D.C. cable derating

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3helect

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Do I need to derate cables for conduit fill (80% for three or more) for D.C. battery banks? Are they considered current carring conductors when its stored power? (i.e. UPS batteries) I can see the logic for A.C. with the field creating heat issues. But I'm not sure with dc.
 
I agree that the NEC does not differentiate between AC and DC when it comes to Table 310.15(B)(2)(a).

Just curious, what is the science behind this requirement? Is the heat generated simply from current moving through a wire with some resistance, or is it something related to the multiple magnetic fields produced around the wires as a result of the current? I think I might be overthinking it.
 
drbond24 said:
Current passing through resistance creates heat. It doesn't matter whether that current is direct or alternating.
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True. But it does make a difference to voltage drop. It isn't particularly significant for smaller cables but can be a consideration for larger cables.
For 300 mm^2 (no exact conversion but it would be about the conductor size as 600 kcmil if you had such a thing) the difference in voltage drop from DC to AC is about 30%.
 
Besoeker said:
True. But it does make a difference to voltage drop. It isn't particularly significant for smaller cables but can be a consideration for larger cables.
For 300 mm^2 (no exact conversion but it would be about the conductor size as 600 kcmil if you had such a thing) the difference in voltage drop from DC to AC is about 30%.
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One day you're quoting Shakespeare about the virtues of brevity, and the next day you're posting answers to questions that the OP never asked. I've got a lot of these guys figured out, but I'm still working on you. ;):D
 
Well here something to think about just to add .

Effective value of ac wave dc voltage is really peak voltage at all times constant and ac reaches its peak value only for a fraction of each cycle if we connect a lamp first to dc and then to ac the lamp will light up more brilliantly when connected to the dc .

Heres why dc remains lets say at 100 volts constant whereas ac voltage reaches 100 volts at two points during the cycle in order for the lamp to light with equal brilliance on ac to dc we must raise the ac voltage to 141 volts.


So i agree current yes conductor wise per NEC . Voltage plays a part with ac to dc power so is the conductor running hotter at the same voltage ac to dc and ya voltage drop would be a big part when dc is used.
 
ohmhead said:
Heres why dc remains lets say at 100 volts constant whereas ac voltage reaches 100 volts at two points during the cycle in order for the lamp to light with equal brilliance on ac to dc we must raise the ac voltage to 141 volts.
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The lamp will have equal brightness on 100Vdc or 100Vac rms.
The rms is the effective value. 141V is the peak.
The thermal inertia of an incandescent lamp is usually sufficient to avoid discernable flicker.
 
iwire said:
Now theres a term I have never seen used before.:cool:
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Learning new stuff here is a no-cost option.
Thank you, Mike Holt.
:cool:
Sometimes it gets bandied around in my field of work in place of transient thermal impedance. It's simpler if less precise. I like simple 'cos I are.
Idiot grin......:grin:
 
Besoeker said:
The lamp will have equal brightness on 100Vdc or 100Vac rms.
The rms is the effective value. 141V is the peak.
The thermal inertia of an incandescent lamp is usually sufficient to avoid discernable flicker.
Click to expand...


Well yes but we were using peak only in example not RMS good point .;)
 
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