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Author Topic: 50amp outlets  (Read 24946 times)

Jamin Lynch

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50amp outlets
« on: September 19, 2013, 10:21:46 am »

Another issue that often comes up is when the only available power at a venue is an old style 50amp outlet with no ground.

What's a good option to get you safely through the night?

Thanks
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RYAN LOUDMUSIC JENKINS

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Re: 50amp outlets
« Reply #1 on: September 19, 2013, 10:49:54 am »

Another issue that often comes up is when the only available power at a venue is an old style 50amp outlet with no ground.

What's a good option to get you safely through the night?

Thanks

Are you sure it as no ground or is it no neutral?  In almost 25 years running audio, I have never come across a 50 amp without a ground.  The 50 amp 125/250 volt receptacle does exist in the Nema 10-50R but I have never come across one.  Are you working in some really old venues?   We do have an old hotel here that had a couple 10-30R recepticals that they supossedly were upgrading and may already be done with but no 50s.
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Jamin Lynch

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Re: 50amp outlets
« Reply #2 on: September 19, 2013, 11:03:46 am »

Are you sure it as no ground or is it no neutral?  In almost 25 years running audio, I have never come across a 50 amp without a ground.  The 50 amp 125/250 volt receptacle does exist in the Nema 10-50R but I have never come across one.  Are you working in some really old venues?   We do have an old hotel here that had a couple 10-30R recepticals that they supossedly were upgrading and may already be done with but no 50s.

Yep. 2 hots and a neutral.



« Last Edit: September 19, 2013, 11:07:14 am by Jamin Lynch »
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Mike Sokol

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Re: 50amp outlets
« Reply #3 on: September 19, 2013, 11:14:28 am »

Another issue that often comes up is when the only available power at a venue is an old style 50amp outlet with no ground.

What's a good option to get you safely through the night?

Thanks

Hmmmm..... I see a lot of 3-wire 30-amp/240-volt dryer outlets with a single ground/neutral contact in older buildings, but I can't remember seeing any 3-wire 50-amp/240-volt outlets in recent memory. But they're entirely possible. Since you're working from a new ground-neutral bonding point, any current on the line conductor will create a voltage drop in that side of the line, which will cause an equal and opposite voltage drop in the neutral conductor. Since your ground wire is also the neutral wire in that case the safety ground voltage will move around a lot, perhaps as much as 10 volts or so.  That suggests you'll want to avoid connecting some of your sound or stage gear to any other receptacles in the building unless you use audio isolation transformers such as a Whirlwind ISO-2 or similar, and you'll certainly want good DI's with a ground lift switch in the "lift" position. If you do cross connect something with shied isolation, lets say the amp rack connected to the 30 or 50-amp/240-volt 3-wire outlet and the mixing console to a handy Edison outlet on the back wall, you'll create a classic ground loop problem and lots of hum. In fact, I think this would be an excellent GLID (Ground Loop Inter-modulation Distortion) situation where power being pulled from the power amp rack will induce 60-Hz ground loop hum that you'll hear only during bass transients (Don't Doubt the GLID). ;D 

So, the basic rule is it. Either EVERYTHING is powered from your own distro connected to this 3-wire, 50-amp/240-volt outlet, or you need to provide audio isolation transformers between gear on the two power feeds. And you might be able to get away with pin-1 lifts on the XLR inputs depending on the CMRR of your balanced inputs on the amp rack. But I personally would avoid cross connecting gear powered between standard Edison wall outlets and your 3-wire distro in general. Too many things to go wrong.   
« Last Edit: September 19, 2013, 11:23:04 am by Mike Sokol »
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Mike Sokol

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Re: 50amp outlets
« Reply #4 on: September 19, 2013, 11:50:47 am »

Yep. 2 hots and a neutral.
Note that the neutral is also the ground as well. So you have a double-bonded G-N in your power system.
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Steve M Smith

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Re: 50amp outlets
« Reply #5 on: September 19, 2013, 01:19:36 pm »

You Americans certainly have a strange assortment of power outlets!


Steve.
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Rob Spence

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Re: 50amp outlets
« Reply #6 on: September 19, 2013, 02:18:24 pm »

Are you sure it as no ground or is it no neutral?  In almost 25 years running audio, I have never come across a 50 amp without a ground.  The 50 amp 125/250 volt receptacle does exist in the Nema 10-50R but I have never come across one.  Are you working in some really old venues?   We do have an old hotel here that had a couple 10-30R recepticals that they supossedly were upgrading and may already be done with but no 50s.

I did a gig a few years back that had a 10-50R next to a sub panel. I made them replace it with a 14-50R before I would use it (discovered weeks before the gig).

My electric range came with a 10-50P on it. I had the cord set replaced before I could use it.

They are pretty common up here in the northeast. Of course, all newer construction has the 4 wire versions.



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Mike Sokol

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Re: 50amp outlets
« Reply #7 on: September 19, 2013, 03:42:41 pm »

You Americans certainly have a strange assortment of power outlets!
Steve.

Yes we do. I've worked for a British sound company here in the US for 20 years, and always laughed at our US power outlets.
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Jonathan Johnson

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Re: 50amp outlets
« Reply #8 on: September 19, 2013, 03:58:30 pm »

Yep. 2 hots and a neutral.



This is a typical older style range receptacle conforming to the NEMA 10-50 spec. You'll notice that right on the front, it says "50A 125/250V" and "NEMA 10-50R."  In the typical application with a slide-in kitchen range, the neutral (the lower, vertical slot in this picture) is actually a shared neutral/ground, and the chassis of the range is bonded at the range's terminal block to the neutral+ground wire in the cordset. (Note that the NEMA 10-30 used to be used for clothes dryers similarly to ranges; this has been replaced with the NEMA 14-30.)

Current electrical code does not permit this style of plug to be used anymore; for new construction, your kitchen range must use a NEMA 14-50 receptacle and plug which provides separate neutral and ground contacts.

Here's a useful chart (courtesy Wikimedia) which applies to North America:

« Last Edit: September 19, 2013, 04:24:16 pm by Jonathan Johnson »
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Jonathan Johnson

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Re: 50amp outlets
« Reply #9 on: September 19, 2013, 04:08:44 pm »

For high-amperage receptacles (30A or higher) the ground pin is typically installed at the top of the receptacle as most cordsets with right-angle plugs will have the ground pin opposite the cord entrance.

That's a fancy way of saying "so the cord be hangin' down."

For 20A and 15A receptacles, the debates for whether the ground pin should be up ("safety!") or down ("my wife says it needs to look like a face!") are endless. To my knowledge, the National Electrical Code gives no requirements for receptacle orientation.
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