480v phase to phase but 120 to ground.

[Sparky Adam]

Hello,

I am working on a high rise residential building that is fed with 480v. When I went in to disconnect the water pumps I tested the lugs that fed the breakers for the pumps in their control enclosure and found that from phase to phase I had 480 volts, but when I tested from the can to each of the phases my tester read 120 volts. What kind of system is this? I am not familiar with anything like this.

Thanks!

 

[GoldDigger]

What kind of tester are you using?
You cannot have a solid 120V from phase to ground in combination with 480V phase-to-phase.
You have an ungrounded 480 delta system and what you are measuring to ground is just capacitive current across the resistance of your tester.
This type of system requires a ground detector under current NEC.
Is this the direct POCO feed or is it coming from a transformer (SDS)?

 

[Sparky Adam]

I am just using a Knopp tester. This is the feed directly off of the main switchgear. Can you elaborate on capacitive resistance in my tester and why that would happen.

Thanks!

 

[GoldDigger]

I am just using a Knopp tester. This is the feed directly off of the main switchgear. Can you elaborate on capacitive resistance in my tester and why that would happen.

Thanks!

If you are using the K-60, it is putting a 4K load on the terminals.
. So to read 120 volts this means that the circuit can only supply "stray" current of about 30 milliamps.
Since the feed is either ungrounded or high impedance grounded, you are not reading the actual voltage.

 

[Sparky Adam]

Thanks a bunch! I love a job where I am always learning new things. Can you point me to the code section that has the requirement for the ground detector on this system?

 

[augie47]

Thanks a bunch! I love a job where I am always learning new things. Can you point me to the code section that has the requirement for the ground detector on this system?

250.21(B) in the '08 Code.
I would suggest that you investigate a little to confirm your findings. It has been my experience on an ungrounded system to actually read 120v on all three phases is rare.

 

[GoldDigger]

250.21(B) in the '08 Code.
I would suggest that you investigate a little to confirm your findings. It has been my experience on an ungrounded system to actually read 120v on all three phases is rare.

Sparky: Actually, on second thought, it is more likely that the "can" you are using as a reference point is not effectively grounded! That would produce a symmetric reading from all three phases.

 

[augie47]

Sparky: Actually, on second thought, it is more likely that the "can" you are using as a reference point is not effectively grounded! That would produce a symmetric reading from all three phases.

good point

 

[texie]

That has to be more to this. It is extremely unlikely to have an ungrounded 480 delta system in an environment like this. I think you may be right that the OP has a 480 grounded Y and there is an unbounded enclosure he is using as reference point to check voltage. There needs to be more research for sure, both for safety and to like look like a fool.

 

[mgookin]

Just curious: Are we talking about booster pumps for potable water or fire pumps?