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How much transient power can you really pull from an amp on a 15 or 20 amp circuit?

post #1 of 14
Thread Starter 
I've engaged in a useless argument with a guy at work who claims you can't pull more than 20 amps from a 20 amp breaker without it tripping.

He says it's impossible my Crown XLS-5000 watt amp is using 5000 watts because i've got it hooked to a 20 amp line that can't support that much power.

I've told him that the power use is transient in nature, limited to dynamic peaks, and that while I could trip the breaker easily with the Crown XLS-5000 amp on my 20 amp circuit playing sine waves or compressed music at high volumes ( I have on several occsasions) -- this isn't an issue while movie watching, even though the Crown may possibly be pulling more than 5,000 watts at times without tripping the breaker.

Does anyone have any good reference material to read on this subject?

How much power can you pull off a 15 amp circuit before tripping it --- how about a 20 amp circuit/breaker?

I do understand it's just momentary draw that the breakers allows to pass before they trip - but what types of amp current are we talking is feasible?

I've read this stuff over the years in different threads, but finding any of it is difficult without knowing the proper search terms, or even terminology in which to get meaningful results.

Links to other discussions you are familiar with would be fine...

Thanks!
post #2 of 14
Some of the initial transient power is supplied by the capacitors in the amp's internal power supply, so less of a spike is seen by the circuit breaker. Of course, that really means that the total load is spread out over a longer period of time.

Modern thermal-magnetic circuit breakers trip because they get too hot. If the overload is brief enough, the circuit breaker won't get hot enough to trip. For details, see the "Trip Unit" discussion which starts on page 14 of
http://www.industry.usa.siemens.com/services/us/en/industry-services/training/self-study-courses/quick-step-courses/Documents/circuit_breakers.pdf
post #3 of 14
I found this article interesting. More info than my head could handle at once but it really puts things in perspective



http://www.audioholics.com/audio-amplifier/the-all-channels-driven-acd-amplifier-test
post #4 of 14
There is a formula: Volts x amp = watts, so 120 volt average for a home times 20 amp circuit will come out to 2400 watts. A 15 amp circuit can go up to 1800 and a 20 amp circuit can go up to 2,400 watts which is the trip point of the circuit. They can go slightly higher for a quick transient. You can't get near 5000 watts out of a 20 amp circuit. In my room I run a Klipsch RF 7 HT, Pioneer Elite avr, Acurus 5 channel amp, TV, BDP, cabel box, Behringer 3000 DSP amp and two 18 in. sub of a 15 amp circuit and at -20 the system uses 1.7 amps or around 120 watts. In a movie at Reference level with sub 20 Hz LFE I may go up to around amps 8 on rare occasions. I also have to more 18 in subs on a second 15 amp circuit and another Behringer amp.

It also depends on room size with subs. Larger rooms will make the subs work harder. But, this system was in a room over 7500 cu ft. Massive gobs of power just don't really happen with regular HT use. I mention the other two 18 in. sub because this increased headroom in the room. When I only had 2 subs the amps shot up one time to close to 15 amps while watch The Grey, which has some 5 Hz content. This movies pushes my watt/amp meter higher than any other movie and I have all the 5 star LFE movies for the most part.

BeeMan or Basshead in the sub forum may have the exact numbers for the OP. The DSP unit are class D amp that use less power. Bash amp don't use much power like Class A/B amps for subwoofers.
Edited by derrickdj1 - 1/27/14 at 6:22pm
post #5 of 14
OP, your friend is basically right. The circuit will be limited to 15/20 amps depending on the breaker and its response time. Further though, is that the "5000" is what the amp delivers not what it draws from the feed circuit. As others indicated, "transients" can readily be "served" by storage elements in the amp's circuitry along with other design elements along with "wonderful marketing hype". The spec. for the amp is set at 20 ms burst which is relatively short. In addition the output voltage isn't limited to 120 volts but is maxed at 180.
In terms of input power, check the fuse on the unit, that's its limit before any "magic" is done by the internal circuitry / design or marketing.
post #6 of 14
Selden's points are right on, but to elaborate, indeed transient power draw can easily exceed the max amp rating of a breaker or the wattage rating of an amp. It's the high sustained loads that usually trip the breaker. But even 2500 watts might take a few minutes to trip a 20 amp breaker, typically a little less for a fuse.

If you find you're stuck with a circuit that trips on spikes from that huge Crown, you might consider a motor rated breaker because they respond slower to spikes.

BTW, as for power in and out, wattage is the same minus inefficiencies. It's voltage and amperage that are inversely proportional for inputs and outputs.

Budwich, your sentence about output voltage doesn't make any sense or it was a typo.

For the OP, maybe you like saying your Crown XLS 5000 is rated at 5000 watts, but if you're using it in stereo and powering 8 ohm speakers then we've solved your puzzle, it's rated at 1100 watts per channel or 2200 total, easily within the range of a 20 circuit. The clue is the power cord receptacle on the back of your amp. If it has a UL label it won't draw 5000 watts on a household circuit.

Anecdotally I can tell you about testing my Sunfire True Sub with a hot/neutral splitter, an amp clamp and my Fluke multimeter. With kettle drum strikes in Aaron Copeland's Fanfare for the Common Man, initially I was seeing 15/16 amps but finished with a high of 21 but the AVR's volume was never more than about 75%. So the math says Bob Carver's little candy bar size amp in that sub was indeed rated realistically at 2700 watts. But the point remains, they were only transient.
post #7 of 14
Thread Starter 
I'm using the Crown XLS-5000 at 2ohm stero on a pair of JTR caps and there is no doubt in my mind it puts out 2500 watts to each subwoofer, having used and experienced a lot of different subs and amps.
post #8 of 14
Quote:
Originally Posted by Archaea View Post


He says it's impossible my Crown XLS-5000 watt amp is using 5000 watts because i've got it hooked to a 20 amp line that can't support that much power.

What makes you think your dissipating 5000 watts?
post #9 of 14
Quote:
Originally Posted by Archaea View Post

I'm using the Crown XLS-5000 at 2ohm stero on a pair of JTR caps and there is no doubt in my mind it puts out 2500 watts to each subwoofer, having used and experienced a lot of different subs and amps.

I can certainly understand why you are having a "useless arguement" with your friend... :-)
post #10 of 14
Quote:
Originally Posted by Archaea View Post

I'm using the Crown XLS-5000 at 2ohm stero on a pair of JTR caps and there is no doubt in my mind it puts out 2500 watts to each subwoofer, having used and experienced a lot of different subs and amps.

Just for fun you might want to actually do some measurements related to that.

The first thing to consider is the possibility that your speakers are not perfect 2 ohm loads.

Here is the impedance curve of a fairly typical subwoofer - I can't find one for exactly a JTR Captivator:



To try to get this approximate evidence into our current context let's say that the flat part of the curve to the left of the peak goes down to 2 ohms.

So, what is the average impedance of this speaker while playing music? Well that big peak at 28 Hz has something to do with it. Its right in the middle of our operating frequency range if we consider typical music or drama. I'll invoke the usual rule of thumb that the average impedance of a speaker is about 150% of its rated impedance and let someone who disagrees with the rule of thumb do a credible job of disproving it. ;-)

So our best estimate and approximation of the 2 ohm speaker is a 3 ohm resistor which does a dramatic job of cutting down on the average power delivered to the speaker by the amplifier.

Now, lets consider bass music waveforms. An important characteristic of music wave forms is their crest factor or peak-to-average ratio. The crest factor of pure sine waves used on a test bench is 3 dB, and that is how the amp is rated. The absolute minimum crest factor of music is about 6 dB and goes up from there. Simply plugging in the absolute minimum crest factor cuts the power at least as dramatically as the average impedance of the speaker did.

Put crest factor and average speaker load impedance concepts together and the 5 Kw amp is putting out maybe a quarter of its rated power with sine waves and into a load resistor whose resistance is the same as the nominial impedance of the speaker. That assumes that every wave of the signal maxes out at the peak unclipped voltage that the amp can put out. Since when have any of us seen bass music or drama waveforms that go to the max, cycle after cycle, all afternoon?

Perhaps you may see why I am skeptical that your XLS 5000 puts out 2500 watts average power to each subwoofer. Average power is what heats heat sinks and voice coils, and trips circuit breakers.

Just for grins one might put a tenth ohm 25 watt resistor or several of them in parallel, in series with the speaker and measure the current it is actually receiving and see.
post #11 of 14
Thread Starter 
Look --- the useless argument started with the fact that he said the breaker 'will not let more than 20 amps pass'. I said more than 20 amps can pass for very short periods of time - I think this is given. The question posed was how much current can pass momentarily on a 15amp circuit? How much on a 20amp circuit.

As to why I think my XLS-5000 amp can actually deliver 500 watts.

I've heard many exceptionally well received amps - including the Speakerpower 4000 watt amps. The Crown XLS-5000 is in line with the output I've experienced on the speakerpower 4000. I've owned and used a couple EP4000 amps, as well as multiple iNuke amps. These EP4000 and iNuke amps have been actually tested and are a known quality. The EP4000 is well tested at 2ohm load. The Crown XLS-5000 puts out SIGNIFICANTLY more SPL than the EP4000 did in two ohm stereo. Gorilla83 measured it at 5-7dB on multiple songs with his pair of caps and videoed the results and uploaded them here. My experience is similar, about 6dB more than the EP4000 in the same 2ohm stereo config. The Crown XLS-5000 is no slouch. We both had two Captivator subs - but generally speaking that 6dB increase is at minimum double the power output of the EP4000 at 2ohm. Gorilla83 could throw 20 amp breakers at will. So can I. So can carp on his CV-5000 with his eight SI-18. That's my testing experience. If you have dynamic peaks in a movie for instance, where the SPL is still very high the breaker won't throw at the same levels that the breaker will throw at sustained playback with lesser dynamic peaks (WOTW comes to mind). That's my experience.
post #12 of 14
Thread Starter 
I stumbled on this post by FOH, which is exactly the kind of discussion I've read about in the past, but couldn't find easily in searches.

http://www.avsforum.com/t/1496782/ep4000-for-279/30#post_23891973
Quote:
Originally Posted by FOH View Post

Also, to accommodate motor loads, and prevent nuisance tripping, a typical residential circuit breaker can easily pass enormous amounts of current past the normal rated size;
For example, a 20 amp circuit can pass 7-8 times the rated 20amp trip amount, .. for up to a second or more.
It will allow up to 3x the rated amount for up to 10sec or so.
And the same 20amp circuit, can allow up to 1.5-2times the rated amount for a period extending as long as 30 seconds.

140-160 amps for a second or so.
60 amps for up to 10 seconds.
30-40 amps for up to 30seconds or so.
post #13 of 14
Quote:
Originally Posted by Archaea View Post

Look --- the useless argument started with the fact that he said the breaker 'will not let more than 20 amps pass'. I said more than 20 amps can pass for very short periods of time - I think this is given. The question posed was how much current can pass momentarily on a 15amp circuit? How much on a 20amp circuit.

Just search out the Square D breaker trip curves for (example) QO 15 or QO 20.

While the breakers can, without doubt, pass more than their rating on a current vs. time curve, there is still voltage drop that can occur during those high demand peaks. That can preclude max power from the amp. Universal SMPS, which is becoming more prevalent, can mitigate the effect of voltage drop to some degree.

If there is any doubt that breakers will allow though more than their rated current without tripping, this should pretty much stifle that bit of the discussion. Here's a scope trace of current during turn-on of one of my amps.


Edited by whoaru99 - 2/3/14 at 10:01am
post #14 of 14
Good stuff. I think the further important aspect is your point about the voltage not actually being shown and not being as high as the "nominal value" which as you point out, the resulting power is not as great as "predicted".
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