Stepdown transformer

[IndeElect]

I am wiring a 120/240 1 1/2 hp pump motor. The closest power I could locate was a spare 277v ckt in a junction for outside lighting. I plan on installing a 3kva 277v to 120v step down xfmr. It has been quite a while since I installed a stepdown, but if I remember correctly my 277v leg would go to H4, primary grounded conductor to H1, 120v secondary to X4, secondary grounded conductor to X1. Also, I am having on heck of a time finding in how to properly ground the secondary, the xfmr is about 100yds from the building. Any help would be greatly appreciated.

 

[winnie]

You might consider using a 'buck-boost' transformer. This would let you use a smaller (cheaper) transformer, and would eliminate the problem of grounding the secondary.

See for example http://www.phaseconverter.com/spbtransformer2448.html or http://www.solahd.com/products/transformers/buckboost/index.html or

-Jon

 

[petersonra]

If you wired the motor as 240V would you even need to ground the secondary of the xfmr at all?

 

[winnie]

If you create an SDS by using an isolation transformer, then I think you would need to ground the secondary and provide secondary OCPD, or you would have to follow all of the rules for ungrounded systems.

On the other hand, with a buck-boost, you don't have an SDS.

Side question: if you had a 277 to 240V transformer, could you tie one of the input legs to one of the output legs, and call it an autotransformer?

-Jon

 

[augie47]

I would tend to agree that a buck n boost might be preferable,

If you decide to use a 277/240-120 transformer and wire the pump 120v I would think you would have to ground it per the SDS rules (250.30)

If you decide to use a 277/240 transformer and wire the pump 240 I think you are in a debatable area. If the transformer has a 120 tap (unused) an argument can still be made that 250.20(B)(1) would require a SDS grounding system. If you did not attach to the 120v tap, I would think you would have to address 250.21(B)..

add it all up and it's buck & boost time

 

[raider1]

I would tend to agree that a buck n boost might be preferable,

If you decide to use a 277/240-120 transformer and wire the pump 120v I would think you would have to ground it per the SDS rules (250.30)

If you decide to use a 277/240 transformer and wire the pump 240 I think you are in a debatable area. If the transformer has a 120 tap (unused) an argument can still be made that 250.20(B)(1) would require a SDS grounding system. If you did not attach to the 120v tap, I would think you would have to address 250.21(B)..

add it all up and it's buck & boost time

I agree.

Chris

 

[petersonra]

I would tend to agree that a buck n boost might be preferable,

If you decide to use a 277/240-120 transformer and wire the pump 120v I would think you would have to ground it per the SDS rules (250.30)

If you decide to use a 277/240 transformer and wire the pump 240 I think you are in a debatable area. If the transformer has a 120 tap (unused) an argument can still be made that 250.20(B)(1) would require a SDS grounding system. If you did not attach to the 120v tap, I would think you would have to address 250.21(B)..

add it all up and it's buck & boost time

I don't see where 250.20(B)1 can possibly apply.

250.20
(B) Alternating-Current Systems of 50 Volts to 1000
Volts. Alternating-current systems of 50 volts to 1000 volts
that supply premises wiring and premises wiring systems
shall be grounded under any of the following conditions:

I don't see how a transformer dedicated to a single device is a premises wiring anything.

Having said that an autotransformer arrnagment is cheaper and avoids the question altogether.

 

[raider1]

I don't see where 250.20(B)1 can possibly apply.

If the transformer secondary can be grounded so that the maximum voltage to ground does not exceed 150 volts the transformer must be grounded, such as in a 240/120 volt secondary.

I don't see how a transformer dedicated to a single device is a premises wiring anything.

Having said that an autotransformer arrnagment is cheaper and avoids the question altogether.

The definition of premise wiring (System) would IMHO, include the wiring to a pump.

Premises Wiring (System). Interior and exterior wiring, including power, lighting, control, and signal circuit wiring together with all their associated hardware, fittings, and wiring devices, both permanently and temporarily installed. This includes (a) wiring from the service point or power source to the outlets or (b) wiring from and including the power source to the outlets where there is no service point.
Such wiring does not include wiring internal to appliances, luminaires, motors, controllers, motor control centers, and similar equipment.

Wiring internal to a motor, appliance etc.. would not be premise wiring, but the wiring leading to the outlet that supplies the pump would be premise wiring.

Chris

 

[augie47]

I don't see where 250.20(B)1 can possibly apply..

I guess it's a matter of opinions. If he installs a 277/240-120 Transformer it seems to me he has "a system that can be grounded....." as defined in 250.20(B)(1) below:

(B) Alternating-Current Systems of 50 Volts to 1000 Volts. Alternating-current systems of 50 volts to 1000 volts that supply premises wiring and premises wiring systems shall be grounded under any of the following conditions:
(1) Where the system can be grounded so that the maximum voltage to ground on the ungrounded conductors does not exceed 150 volts

I don't see how a transformer dedicated to a single device is a premises wiring anything.

Again, we will have to have a different opinion. When I read the NEC definition of premises wiring it seems to only exclude INTERNAL wiring of appliances, MCCs etc. I would not consider this internal wirinng.

Having said that an autotransformer arrnagment is cheaper and avoids the question altogether.

{Sorry, Chris. I was composing as you answered, but I'm glad to see we agree}

 

[LarryFine]

Indy, welcome to the forum!

I am wiring a 120/240 1 1/2 hp pump motor.
I plan on installing a 3kva 277v to 120v step down xfmr.

1. All other things being equal, I'd opt for 240v operation over 120v operation in almost every application.

2. I agree about using a buck/boost, but, I'd ground an SDS when possible, even if not required by code.