High current & warm wires

[G._S._Ohm]

I got the example problem working in a spreadsheet. There were some typos in it which I fixed.

3 >enter number of conductors
0.09 > enter insulation thickness in inches
0.292 > enter conductor diameter [DC] in inches
0.576 > enter R-value of insulation on a per inch basis
3.2 >enter A' value
0.19 >enter B' value

2.15 >enter factor from table
0.95 >enter e value
1.315 >enter conduit OD
194 >enter Rdc at 75 C in uOhms
90 > enter TC, conductor temp in C
40 > enter TA, ambient temp in C

0.120 =calc'd I value in kiloamps


So, in principle, by using the OPs values I should be able to tell how hot the conduit should be. Or, given the conduit temp, the current flow necessary to get that temp.

 

[mike33]

Thank you for all the help.

The lamps are 54w T5.
I use a Fluke T5-600. I had never heard of TRMS, but now I want one
I don't have a way to measure the temp of the wires or the EMT. But I have been wanting to buy one those infrared thermometers too :grin:
The fixtures are rated for 120 or 277, but I'm told there's no 277 in the building.

I went back today. When I bypassed all switching in the junction box my 19.5A load became around 16A.

At the last fixture in a run I measured:
98V hot to neutral
104V hot to EMT (ground)
4V neutral to ground

My foreman thinks this isn't a concern because he wants to take one or two fixtures off that circuit and add it to another circuit using a contactor. I liked the suggestion of using contactors for all switching and shared it with him, but it sort of fell on deaf ears.

Is there a risk to the ballast running it at such a low voltage?
Will it affect lamp life?

In one 3/4" pipe we have 12 #12 wires. (4 ungrounded conductors and 8 travelers). Do I need to derate 50% for this? If so, than all four circuits would need to be on 15AMP breakers?

 

[broadgage]

At present, the current exceeds that permitted for continous operation on a 20 amp circuit. If the lights will be operated for more than 3 hours, as is likely, then something needs to be done.
Transfering some lights to another circuit, or contactor switching would be the obvious answers.

Apart from other concerns, what about the cost of the wasted energy ?
It appears that several hundred watts are being wasted, not much point in installing energy efficient fictures if 20% of the energy paid for is being lost in the wiring.
The energy wasted could be worth several hundred $ a year.

 

[ptonsparky]

What is the measured voltage at the panel?

 

[G._S._Ohm]

For 5 ea. 12 AWG copper THHN in your conduit I get 90 C conductor temp. with 0.037 KA flowing and a conduit surface temp. of 68 C for a 40 C ambient.

For 0.0195 kA I get a TC-TA of 13 C so for a T ambient of 40 C your conductor temp would be 53 C.

I used some values from the '99 NEC but I doubt these physical constants for cables has changed.

???

 

[hurk27]

What is the measured voltage at the panel?

Well from these measurements:

At the last fixture in a run I measured:
98V hot to neutral
104V hot to EMT (ground)
4V neutral to ground

I can conclude that it should be 106-108 volts if there is no load on the EGC, he has a 2 volt error between the hot to neutral measurement and the hot to EGC measurement which should give you a 6 volt measurement on the neutral to ground reading.

 

[hurk27]

Never mind the above, I forgot the hot is a longer run then the neutral.

 

[mike33]

measured 122volts at panel - hot to ground.

I'm working with the big boss tomorrow so I will have a chance to drop some of my newly acquired Mike Holt Forum knowledge on him thanks guys!

I told him briefly on the phone that I thought voltage drop was a problem (then explained how that affects amperage) and he was impressed and wants to see my numbers and suggestions.

I am going to stress the wasted energy spending as well.

 

[mike33]

Derate for travelers?

Derate for travelers?

Do I count all wires in a raceway (including travelers) when considering for derating?

 

[PetrosA]

Thank you for all the help.

The lamps are 54w T5.
I use a Fluke T5-600. I had never heard of TRMS, but now I want one
I don't have a way to measure the temp of the wires or the EMT. But I have been wanting to buy one those infrared thermometers too :grin:
The fixtures are rated for 120 or 277, but I'm told there's no 277 in the building.

I went back today. When I bypassed all switching in the junction box my 19.5A load became around 16A.

At the last fixture in a run I measured:
98V hot to neutral
104V hot to EMT (ground)
4V neutral to ground

My foreman thinks this isn't a concern because he wants to take one or two fixtures off that circuit and add it to another circuit using a contactor. I liked the suggestion of using contactors for all switching and shared it with him, but it sort of fell on deaf ears.

Is there a risk to the ballast running it at such a low voltage?
Will it affect lamp life?

In one 3/4" pipe we have 12 #12 wires. (4 ungrounded conductors and 8 travelers). Do I need to derate 50% for this? If so, than all four circuits would need to be on 15AMP breakers?

I took a look at the specs and neither the T5-600 or 1000 are True RMS instruments. What this will mean is that the more harmonic distortion in the mains, the lower the values will be that your meter reads. So where you read 19.5A, it may in fact be closer to 22-23A as the breaker and wires see it since you're dealing with a circuit with non-linear loads (electronic ballasts on the T5 fluorescents). Your voltage readings are likely low as well. Check out this link for a better explanation:

http://www.allaboutcircuits.com/vol_2/chpt_1/3.html

If you do get an IR thermometer the most important place to check the temp will be the breaker. It's really difficult trying to get a reading off a #12 THHN

It sounds like your foreman is going to be hard to convince. Ultimately the strongest argument (more than voltage drop or effect of low voltage on the ballasts or lamps) will be that your circuits and the wiring are overloaded because of the voltage drop = code violation and liability. The breaker will start tripping sooner or later and the wires will get damaged over time. As others have mentioned, there are only two efficient ways to deal with that - increase the wire size or use contactors to shorten the runs. You don't mention whether the 5 circuits share any neutrals, but I would highly recommend you check the amperage on them if they do. There's a high probability that they're more overloaded than the phases.

 

[broadgage]

Do I count all wires in a raceway (including travelers) when considering for derating?

Common sense says that of a pair of travelers, only one should be counted, since only one of a pair can carry current at the same time.

 

[Electric-Light]

Is there a risk to the ballast running it at such a low voltage?

Yes. Ballasts are usually rated. 108-305v. Continuous operation at above or below these limits voids warranty. It maintain full regulation within range. It will try to regulate below 108 and current will go up, then at certain point, it can no longer compensate it (compensation "gas pedal" bottomed out) and output will drop...

Also, when it is trying to compensate for output at lower voltage, current will exceed design rating, so operating outside of acceptable tolerance voids warranty.

 

[iwire]

20 A through #14 copper Romex in free air after 15 minutes finally leveled off at a rise above ambient of 2 C. That's it.

20 A should give (20/15)^2 = 1.8x the rise that 15 A should give.

If your current is sinusoidal this kind of heating is really puzzling.

That sounds low, sure would like to know how close your test matched real world conditions.

Rarely is NM 'free air'

 

[G._S._Ohm]

That sounds low, sure would like to know how close your test matched real world conditions.

Rarely is NM 'free air'

Yes, I'll have to check it against the neher mcgrath spreadsheet. It was real world, 20 A through #14 copper Romex. I powered two hair dryers from the same cord from an elec. dryer outlet.

BTW, with an IR meter, be sure it has adjustable emissivity to compensate for the material, surface texture and color.
http://www.engineeringtoolbox.com/emissivity-coefficients-d_447.html

I don't have an IR meter so I used a thermistor under the Romex jacket for my test.

 

[G._S._Ohm]

Actually my spreadsheet says 21 A, for a TC of 24 C and TA of 22 C. For a sloppily run experiment this is almost too close.

I'll have to try this again while running 30 A through this chunk of #14 copper cable. It should roughly give me (30/20)^2 = ~4 C delta T = TC-TA.

 

[mike33]

Thanks Guys!

Thanks Guys!

It worked! I proposed my plan of reducing voltage drop by way of multiple contactors to my boss and he was very impressed. The wasted energy cost and the possible future ballast warranty issues seemed to be most convincing. It helped that my foreman wasn't there that day .

The foreman installed the contactors today and he couldn't just accept the entire revised plan as was given to him, so he decided to leave the 12 wires in one pipe feeding the power side 3 way switches so that all contactors are still on separate circuits -- disregarding the requirement to derate since the "load to the coils will be very low" on that one section of pipe. Oh well

 

[PetrosA]

Congratulations

 

[mbeatty]

It is very good that you continued to follow this through to a safe and acceptable solution. Way to go! :grin:

 

[johnjohn]

You did not mention that the CB was hot as well. And it probably is. If all else is equal you have too many lights on a circuit. Max amps on 20 amp is 16. You will also experience soon nuisance tripping. I would not put more than 5 on a circuit.

 

[gar]

110305-1511 EST

G._S._Ohm:

One way to measure temperature rise of wire is to use the change of resistance of copper with respect to temperature. This technique is also useful to measure average internal temperature rise in a transformer.

For your #14 Romex experiment a 250 ft length will provide enough resistance to allow moderately accurate resistance measurements. Use both wires in the Romex to generate heat, but use only one for the resistance measurement. Make the one wire a 4 terminal resistor to get accurate resistance measurements. #14 copper is about 2.525 ohms per 1000 ft. So 250 ft is about 0.631 ohms. For annealed copper the coefficient at 20 deg C is 0.00393 and 0.00382 for hard drawn copper. For a 10% change in resistance the temperature rise is about 0.1/0.00393 = 25.4 deg C. This is a wire temperature of 114 deg F. for room ambient of 20 deg C.

I seriously doubt that you only had a 2 deg C wire temperature rise in your experiment.

.