In our area the poco has a three wire 3 phase service. The service is 240 volt. Two of the legs to ground read 240 volt. One leg reads o volts to ground and is run to a ground rod at the meter. Yet you need a transformer to get 120 volts. The transformer is 480-240 single phase line voltage. Secondary voltage is 240-120. What kind of a system is this.
Three Phase Question.
- Thread starter voltage37
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charlie
Senior Member
- Location
- Indianapolis
Re: Three Phase Question.
The first part of your question is indicating a 240 volt, corner grounded, delta configuration. The slang is a grounded B? configuration.
The first part of your question is indicating a 240 volt, corner grounded, delta configuration. The slang is a grounded B? configuration.
This part, I don't understand.
milwaukeesteve
Senior Member
- Location
- Milwaukee, WI
Re: Three Phase Question.
For purposes of understanding 3phase, remember that phase refers to the phasing of voltage potential between conductors, and does not refer directly to the conductors.
A single phase 240/120V system will have 3 conductors.
A 3 phase 240V system will have 3 conductors. (they actually look the same)
It depends on the location of the ground point inside the utility transformers. The same two high voltage conductors that on the power pole can give you either service.
There are some quirks about 3 phase grounded B however that need attention.
The 'B' phase is your grounded phase, and because it is grounded does not mean that it is ground. It is a current carrying conductor.
The 'B' phase is identified with white just like a standard neutral in a 240/120V system.
Because the B phase is grounded, be careful when you do or do not open the conductor. Generally, you would bypass a disconnect with your B phase conductor (check NEC for proper, conditions may vary).
Because this is 3 phase, equipment must be rated for either 3phase or specifically for grounded B. (ie Cutler Hammer panels are only rated for Grounded B by using a real 3phase panel and 3phase breakers, however, you would not use the middle buss in the panel, rather the grounded bar in the panel)
Proper Identification is best, since 240V Grounded B and 240/120V panels look identical at first glance.
For purposes of understanding 3phase, remember that phase refers to the phasing of voltage potential between conductors, and does not refer directly to the conductors.
A single phase 240/120V system will have 3 conductors.
A 3 phase 240V system will have 3 conductors. (they actually look the same)
It depends on the location of the ground point inside the utility transformers. The same two high voltage conductors that on the power pole can give you either service.
There are some quirks about 3 phase grounded B however that need attention.
The 'B' phase is your grounded phase, and because it is grounded does not mean that it is ground. It is a current carrying conductor.
The 'B' phase is identified with white just like a standard neutral in a 240/120V system.
Because the B phase is grounded, be careful when you do or do not open the conductor. Generally, you would bypass a disconnect with your B phase conductor (check NEC for proper, conditions may vary).
Because this is 3 phase, equipment must be rated for either 3phase or specifically for grounded B. (ie Cutler Hammer panels are only rated for Grounded B by using a real 3phase panel and 3phase breakers, however, you would not use the middle buss in the panel, rather the grounded bar in the panel)
Proper Identification is best, since 240V Grounded B and 240/120V panels look identical at first glance.
Re: Three Phase Question.
Also remember that article 430.whatever requires a circuit breaker pole or fuse in the grounded conductor of the motor branch circuit. This is partly to protect the grounded pole of the motor controller from short circuit return current and so that the branch circuit device can be sized to act as backup for the overload relay.
Also remember that article 430.whatever requires a circuit breaker pole or fuse in the grounded conductor of the motor branch circuit. This is partly to protect the grounded pole of the motor controller from short circuit return current and so that the branch circuit device can be sized to act as backup for the overload relay.
- Location
- Windsor, CO NEC: 2023
- Occupation
- Hospital Master Electrician
Re: Three Phase Question.
So, in the field, is this grounded B phase called a neutral, or is it common to maintain strict language when addressing this conductor?
A 240V 3? load wouldn't notice that B? was at zero potential, would it?
If you have a midpoint-grounded delta sitting next to a corner-grounded delta, I'm guessing that the two motors are identical, right?
That's cool.
So, in the field, is this grounded B phase called a neutral, or is it common to maintain strict language when addressing this conductor?
A 240V 3? load wouldn't notice that B? was at zero potential, would it?
If you have a midpoint-grounded delta sitting next to a corner-grounded delta, I'm guessing that the two motors are identical, right?
That's cool.
Re: Three Phase Question.
voltage37 said:
georgestolz said:
voltage37 said:
You have a single phase transformer with a center tap secondary. Very common on this system.
georgestolz said:
No, we just call it the B phase.
3 phase is 3 phase is 3 phase. Motor doesn't care as long as the three phases are 120 degrees apart.
- Location
- Windsor, CO NEC: 2023
- Occupation
- Hospital Master Electrician
Re: Three Phase Question.
Thanks, tshea.
Thanks, tshea.
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