Help!! Delta... 3 phase 240v!!

TNGuy81

Member
Location
Nashville TN
:?:ashamed1:Hello to all first off. I am a commercial electrician and recently landed a job doing more industrial than commercial. Anyway, I do not understand three phase 240v... I get the wild leg B phase is high leg but I don't get how A to ground reads 120v, B - G reads 240volts, and C to ground is 120v. Its almost like single phase 240??...????????. How does a a 2 pole breaker get 240v when B phase is 240v?? Or a motor say wye example, 3 phase in peckerhead would be L1-a, L2-b, L3-c, 4-7, 5-8, 6-9.. How do I run 240v 3phase to motor??? Hope that isn't to confusing. I just can't wrap my head around this Delta. Thanks
 

TNGuy81

Member
Location
Nashville TN
thanks

thanks

But wouldn't that be single phase?? @roger, lets say I am landing a standard 2 pole breaker. Would AB = 240v on a delta 3 phase, or BC? Is it possible to run 240v just from B. (theoretically speaking). If B to ground is 240, why do I need A or C?? Or if I landed a single pole on B, would it get 240 or 120v? I'm use to 120/208, 277/480.. Thanks to all
 
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ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
:?:ashamed1:Hello to all first off. I am a commercial electrician and recently landed a job doing more industrial than commercial. Anyway, I do not understand three phase 240v... I get the wild leg B phase is high leg but I don't get how A to ground reads 120v, B - G reads 240volts, and C to ground is 120v. Its almost like single phase 240??...????????. How does a a 2 pole breaker get 240v when B phase is 240v?? Or a motor say wye example, 3 phase in peckerhead would be L1-a, L2-b, L3-c, 4-7, 5-8, 6-9.. How do I run 240v 3phase to motor??? Hope that isn't to confusing. I just can't wrap my head around this Delta. Thanks
B to ground should be 208V. Many places just use the A and C phases with ground as 240V split (single) phase for most of the loads.
 

TNGuy81

Member
Location
Nashville TN
Adave

Adave

Not 208. A-g= 120, B-g= 240, c-g=120
@aDave, thank u sir, that registered. So, regardless of B @ 240v any order of the phases still gets me at 240v, correct? Obviously not considering rotation, I know to switch out a phase/rotate..
 
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ActionDave

Moderator
Staff member
Location
Durango, CO, 10 h 20 min without traffic from wing
Occupation
wire pulling grunt
Not 208. A-g= 120, B-g= 240, c-g=120
@aDave, thank u sir, that registered. So, regardless of B @ 240v any order of the phases still gets me at 240v, correct? Obviously not considering rotation, I know to switch out a phase/rotate..
Yes. Phase to phase is 240V every time.

Got to run for now. The fact this is driving you crazy means you can get this. Hang in there.
 

ggunn

PE (Electrical), NABCEP certified
Location
Austin, TX, USA
Occupation
Electrical Engineer - Photovoltaic Systems
Not 208. A-g= 120, B-g= 240, c-g=120
@aDave, thank u sir, that registered. So, regardless of B @ 240v any order of the phases still gets me at 240v, correct? Obviously not considering rotation, I know to switch out a phase/rotate..
How can that be? In 240 three phase high leg with a grounded center tap between A and C, A and C are 120V to ground and B to ground is 208V. The only way for any phase to be 240V to ground is if the system were corner grounded on one of the other phases, but then two phases would be 240V to ground and the third would be 0V to ground.

Is everything I know wrong? It could happen... :D
 

GoldDigger

Moderator
Staff member
240V wye? Is there such a thing?
Of course there is such a thing.
But whether it is common in the US is another question.
Generally not found. More common are 240 delta (corner ground or ungrounded), 240Y/139 and 208Y/120.

220 wye is common in EU. Which can also be described as 380Y/220, although a residential service may only get one phase.
 

jaggedben

Senior Member
In the interest of answering questions directly...
:?:ashamed1:Hello to all first off. I am a commercial electrician and recently landed a job doing more industrial than commercial. Anyway, I do not understand three phase 240v... I get the wild leg B phase is high leg but I don't get how A to ground reads 120v, B - G reads 240volts, and C to ground is 120v.
Check your voltages again. B-G should be closer to 208V as others have mentioned above. (That is, if this is a typical High Leg Delta, which you seem to be describing, as in the picture roger posted.)

Its almost like single phase 240??...????????.
On A and C, yes, it is just like that. B is only there for three-phase equipment.

On many high-leg systems I've seen, the electrician who installed it put tape or some other marking on all the stabs on the B busbar. That's to discourage someone from installing any breakers to that phase if they don't really know what they're doing.

How does a a 2 pole breaker get 240v when B phase is 240v??
Check your phase to phase voltages. They should all be 240V.

If your equipment requires a neutral then you can only plug that 2-pole breaker in on the A and C busbars.

Or a motor say wye example, 3 phase in peckerhead would be L1-a, L2-b, L3-c, 4-7, 5-8, 6-9.. How do I run 240v 3phase to motor??? Hope that isn't to confusing. I just can't wrap my head around this Delta. Thanks
If the motor requires a wye hookup then you can't use it on this system. A 3phase motor that you want to hook up to this system needs to not require a neutral. Just the 3 hot wires and ground.
 
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