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Author Topic: Chasing power again....  (Read 5037 times)

William Schnake

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Re: Chasing power again....
« Reply #10 on: January 01, 2018, 11:43:10 AM »

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Craig Hauber

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Re: Chasing power again....
« Reply #11 on: January 01, 2018, 05:15:41 PM »

The "problem" is that you are dealing with a dynamic signal, NOT lights or heaters or fans.

The actual current draw will vary with the music, how loud the players are, dynamic range of the music etc.

Yes, the backline will draw some idle current, tha is easy enough to figure out IF you know the specific gear being used.

But the music signal current draw will be constantly changing, so much harder to figure out.

It also depends on the particular breaker being used, how often it has been tripped already etc.  Some trip faster, earlier than others.

YES, it gets very complicated when you try to get specific.

NOW, lets just make it a bit more complicated.

YOUR cable is 150' 12ga.  How long and what gauge is the cable from the breaker to the outlet you are plugging into?

THAT is the total length of the cable over which there will be loss, NOT just your final piece--------------

Fun isn't it :) :)

So true and often overlooked.  Our little town's park has the ball-diamond snack-shack served by one 125' of direct-buried 12-3.  Also the same breaker serves 2 other buried lines for a yard-light and lightbulbs in the bathroom building. (all on one 15A breaker -it was originally intended for lighting only)

So always verify your power if you have to rely on it -and that may mean having a hired electrician pull panel-covers and check connections.  Older outdoor receptacles can really benefit from frequent replacement with a cleaning and re-torque of connections.  The same applies to outdoor service panels and breakers.
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Craig Hauber
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Lyle Williams

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Re: Chasing power again....
« Reply #12 on: January 01, 2018, 11:42:31 PM »

The power loss is irrelevant.  You need to be able to pull 7.5x the 20A breaker capacity for an instant trip.

Assuming a 1/2 ohm source impedance plus 1/2 ohm for the 150' 12G extension, you will get 115A through a short, insufficient for the 150A required for an instant breaker trip.

If the rules are the same there as here, your 150' 12G extension would not be legal.

A smaller breaker supplying the 12G lead would make it legal, a dual pole GFCI would probably make it safe (but probably not 100% legal.)

I am not an electrician, but I do run up against similar site power constraints all the time.
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Stephen Swaffer

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Re: Chasing power again....
« Reply #13 on: January 02, 2018, 01:00:23 PM »

I am curious (professionally) where this restriction is articulated?  I am a licensed electrician-and have yet to run into a situation where an inspector tied over current protection requirements to a circuits length.  Even at 4 times the current trip time will 4-5 seconds-not really enough time to cause overheating.  And I've seen #14 carry 150-200 amps long enough to melt the cable without ever hurting the copper-so that is not a concern.

I'm not arguing and not saying some an AHJ hasn't made a rule just curious who the authority is?
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Steve Swaffer

Ivan Beaver

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Re: Chasing power again....
« Reply #14 on: January 02, 2018, 01:18:30 PM »

I am curious (professionally) where this restriction is articulated?  I am a licensed electrician-and have yet to run into a situation where an inspector tied over current protection requirements to a circuits length.  Even at 4 times the current trip time will 4-5 seconds-not really enough time to cause overheating.  And I've seen #14 carry 150-200 amps long enough to melt the cable without ever hurting the copper-so that is not a concern.

I'm not arguing and not saying some an AHJ hasn't made a rule just curious who the authority is?
Agreed,

 I have never had any body ask for that test, on an install or portable situation.

But I guess they could, but then, lots of situations could be up for question, if you ask the right question or demand a specific result.
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A complex question is easily answered by a simple-easy to understand WRONG answer!

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Tim McCulloch

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Re: Chasing power again....
« Reply #15 on: January 02, 2018, 02:56:15 PM »

I am curious (professionally) where this restriction is articulated?  I am a licensed electrician-and have yet to run into a situation where an inspector tied over current protection requirements to a circuits length.  Even at 4 times the current trip time will 4-5 seconds-not really enough time to cause overheating.  And I've seen #14 carry 150-200 amps long enough to melt the cable without ever hurting the copper-so that is not a concern.

I'm not arguing and not saying some an AHJ hasn't made a rule just curious who the authority is?

Lyle is not in the USA...
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Tim McCulloch

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Re: Chasing power again....
« Reply #16 on: January 02, 2018, 03:01:08 PM »

Mike, I was in a very similar situation several years ago but the venue was dirt race track.  They did 2 runs of #12 Romex (at least 100ft) from a concession stand.  First kick drum hit dropped the voltage to <95V and all my digital EFX froze up.  It was a painfully long set.

Insist on a real diesel generator from Multi-Quip, Kohler or CAT.  Sunbelt, Volvo Rents... all of the usual places have them.  If the producer/promoter balks, ask them how much they'd have to refund to punters, sponsors, etc and how much they'd still have to pay to the venue, security, insurance and sanitation...
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"If you're passing on your way, from Palm Springs to L.A., Give a wave to good ol' Dave, Say hello to progress and goodbye to the Moonlight Motor Inn." - Steve Spurgin, Moonlight Motor Inn

Lyle Williams

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Re: Chasing power again....
« Reply #17 on: January 02, 2018, 03:13:03 PM »

I am curious (professionally) where this restriction is articulated?  I am a licensed electrician-and have yet to run into a situation where an inspector tied over current protection requirements to a circuits length.  Even at 4 times the current trip time will 4-5 seconds-not really enough time to cause overheating.  And I've seen #14 carry 150-200 amps long enough to melt the cable without ever hurting the copper-so that is not a concern.

I'm not arguing and not saying some an AHJ hasn't made a rule just curious who the authority is?

I am in Australia and the ASNZ 3xxx series of electrical rules apply.  The sections on shows and carnivals and the sections on temporary construction site power both contain limitations on extension cord gauge/le gth based on a need to trip the breaker without delay.

I would have thought that the electrical principles were the same all over.  The breaker needs to clear a fault without delay.

Does the US allow arbitary length extension cords?



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Tim McCulloch

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Re: Chasing power again....
« Reply #18 on: January 02, 2018, 04:49:57 PM »

I am in Australia and the ASNZ 3xxx series of electrical rules apply.  The sections on shows and carnivals and the sections on temporary construction site power both contain limitations on extension cord gauge/le gth based on a need to trip the breaker without delay.

I would have thought that the electrical principles were the same all over.  The breaker needs to clear a fault without delay.

Does the US allow arbitary length extension cords?

I'll let Stephen quote the NEC but the other factor is our national Occupational Safety & Health Administration which governs workplace practices and is, I think, where job site extension cord lengths are addressed.
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"If you're passing on your way, from Palm Springs to L.A., Give a wave to good ol' Dave, Say hello to progress and goodbye to the Moonlight Motor Inn." - Steve Spurgin, Moonlight Motor Inn

Stephen Swaffer

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Re: Chasing power again....
« Reply #19 on: January 02, 2018, 05:57:36 PM »

The NEC specifically states that coordination with over current is taken into account in ampacity tables.  The only reference to length, that I am aware of, is in the articles in the branch circuits which recommends a maximum 3% voltage drop on feeders or branch circuits, 5% total (a non-enforceable recommendation).  Keep in mind that a NEC compliant circuit is supposed to be minimully safe-not necessarily a good design.

Honestly, I was curious.  There are times when I want documented rules to back me up and justify doing things right-helps with the budget at times.

Practically speaking, if you have a 5% voltage drop at a 20 amp load, then you have a .3 ohm circuit impedance.  A 20 amp Square D QO breaker will pass 6 times (120 amps) for 2 seconds before it trips.  With a .3 ohm impedance, that is a 36 volt drop, which means your voltage is down to 84 volts-and if you have any digital gear your system is going to drop out unless it can ride through a brown out like that.

That is why I suggested a test.  Perhaps 5% at full load is too much for audio-1-2%?  maybe your OK.  As Ivan always says, it depends.  If you have 3% drop, run 15 A continuously and have 4X peak and your starting voltage is 125 volts, you'll drop from 125 volts to 114-and everything will be happy.  The bottom line is a test tells you what you actually have available.

If I was serious about making a living doing what you guys do, I'd have an easy way to load test any circuit I planned to use-including that genny that was just bounced down the road for 20 miles giving a vibration test to every electrical connection in the thing. Even a circuit close to the panel can have a connection that seems good until you load it down. The least expensive would be something close to a full load-or even a 125% load and a good meter.
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Steve Swaffer

ProSoundWeb Community

Re: Chasing power again....
« Reply #19 on: January 02, 2018, 05:57:36 PM »


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