Corner Grounding or Center Tap Grounding of 240 Delta Secondary

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I am installing a 480X240 Delta Delta transformer. The secondary has one winding that has a center tap brought out (X4). I'd like to ground the system to maintain a solid reference to ground but I do not have any use for a neutral conductor as we are only feeding straight three phase equipment. Should I corner ground this system or ground X4 and simply not utilize it?

Thanks
 
I would suggest using the corner ground. My reason might not be that important, but here it is: With a corner ground, the highest voltage to ground in the secondary system will be 240 volts. With a center tap ground, the highest voltage to ground will be higher. Sorry, but my brain is not working this morning, or I would be able to tell you how much higher. It?s either 41.4% higher, or 50% higher, or 73% higher, but I don?t remember which.

If you had been talking about a 120 volt delta secondary, this would have been a much greater reason for using the corner ground. That is because in a center-tap ground configuration, the highest voltage to ground would be over 150, and that puts you in a different row of Table 110.26.
 
Thanks for taking the time to respond. As you said with a corner grounded system I would get 240 to ground on two legs and zero on the grounded leg.

The fact is that by grounding the center tap I would have 120 volts to ground on two of the legs and 208 on the remaining. The 208 leg would be my "red leg" or "bastard leg". I've heard numerous other terms used. Anyway, my thought was that it seemed more practical, and in my mind safer, to have reference to ground on all three legs as opposed to only two. What do you think?
 
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charlie b said:
I would suggest using the corner ground. My reason might not be that important, but here it is: With a corner ground, the highest voltage to ground in the secondary system will be 240 volts. With a center tap ground, the highest voltage to ground will be higher. Sorry, but my brain is not working this morning, or I would be able to tell you how much higher. It’s either 41.4% higher, or 50% higher, or 73% higher, but I don’t remember which.

If you had been talking about a 120 volt delta secondary, this would have been a much greater reason for using the corner ground. That is because in a center-tap ground configuration, the highest voltage to ground would be over 150, and that puts you in a different row of Table 110.26.
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Sorry, Charlie. The high-leg voltage is greater than the other two in a center-grounded system, but all three lines are still lower than with a corner ground.

A corner-grounded Delta gives you two 240v-to-ground lines and a grounded line; a center-grounded Delta gives you two 120v-to-ground lines and a 208v-to-ground line.
 
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Sorry it wasn't more obvious: the center ground.

Why, besides the lower voltage? More versatile in the future.
 
MRG, Article 200.2 requires a grounded conductor in all premises wiring.If you choose the high leg delta, you'll need to pull a grounded conductor anyway. If you choose the corner grounded delta, you will lose your three phase power that you need.
Rick
 
RUWired said:
MRG, Article 200.2 requires a grounded conductor in all premises wiring.If you choose the high leg delta, you'll need to pull a grounded conductor anyway. If you choose the corner grounded delta, you will lose your three phase power that you need.
Rick
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Rick

What do you mean by lose your 3 phase power? Three phase power is available here just as in other systems. Grounding one phase doesn't affect the 3 phase configuration at all.

Jim T
 
RUWired,
200.2 goes on to point to many instances that do not require an electrical system to be grounded.
Article 250.20 (B) (1) states that "AC systems are require to be grounded if the maximum voltage between ungrounded conductors to ground will not exceed 150 volts". This is not the case with this 240 delta. I will have 208 to ground if I ground the center tap or 240 volts to ground if I ground the corner.
Article 250.21 (4) states "where this system is not grounded, ground detectors shall be installed on the system". Ungrounded delta systems like this are all over the place. In fact, the transformer I am installing is replacing an existing ungrounded delta system. Problem is, the existing system does not have the required ground detector system and I don't wish to deal with ground detectors. That is one of the reasons I wish to ground the new system.
On the other point, I will stiill have 240 volts 3 phase between phases.
I do appreciate all of your replies folks.
 
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RUWired said:
Jim,I thought that if you are grounding one leg, then it became the grounded conductor.
Rick
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It does, but it has no effect on the 3 phase power.
 
Corner Grounding versus Center Tap Grounding

Corner Grounding versus Center Tap Grounding

On a corner grounded system the grounded conductor is NOT a neutral and carries full load current when the system is carrying full balanced load. You should have a circuit breaker pole in this conductor on any circuit because it needs to be protected against overcurrent. For a motor branch circuit a fuse or circuit breaker pole in the grounded conductor is required. My installations have always used panelboards that have 3 phase busbars with the grounded conductor on one of them so that 2-pole and 3-pole circuit breakers must be used.

If the available short circuit current at the secondary does not exceed 5,000 amps you can use 2-pole circuit breakers that are rated for use on 240 volt ungrounded and corner grounded systems such as a SquareD QO220H. You can also use 3-pole circuit breakers that are rated for use on ungrounded systems.

Better practice with 240 volts corner grounded is to use breakers that are rated 277/480 volts as that gives a superior short circuit rating.

The secondary neutral terminal of a 3-wire delta primary 4-wire delta secondary transformer should not be used for any purpose whatsoever. More than 5% current going to the neutral terminal acts like a turn-to-turn short circuit and causes circulating current in the transformer windings. This can also occur if a 3-wire delta primary 3-wire delta secondary transformer has a heavy single phase load on the secondary. This was figured out after Dd transformer banks starting blowing up. A 3-phase transformer with a 3-legged core is more susceptible to circulating current than 3 single phase transformers or a 3-phase transformer with a 5-legged core.

A better way to get 120/240 volts 4-wire delta off of a 480 volt system is to use 2 single phase transformers in the 3-wire T-primary 4-wire T-secondary configuration. The primary "neutral" point is not connected to ground or the system neutral and must be fully insulated from ground. Conventional primary and secondary protection is required. This does not have fully balanced voltage as does a 3-wire floating neutral wye primary 4-wire delta secondary transformer. The Tt transformer also has more core and coil KVA that a Yd transformer.

A 2-pole circuit breaker that is connected high leg to low leg MUST be rated for use on corner grounded or ungrounded systems.

If you decide to use three 277 volt primary 120/240 volt single phase transformers in the 3-wire wye 4-wire delta configuration there are several things that you must watch. One of them is that the primary neutral point must be insulated from both ground and the primary system neutral. The other is that you need 5 supplemental overload relays because conventional primary/secondary protection does not protect the transformers against overload. You need 1 supplemental relay in each primary lead. You also need a supplemental relay in each half of the center tapped secondary. The contacts of these relays need to be connected in series with the hot wire going to the coil of a primary contactor. The primary contactor needs to be selected on the basis of the primary inrush current which requires a bigger contactor than a motor of similar KVA.

Mike Cole mc5w at earthlink dot net
 
Mike,
You should have a circuit breaker pole in this conductor on any circuit because it needs to be protected against overcurrent.
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It doesn't need to be protected any more than any other grounded conductor. The current in the grounded conductor must also run in one of the ungrounded conductors. If the current is excessive the OCPD in the ungrounded conductor will protect the grounded conductor. One of the advantges of a corner grounded system is that you can (if the equipment is so listed) use cheaper single phase equipment on the three phase system.
For a motor branch circuit a fuse or circuit breaker pole in the grounded conductor is required.
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No, overload protection for the motor is required in all three phases, but this does not require the use of a fuse or circuit breaker in the grounded phase. Standard overload relays in a motor starter will serve the purpose.
Don
 
mc5w said:
On a corner grounded system the grounded conductor is NOT a neutral and carries full load current when the system is carrying full balanced load. You should have a circuit breaker pole in this conductor on any circuit because it needs to be protected against overcurrent. For a motor branch circuit a fuse or circuit breaker pole in the grounded conductor is required. My installations have always used panelboards that have 3 phase busbars with the grounded conductor on one of them so that 2-pole and 3-pole circuit breakers must be used.
Click to expand...

And the last part of your post, that I don't quote because it's very long, shows that you obviously have a good background in transformers. I understood one of the points you were making was that a center tapped delta can not handle a large load on the grounded neutral center tap. Is that correct ?

I do have to agree with Don that some of your statements in the beginning of your post were not correct.

don_resqcapt19 said:
Mike,

mc5w said:
You should have a circuit breaker pole in this conductor on any circuit because it needs to be protected against overcurrent.
Click to expand...

It doesn't need to be protected any more than any other grounded conductor. The current in the grounded conductor must also run in one of the ungrounded conductors. If the current is excessive the OCPD in the ungrounded conductor will protect the grounded conductor. One of the advantges of a corner grounded system is that you can (if the equipment is so listed) use cheaper single phase equipment on the three phase system.

mc5w said:
For a motor branch circuit a fuse or circuit breaker pole in the grounded conductor is required.
Click to expand...

No, overload protection for the motor is required in all three phases, but this does not require the use of a fuse or circuit breaker in the grounded phase. Standard overload relays in a motor starter will serve the purpose.
Don
Click to expand...

Your statement that "For a motor branch circuit a fuse or circuit breaker pole in the grounded conductor is required." is against the NEC. 240.22 does not allow fuses on a grounded conductor and only allows a multipole breaker if it "is designed so that no pole can operate independently". It is a safety hazard to have the grounded conductor OCD open while the ungrounded phase conductors are still energized.

When using fuses you are required to put a "slug", which is a manufacturer listed jumper, in the fuse holder for the grounded conductor. A fuse in that holder is a violation and a real safety issue.

Also your other point that "On a corner grounded system the grounded conductor ..... You should have a circuit breaker pole in this conductor on any circuit because it needs to be protected against overcurrent." The OCD on the grounded conductor will not operate when there is a fault in that conductor. That conductor is already grounded. A fault wouldn't change anything.

The only time an issue would arise is during a phase to phase short. And when/if that occures the OCD on the ungrounded phase conductor will operate and deenergize the circuit.

As far as your statement that "On a corner grounded system the grounded conductor is NOT a neutral". That statement is disputable.

http://www.mikeholt.com/code_forum/showthread.php?p=602259#post602259

David
 
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Getting a little off topic here as the original question was what apporach would you take if you didn't require a neutral but want to ground the system. All of these posts are interesting and seem to be generating some good discussion. My take on overcurrent protection is this:
240.22=No OCD shall be in series with a conductor that is intentionally grounded unless The OCD opens all conductors including the grounded conductor and no ploe can be operated independently. So circuit breakers yes, fuses no.
It also goes on to say the grounded condutor can be in series with an OCD where it is required by 430.36 or 430.37.
430.36= Where fuses are used for motor overload protection, a fuse shall be inserted in each ungrounded and grounded conductor if the circuit supplying the motor is a 3wire 3phase system.
So when a corner grounded system is supplying a motor for overload protection, fuses are OK in all three legs including the grounded conductor.
430.37= Where the motor being supplied is 3phase, 3 wire, three overloads are required. So, one overload device in each phase condutor means the grounded conductor also.
 
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MRG said:
Getting a little off topic here as the original question was what apporach would you take if you didn't require a neutral but want to ground the system. All of these posts are interesting and seem to be generating some good discussion. My take on overcurrent protection is this:
240.22=No OCD shall be in series with a conductor that is intentionally grounded unless The OCD opens all conductors including the grounded conductor and no ploe can be operated independently. So circuit breakers yes, fuses no.
It also goes on to say the grounded condutor can be in series with an OCD where it is required by 430.36 or 430.37.
430.36= Where fuses are used for motor overload protection, a fuse shall be inserted in each ungrounded and grounded conductor if the circuit supplying the motor is a 3wire 3phase system.
So when a corner grounded system is supplying a motor for overload protection, fuses are OK in all three legs including the grounded conductor.
430.37= Where the motor being supplied is 3phase, 3 wire, three overloads are required. So, one overload device in each phase condutor means the grounded conductor also.
Click to expand...

I'm in agreement with almost everything you said. The exception is that you're not making the needed differentiation between OCD and OL.

For instance, this sentence, "It also goes on to say the grounded condutor can be in series with an OCD where it is required by 430.36 or 430.37." is not exactly right. The correct way is, "It also goes on to say the grounded conductor can be in series with an OL where it is required by 430.36 or 430.37."

OCD - Over Current Device is a fuse or circuit breaker placed before the conductor distribution branch circuit home runs to protect the conductors. OCD is what we're talking about when we're talking about the design or installation of a system.

OL - Over Loads, which as commonly called "heaters", are devices that can use a number of different means to prevent an overload on the equipment. OL can be thermal dissimilar metals that bend at different rates and break connection when heated. These automatically come back together after they cool down and are ready to conduct again. The only reason the motor doesn't usually come back on when these are used is because the holding contact on the control circuit drops out when the "heater" first opens. OL only comes into the conversation when we start talking about specific equipment/motors. It's not part of the system or transformer conversation.

This sentence is correct, "So when a corner grounded system is supplying a motor for overload protection, fuses are OK in all three legs including the grounded conductor." as long as everyone involved understands that we're not talking about OCDs.

David
 
dnem,
You're right. I stand corrected.
I know the difference between overcurrent device and overload device. I simply misread the article as I was typing. No doubt the interchanging of OCD and OL makes for a major misconstrusion of the rule and it is important to note that difference.
The mistake having been acknowledged, the point is, there are situations where fuses are allowed in series with a grounded conductor, and the grounded conductor of a corner grounded delta system is just another phase conductor when it comes to fuses for motor overload protection. Also when using devices other than fuses for overload protection of a three phase motor, the overload device must be installed in the grounded conductor just as the ungrounded.
Thanks for picking up on the mistake.
 
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