Using transformer to drop down from 240/480 service to 120 v lights and current calcs

[Plano12345]

Hi all. You all have really helped me a few times, and I'm hoping you can help me one more time.

On a project I'm working on, there are lights for aesthetic purposes being proposed at various locations along the corridor. The electrical services being used are a 240/480 variety. After talking with construction, they have decided on using 120 v/ 75 w lights.

The lighting contractor is stating that for every location of these 120 v/ 75 w lights, we have room to install a small transformer to drop down to 120 volts. In this design, all of these 120 v lights are installed on the same circuit breaker at the proposed electrical service (with no other light fixtures on them).

While I'm familiar with some basic design, I'm not familiar with installing a transformer between a service and a light fixture when it comes to voltage drop calculations. To pick out the correct transformer size, I'm assuming it's no different from figuring out an electrical service. Also, what do I use for voltage drop/current a) from the service to the transformer and b) from the transformer to the light fixture?

I'm just making sure I'm setting up the proposed electrical service with the correct current for the circuit breaker.

For the sake of math, I have 5 light fixtures at 120 V / 75 W, each with their own transformer to drop from 480 volts to 120 volts. What current should I be using at the main electrical service for the breaker?

 

[kingpb]

75W is 75W. When you pick the voltage, you can calculate the current. However, I venture to guess you are using 75W lamps, which may not tell the whole picture on power requirements. You need to know the input power requirement for each fixture.

Next issue is that you are going from single phase 120V (one hot and one neutral) to single phase 480V (two hots, and no neutral). I assume single phase 480V because the OP said 480/240.

You will need to figure the required KVA at 120V and use the transformer KVA size to determine current at 480V.

Based on rough numbers provided; 5 x 75 = 375W @ 120V, is 3.2A 375W at 480V is 0.8A.

Probably going to need to use a 500VA transformer based on your numbers.

 

[Plano12345]

75W is 75W. When you pick the voltage, you can calculate the current. However, I venture to guess you are using 75W lamps, which may not tell the whole picture on power requirements. You need to know the input power requirement for each fixture.

Next issue is that you are going from single phase 120V (one hot and one neutral) to single phase 480V (two hots, and no neutral). I assume single phase 480V because the OP said 480/240.

You will need to figure the required KVA at 120V and use the transformer KVA size to determine current at 480V.

Based on rough numbers provided; 5 x 75 = 375W @ 120V, is 3.2A 375W at 480V is 0.8A.

Probably going to need to use a 500VA transformer based on your numbers.

Assumptions were right. Thanks for the help. Already spoke with the manufacturer so I know the current requirements for the 120 v lights. My biggest worry was what current should I use from the transformer to service, which you answered. Thanks!!!

 

[texie]

Hi all. You all have really helped me a few times, and I'm hoping you can help me one more time.

On a project I'm working on, there are lights for aesthetic purposes being proposed at various locations along the corridor. The electrical services being used are a 240/480 variety. After talking with construction, they have decided on using 120 v/ 75 w lights.

The lighting contractor is stating that for every location of these 120 v/ 75 w lights, we have room to install a small transformer to drop down to 120 volts. In this design, all of these 120 v lights are installed on the same circuit breaker at the proposed electrical service (with no other light fixtures on them).

While I'm familiar with some basic design, I'm not familiar with installing a transformer between a service and a light fixture when it comes to voltage drop calculations. To pick out the correct transformer size, I'm assuming it's no different from figuring out an electrical service. Also, what do I use for voltage drop/current a) from the service to the transformer and b) from the transformer to the light fixture?

I'm just making sure I'm setting up the proposed electrical service with the correct current for the circuit breaker.

For the sake of math, I have 5 light fixtures at 120 V / 75 W, each with their own transformer to drop from 480 volts to 120 volts. What current should I be using at the main electrical service for the breaker?

Your question implies you are only concerned with calculating primary current. I might suggest that you have much more to consider than the line current as there are a number of NEC issues to address. My first question would be how you have a 240/480 service. While possible, it is very unlikely. Second, you can't just install a transformer to supply a load without applying all the required rules regarding transformers. This would include all sorts of grounding/bonding and OCPD rules.

A more standard situation would be a 480/277Y service and the lighting would be 277 volt.

Maybe you could provide more details for this unusual installation?