated breakers.
 

sdbob

Senior Member
Thanks for your input Don. And I first saw that 4th wire about 6-7 years ago, which was the first time I stumbled across this system. Check out this page from the SDG&E Service Manual. Again, my assumption has always been this redundant wire is provided in the event an electrician unfamiliar with the corner grounded system is too squeamish to land a 480 Volt phase conductor to ground.
 

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sdbob

Senior Member
I have never seen that extra conductor.
Even without a customer side grounding connection, the current would not be flowing through the earth unless there is a ground fault.
\

And I'm guessing there's a ground fault. The OP stated "Lamps on poles". Which led me to assuming they're fed underground, and it wouldn't be difficult at all to get current leaking into a water filled conduit with 480 volts pushing it. Given the voltage fluctuations he was experiencing combined with complaints of shock despite the fact that breakers feeding those poles were off, my first thought would be current leaking to ground somewhere else trying to get home. But who knows, lol? That's why Im really hoping the OP surfaces and fills in the blanks.
 

kwired

Electron manager
Location
NE Nebraska
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And I'm guessing there's a ground fault. The OP stated "Lamps on poles". Which led me to assuming they're fed underground, and it wouldn't be difficult at all to get current leaking into a water filled conduit with 480 volts pushing it. Given the voltage fluctuations he was experiencing combined with complaints of shock despite the fact that breakers feeding those poles were off, my first thought would be current leaking to ground somewhere else trying to get home. But who knows, lol? That's why Im really hoping the OP surfaces and fills in the blanks.

There may be a ground fault, but I still think the biggest problem is open grounded conductor someplace, and it is complicated by improper grounded to grounding conductor bonding beyond the main bonding jumper.
 

ptonsparky

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There may be a ground fault, but I still think the biggest problem is open grounded conductor someplace, and it is complicated by improper grounded to grounding conductor bonding beyond the main bonding jumper.

That is the problem in a nutshell. I don't know why the concept is so hard for so many, be it a corner ground or a single phase residential.
 
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