# 15 Amp circuit capacity

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• posted on October 27, 2003, 3:29 am
Hello, I'm adding some recessed lighting in my basement. The wiring that is there consists of some 14/2 wire and 12/2 wire for the existing recessed lights. There are 4 of them right now. I was wanting to add 6 more on one 15 amp circuit. The bulb themselves would be 65 watt spots. Will one 15 amp circuit handle 6 lights? Can I get away with using 14/2 NM wire or should I use the heavier 12/2 on the new circuit? When all is done I would have 2 -15 amp circuits that have 6 recessed lights each. Is this safe?
Thanks in advance, Joseph Indiana
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• posted on October 27, 2003, 4:37 am

A fifteen ampere circuit will carry a continuous load such as lighting of 1440 volt / amperes. A twenty ampere circuit will carry 1920. Do the math. Divide the circuits available VA capacity by the VA of the installed lamps and you will get the number of lamps that the circuit will carry. As long as the lamps are incandescent the wattage of the lamps can be used as the VA of the lamps. -- Tom
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• posted on October 27, 2003, 6:57 am
Tom Horne wrote:

Hi, VA is not equal to Watts. Reason? Power factor, Cosine Phi. As far as math is concerned. Tony
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• posted on October 27, 2003, 11:13 am
wrote:

As far as a purely resistive load such as an incandescent lamp is concerned, they are indeed equal.
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
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• posted on October 27, 2003, 3:45 pm
Doug Miller wrote:

Hi, Since you used word PURELY, even the filament of incandescent bulb has inductance, how could it be pure R without X? I still stick my gun on my statement. Watts is not equal to VA, better give some margin. Tony
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• posted on October 28, 2003, 3:38 pm
wrote:

OK, why don't you do the math, and tell us how much difference there is between watts and volt-amperes for, say, a 60-watt incandescent light bulb?
-- Regards, Doug Miller (alphageek-at-milmac-dot-com)
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• posted on October 28, 2003, 4:11 pm
Doug Miller wrote:

Hi, Also wiring has inductance. If I install 15A circuit, I'd let it carry ~13A and have peace in mind. Are you going to give full load in your case then? Tony
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• posted on October 28, 2003, 7:03 pm

concerned,
bulb?
Tony, your argument holds about as much water as a tiny piece of pork (fat) in a can of "Pork" and Beans.......interesting how the pork is always listed _first_.
The reason why the NEC limits a continuously loaded lighting circuit (defined by the NEC as a circuit expected to remain on for 3 or more hours) to 80% (that's 12 amps for a 15 amp circuit) is because the circuit breaker will start to nuisance trip when left on for an extended period of time if the full 15 amps is applied continuously, _not_ some ridiculous argument about the minuscule amount of inductance in an incandescent lamp.
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• posted on October 28, 2003, 10:24 pm
On 28 Oct 2003, volts500 wrote:

Uh huh (snicker). And any discussion of this subject (inductance, AC power factor and phase relationships, Xl and Xc) is beyond your area of knowledge, you've already told us that when I brought it up months ago.
I get it now, Volts. You donut understand it, it becomes "ridiculous" and "miniscule".
Stick to wiring whips for air conditioners, you won't strain yourself that way.
--
Baisez-les s'ils ne peuvent pas prendre une plaisanterie
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• posted on October 28, 2003, 11:46 pm

Yeah, I remember tried to tell us that a toaster is an inductive load and a fan motor was a capacitive load. BRAHAHAHAHAHA, I'm still laughing about that. Doesn't surprise me that you also think that an incandescent lamp is a significant inductive load just because the filaments are coiled. ROTFLMAO.
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• posted on October 29, 2003, 1:06 am
I-zheet M'drurz wrote:

Hi, I had an electrician who wired emergency shut down of a big computer room from blue print. When test time came after completing the wiring, I pushed big red button, nothing happened. Ooops! nothing went down. This so called licensed electrician couldn't even read simple relay logic mixed with 24V and 120V control circuits. I had to tell him what to do. Maybe this guy is his brother or co-worker. This kind of folks are trouble makers in the field. I am retired but I still keep my basic knowledge. I don't invent or bend rule of physics. Tony
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• posted on October 29, 2003, 5:21 am

That works two ways don't ya know. I've seen plenty of stamped and signed blueprints that were not correct......everybody makes mistakes. A shunt trip is soooooooooo hard to wire, huh Tony? _DO_ tell us how the HVAC shutdown and the fire dampers were incorporated into the deal.

Make me wonder how _you_ managed to get on top of last year's list of AHR's "Who's Who of Useless Posters."

Geez, you sure about that Tony? _DO_ tell us how that "air" core resulting from coiling the ni-chrome wire for the incandescent lamp filament creates that overwhelming inductance. Funny how the engineers and other professionals who write the NEC consider inductance to be insignificant when making calculations for resistive loads......or is it just that they are morons too?
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• posted on October 29, 2003, 1:00 am
volts500 wrote:

Hi, 15A breaker won't trip at 15A. Depending on what kinds, it has to be over 15A. The delay time is different. Just like fuses, fast blow vs. slow blow. Like 15A fuse does not blow at 15A. Hardly any electrical load is pure resistance. They're mostly inductive load. Never saw a phase correcting capacitor banks in commercial buildings? Inductance causes surge when power is turend on. Tell me one pure resistive load in any utility grid. In non-DC circuit we talk about impedance(combination of resistance and reactance; sum of inductive and capacitive reactance) As an example, again I emphasize example, not real figures, a 60W light bulb has 6 Ohm resistance and 1 micro Henry reactance. Impedance at 60Hz is root of 6+(2 x Pi x 60 x 1/1000,000). Definitely more tahn 6 Ohms. This thread is closed for me. Tony
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• posted on October 29, 2003, 4:07 am

oh my god, you're right!!!!! it's 6.0003 ohms!

yah, I think it is...
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• posted on October 29, 2003, 4:41 am
volts 500 wrote:

(fat)
listed
hours)
breaker
if
Tony Hwang wrote:

It most certainly will trip, given time. That's the whole purpose of the NEC requirement to limit a continuous load to 80% capacity.....prevent nuisance tripping.

I've installed plenty of them. Try a synchronous condenser some time. _DO_ tell me the _correct_ required wire size (THWN) for a 100 kVAR capacitor bank connected to a 3 phase 277/480 volt Wye connected system........that is, if you know how to calculate amps for the given kVAR.

Tell me one pure inductive load in any utility grid.

I can see why.
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• posted on October 29, 2003, 3:06 pm

time
argument
I had a 15 amp breaker that tripped when the load was 20 amps (that's what I calculated, with computers, TV, lights and a 1100 watt blowdrier). I asked the electrician about it and he said breakers trip at different currents, depending on how long they stay at that current. since a breaker works by heating, it will take let's say 10 seconds to trip at 20 amps and maybe a minute to trip at 17 amps, but yes, if left at 15 amps constantly it will trip after a while because it heats up.
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• posted on October 29, 2003, 12:36 am
Have you ever measured the inductance of a lamp filament? It's insignificant.
Boden
Tony Hwang wrote:

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• posted on October 29, 2003, 4:02 am

at 60 Hz, the inductance of a filament is irrelevant. We're not in the microwave range...
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• posted on October 29, 2003, 5:13 am
j j wrote:

Hi, Knit picking. I am RF guy. Tony, VE6CGX
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• posted on October 29, 2003, 3:08 pm

then you know that engineering is based on approximations. at RF frequencies the inductance can't be ignored, but for a power engineer, at 60 hertz, the load is purely resistive. the difference between 6 ohms and 6.0003 is irrelevant in this context.