LED Lighting Design 1

[CuriousElectron]

Hi Guys,

Do any of you have reference information/examples you can share regarding performing voltage drop calculation on a lighting system consisting of LED light fixtures. I understand light fixtures in an industrial plant are installed in parallel and we can take the light that is installed the farthest from the supply panel and use it as a conservative wire sizing verification.

Should LED fixtures be treated same as fluorescent lights in this type of calculation?

Gracias!

 

[itsmoked]

Start by reading this thread.
thread240-432503

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Check the NEC.
In Canada the combined voltage drop of feeders and branch circuits must not exceed 5% of the circuit nominal voltage.
I understand that the NEC has a similar rule concerning voltage drop.
Protection and minimum wire size go hand in hand.
Consider a circuit protected by a 20 Amp breaker. The minimum wire size is #12 AWG.
A long branch circuit with a distributed load may originate with #8 AWG wire due to voltage drop concerns.
Some distance from the supply where the distributed load has dropped, the wire size may be dropped to #10 AWG.
At some further distance the wire size may be dropped to #12 AWG but in no case may the wire be sized below #12 AWG regardless of the load.
For very long circuits it may be required to calculate the voltage drop for every section of the circuit between loads.

As far as light output from LED fixtures, Many of the drivers are voltage agile and will deliver rated output as long as the supply voltage is above 100 Volts.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CuriousElectron]

Thanks, guys!

Is it normal to use a double-pole breaker to supply lighting fixtures installed on the same elevation? I'd think this approach would help with voltage drop, since I'd have two parallel circuits coming off the same breaker that would split the total load. I'd bring out two neutrals.

Regards,
EE

 

[waross]

Single phase or three phase?
For best safety; two single pole breakers and two neutrals.
For best efficiency; one double pole breaker and a shared neutral.
Not recommended; two single pole breakers and a shared neutral.

Safety; two breakers and two neutrals. Either circuit may be made safe by turning off the corresponding breaker. Only one half of the area is without light.
A two pole breaker; two circuits must be turned off to prevent back feed shocks from open neutrals, twice the area is without lighting.

Efficiency; For the sake of explanation consider two concentrated loads.
The resistance of each conductor is 0.5 Ohms.
Voltage 120 Volts.
Each load draws 10 Amps.
If each circuit has a neutral; the voltage drop is 5 Volts on each conductor for a total drop of 10 Volts.
10V/120V = 8.33% voltage drop.
The losses for each circuit are 10 A x 10 V = 100 Watts for total losses of 200 Watts.
If there is a shared neutral; when both circuits are energized there is no current on the neutral. the voltage drop is 5 Volts on each hot conductor for a total drop of 10 Volts.
10 Volts x 10 Amps = 100 total watts losses.
240V/10V = 100 Watts total.
10V/240V = 4.2% Voltage drop.
Two single breakers and a shared neutral are not recommended.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[CuriousElectron]

Thanks waross for wonderful insight!

I had to do vector analysis to convince myself no current would flow in the neutral if both loads are equal. Unfortunatly, I dont think it is practical for my application to have a shared neutral, as I have to run 2 wires to corridor A and 2 wires to corridor B from the AC panel on the production floor.
Still better then having one single pole breaker supplying all the loads.Then there is combined inrush on LEDs I'd have to worry about..

 

[waross]

In that case, use two single pole breakers and two neutrals. You don't want the lights to go out in corridor "B" when you isolate corridor "A" for repairs.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PHovnanian]

"Two single breakers and a shared neutral are not recommended."

Not code either, IIRC.

"I had to do vector analysis to convince myself no current would flow in the neutral if both loads are equal."

For LEDs, there are going to be power supplies with their associated non-linear characteristics and harmonic currents. The fundamental frequency load currents might cancel, but something is still going to flow. There's also the unbalanced load case, where one fixture fails or the maintenance person removes it.

 

[waross]

PHovnanian;
Many years ago independent breakers with a shared neutral was common.
I remember trying to explain something to an apprentice and seeing his eyes glaze over.
He was back 1/2 hour later white faced and shaken.
"What was that you were saying about a back feed on an open neutral?"
Sometimes you have to wait for teachable moments. grin

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PHovnanian]

"Then there is combined inrush on LEDs I'd have to worry about.."

That's the inrush of the LED power supply (charging a cap). Well-designed supplies incorporate some sort of PTC resistor to limit this inrush. The result being a slightly unnerving start up time when you flip the light switch.

 

[itsmoked]

My living room is lit with 23 LEDs run by a non-PFC driver and it has that same disconcerting delay so I don't think that's the purvey of only PFC drivers. I bend over, open the screwy folding drawer on a father-in-law cabinet, hit the button on a powerstrip, close the screwy complex door, stand back up, and about then the array goes on.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[PHovnanian]

"I don't think that's the purvey of only PFC drivers."

It's not a property of power factor correction. The PTC resistor is usually included to keep from blowing up cheap capacitors due to inrush.

 

[CuriousElectron]

Have any of you guys utilized an external inrush current limiter in LED lighting circuits? I'm wondering if it would be feasible to install it in a JB. The typical thermistor that I've seen sold online doesn't look like it is rugged enough to wire into a circuit by itself, but is rather made for plugging into a TB or a module.

Any thoughts are appreciated.

Thanks,
EE