DOUBLE DERATING PER CODE ?

[EEAOC]

I am working on emergency egress lighting design for a half a mile tunnel. The temperature is about 154 Deg. F. Because of this higher temperature, the 20 amp circuits usually takes No 12 AWG were derated to No 10 AWG per NEC.

Again if I derated the same 20Amp circuits for 5 % Voltage Drop per NEC, the 20 Amp circuits sized to 4/O which is almost makes it too large to be connected to a light fixture that is made for No 12 wiring.

If anyone had similar experience, please let me know what should be best approach under this condition. Should I ignore the voltage drop and go with No 10 AWG that is rated for the heat conditions only?

 

[dpc]

If your voltage drop calcs show that you need #4/0 to keep voltage drop at 5%, that pretty much trumps the temperature de-rating. How in the world could you ignore that?

Trying to run a 20A (120V?) circuit for 2500 feet is generally never going to work. You need to run a higher voltage, like 480V or 600V.

 

[rbulsara]

Or use booster (auto) transformers at regular intervals...

 

[EEAOC]

Is there a manufacturer who makes a PAR lamp or any Incandescent lamp at 240 Volts? if posible in medium base?

Is there a local [US] manufacturer who makes a booster transformer for this purpose ?

 

[itsmoked]

Yes certainly there are 240V PARs. But this whole thing is starting to sound pretty bizaar. Why would you need to light a tunnel that is too hot for a human to use? Why would you want to add to that heat with incandescent lighting?

LED replacements for incandescent lights cost a bloody fortune. (I know, I have a $300 one shining into my aquarium) They aren't stunningly bright either but they could greatly reduce your wiring. (I mean GREATLY!) Floodlights that are 2.5W - stuff like that. Do try one before committing to them though. They'll probably last 10 times longer too.

http://www.whiteled.net/datasheet.asp

http://www.ledtronics.com/datasheets/25mm_medbase_index/25mm_med_index.htm

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[jghrist]

The load current and voltage drop for 240 volt lights will be the same as for 120 volt lights on a balanced 3-wire circuit.

 

[EEAOC]

I don't have a choice. These are steam tunnels. The lights are meant for emergency egress to meet life safety code in the US.

I cannot have anything with the ballasts because of the heat.
There are no PAR lamps at 240 V in the US. Howeeverthere are regular incandascent lamps are avaialable at 240 V.

The voltage drop may be the same at 240 V, but I am reduce the wire size so it can be installed with out special orders.

Even with 240 V, my wire size at 5% V drop is about 2/0.
which is too large for a 20 amp lighting circuit.

 

[Laplacian]

Could you use a constant current transformer the way steet lighting used to be fed? No voltage drop to worry about. Besides the runway lighting transformers, there is still one manufacturer of the old Westinghouse type constant current transformers. I believe they are called specialty transformers inc. (

http://www.specialtytransformers.com/CCRs.htm)

I don't know where you get the fixtures and lamps though, but I'm sure they are still available.

 

[dpc]

I think you could run 480V or 600V and put in some mini-power centers to distribute power at 120V at intervals.

This will be far better than trying to boost the 120V power as you go.

 

[waross]

I agree with dpc completely.
You don't have to double de-rate.
I would calculate the maximum run length with #10 wires and see how many mini distribution centers you need. If the lighting is going to be on at all times, consider a three wire circuit. You may then use 120 volt lamps, but calculate the voltage drop at 240 volts. You will be able to go four times as far on your #10 cables, with the same load. However, more length means more load so you won't get four times the length, but you will be able to run your circuits considerably farther with three wire circuits.
Once I had done the calculations and pricing with #10 and Mini distribution centers, I would re calculate with #8s and see if the saving on the number of mini distribution centers needed would ofset the higher cost of the larger wire.
respectfully

 

[jghrist]

What is the actual load amps? You do not have to calculate voltage drop with the circuit rating amps. Also, the NEC does not require a 5% voltage drop. The NEC rule FPN advises that this give reasonable efficiency. You don't have to have an efficient system. If you can boost the voltage at the beginning and live with lower voltage at the lamps (if the lumen output is sufficient), you can get by with more than 5% voltage drop.

I wouldn't put mini distribution centers in a steam tunnel unless absolutely necessary.

Don't forget that most terminals are designed for 60 deg C. Your ambient, without conductor heating is 67 deg C.

NEC Table 9 impedances are at 75 deg C. If you use the full derated capacity of a 90 deg C conductor, you will need to adjust the resistance of the wire for voltage drop calculations.

If the larger conductor is only needed for voltage drop, you can reduce the size at the end to allow termination.

 

[bigbillnky]

EEAOC
I admit I did not do the calculations, but 4/0 seems to be way oversized, all things considered. Voltage drops in the NEC are recommendations only, not a requirement. Keep in mind that local codes CAN require compliance with voltage drop limitations. Refer to the AHJ if in doubt.

Yes, use a high temp cable. If a leak occurred in the steam lines, I would expect the temperature would exceed 154*F and potentially damage standard building wire. It could lead to the failure of the lighting just when the lights would be needed for the safety of the repair crew. I would not attempt to derate THHN to compensate for this ambient temperature.

Installing additional unnecessary equipment in this kind of enviroment will simply create a maintenance nightmare in an attempt to keep a safety system in working order.

Also consider installing circuits from both ends of the tunnel, working toward the middle.



Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[DaveScott]

I've got some ideas:
1. Use mineral insulated cable
2. Use the bigger cable, as you have already calculated, but, local to the light fitting, use a junction box and connect a local small cable to the light. The volt drop in the small cable is minor.

 

[ScottyUK]

Also consider installing circuits from both ends of the tunnel, working toward the middle.

Also consider using a ring system where the string of lamps is fed from both ends, but the supply is from from a single source. It costs an extra run of cable to the remote end, but should make a significant improvement to the volt-drop problem without requiring excessively large conductors. This arrangement is common for socket outlet circuits in domestic and light commercial applications in the UK.

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I don't suffer from insanity. I enjoy it...