3-phase, 3-wire System with an added unbalanced load

[Tank11]

Greetings,
I have an existing installation with a 480-volt, 3-phase service, 3-wire, 400-amps. No neutral is pulled from the utility transformer. All loads are motor loads so no need for the neutral wire.

The electrician wants to connect a 480-240/120-volt transformer for single phase loads; lighting, receptacles, etc.

Do we need to change the service to be 4-wire and pull the neutral? What will happen to the system if this transformer is connected? Will it cause damage or not work correctly?

Any input is always appreciated.

 

[retirede]

If the system had a neutral, you wouldn’t want to connect it to the transformer primary anyway.

 

[infinity]

The electrician wants to connect a 480-240/120-volt transformer for single phase loads; lighting, receptacles, etc.

As retirede mention no you do not need a neutral. If you're concerned with an imbalance due to the addition of a single phase transformer then just use a 3 phase 208Y/120 transformer instead and balance your 120 volt loads.

 

[garbo]

You would not have to change the service. I would go with a 480 to 120/208 volt three phase step down transformer especially if load will be more then 10% of the 480 volt load. If you pull to much power from two of the three phase legs it will cause A voltage imbalance. .Best to keep voltage unbalance to 3% on three phase motors. Consequences of imbalance are overheating, reduced efficiency and shortened motor life.

 

[dkidd]

Greetings,
I have an existing installation with a 480-volt, 3-phase service, 3-wire, 400-amps. No neutral is pulled from the utility transformer. All loads are motor loads so no need for the neutral wire.

The electrician wants to connect a 480-240/120-volt transformer for single phase loads; lighting, receptacles, etc.

Do we need to change the service to be 4-wire and pull the neutral? What will happen to the system if this transformer is connected? Will it cause damage or not work correctly?

Any input is always appreciated.

I thought that you always needed to run a neutral to the service disconnect.

 

[infinity]

I thought that you always needed to run a neutral to the service disconnect.

Good question, a 3 wire, 480 volt Delta system can be installed without a neutral as an ungrounded system.

 

[Little Bill]

I thought that you always needed to run a neutral to the service disconnect.

That was my understanding. The neutral had to be brought to the main service, but didn't have to go any farther if neutral was not needed.

 

[infinity]

That was my understanding. The neutral had to be brought to the main service, but didn't have to go any farther if neutral was not needed.

What if the system has no neutral?

 

[tortuga]

I thought that you always needed to run a neutral to the service disconnect.

What if the system has no neutral?

250.24(C) Requires a grounded conductor to be bonded at the service disconnect
on a 480V delta secondary it would be 'corner grounded' though it is also possible to have a delta hi-leg 480 and nobody bothered running a neutral as it would be 240V to ground.
or if its a truely ungrounded system
250.21(B) requires ground detectors.

 

[augie47]

In this area, 3 phase 3 wire ungrounded 240v and 480v systems were commonplace for a lot of our industrial facilities at one time.

 

[infinity]

In this area, 3 phase 3 wire ungrounded 240v and 480v systems were commonplace for a lot of our industrial facilities at one time.

Same here. Some old industrial buildings have 480, 3-wire, ungrounded, Delta services.

 

[electrofelon]

. All loads are motor loads so no need for the neutral wire.

.

Well if it's a grounded system, the neutral is your bonding/fault path conductor. That is why is required to be run to the service disconnect even if you are not using it as a circuit conductor. Maybe, hopefully this is an ungrounded system.

 

[tortuga]

The requirement for ground detectors goes beck to the 2005 NEC.
Before 1990 the grounding requirements were strange, If I am reading the 1987 NEC right it appears the 1987 NEC and earlier permitted any combination of 480 delta to not be grounded on the NEC side, even if it was grounded on the utility side.
A utility could have a say provided a 480 hi-leg open delta bank (two legs 240 to ground B phase 416 to ground) and the 1987 NEC did not require a neutral to be run into to the service from the bare grounded messenger wire, but it did require it for a 240V hi leg delta to have a grounded neutral run to the service disco.
So, possibly any 3-wire 480 delta service installed before 1990 could really be either ungrounded, corner grounded or hi-leg grounded depending on what the utility thought was best for their system at the time.

 

[kwired]

I thought that you always needed to run a neutral to the service disconnect.

You always need to run the grounded conductor (if the system has one) to the service disconnect. On systems with a neutral conductor NEC as a general rule will require the neutral to be the conductor that is grounded.

Three phase three wire is likely either ungrounded or corner grounded delta. If corner grounded then the grounded phase conductor must be run to the service disconnect, which if you are running any three phase loads it is going to be needed anyway as it is the third phase conductor.

(Sorry for repeating some of what has already been said)