Voltage vs. load calc

[Limart]

Forgive my first clumsy post, but my usual experts are stumped on this one:

Measured existing lighting load at breaker = 8.7A
Measured existing Voltage to said breaker = 90V
(Unknown as to whether voltage was stepped down on purpose or if something else is wrong)
(Unknown specific light fixtures and wattages. Site surveys are difficult to schedule.)

Question: Should the voltage be restored to 120V to the breaker, what becomes of the load should the fixtures remain the same? Is there a calculation to determine this?

Thank you in advance for your assistance.

 

[charlie b]

Lights are resistive loads, so this is a simple Ohm's Law calculation. E = I R. In this case, the resistance will not change. So if you increase E by a factor of 120/90, you will increase I by that same factor. Thus, the original 8.7 amps, times 120, divided by 90, gives you 11.6 amps.

 

[Besoeker]

Lights are resistive loads,

Some are, some are not.
Maybe Limart needs to clarify what sort of lighting it is.

 

[Limart]

Some are, some are not.
Maybe Limart needs to clarify what sort of lighting it is.

Thank you for your prompt replies. I'm hoping that it would be as simple as that. The feed to the breakers is a dimmer that is at 100%. My thinking is that the voltage was dialed back at the dimmer in order to save lamp life at the load. Other than that, the loads are all incandescents.

 

[Besoeker]

Thank you for your prompt replies. I'm hoping that it would be as simple as that. The feed to the breakers is a dimmer that is at 100%. My thinking is that the voltage was dialed back at the dimmer in order to save lamp life at the load. Other than that, the loads are all incandescents.

The dimmer is upstream of the breaker?
What is the voltage fed to the dimmer?

 

[Limart]

The dimmer is upstream of the breaker?
What is the voltage fed to the dimmer?

That is something to be determined. If it is 120V then the dimmer was set to provide 90V out, which is not unheard of in a system of this age. If it's 90V, we have a bigger problem at the transformer.

 

[Besoeker]

Lights are resistive loads, so this is a simple Ohm's Law calculation. E = I R. In this case, the resistance will not change.

Actually, for an incandescent, the resistance changes quite a lot. Lower voltage means lower current, reduced power dissipation, lower filament operating temperature, and lower resistance. It isn't a simple Ohm's Law calculation.

 

[Besoeker]

That is something to be determined. If it is 120V then the dimmer was set to provide 90V out, which is not unheard of in a system of this age. If it's 90V, we have a bigger problem at the transformer.

Where is the transformer in the circuit and why is it a problem?

 

[Limart]

Actually, for an incandescent, the resistance changes quite a lot. Lower voltage means lower current, reduced power dissipation, lower filament operating temperature, and lower resistance. It isn't a simple Ohm's Law calculation.

Is there more information I need to discover in order to determine what the loads truly would be at 120V?

As for the transformer, we are replacing the dimmers and I'm concerned about the new gear being efficient at 90V.

 

[Besoeker]

Is there more information I need to discover in order to determine what the loads truly would be at 120V?

As for the transformer, we are replacing the dimmers and I'm concerned about the new gear being efficient at 90V.

I'm not quite sure I understand your set up. You mention a transformer and dimmer(s). And incandescent lighting.
Incandescent bulbs usually, in my experience, have the rated Voltage and Wattage marked on them. Isn't that your starting point for working our what the loading? At 90V they would take less power.
Not sure now what your question is.....