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Author Topic: 208 Amperage issues  (Read 3545 times)

Dave Sturzenbecher

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208 Amperage issues
« on: December 30, 2004, 01:01:57 am »

Hi guys
I got some issued that haven’t yet caused problems.  Just for giggles the other day I took some amperage readings at the point where my distro plugs into an outlet.  

First off, before I plugged the unit in to the outlet, I checked the voltage across all terminals, in all possible combinations. 210 across hots, 120 from each hot to neutral and from each hot to ground, nothing between ground and neutral.  (Essentially three prong plug with ground clamped to conduit).

Ok, here is the issue, when I took measurements I got 15amps on each of the hots and 30 amps on the neutral.  This kinda scared me.  Now, im not an electrician, but last time I checked that should be a lot closer to zero.

Our Distro was constructed by a certified election, but then rewired when the users of the distro certified that the distro wired wrong.  It was rewired over 6 years ago, and has yet to have a documented problem.  I don’t have a direct picture of the distro.  But the distro has two 208 outlets on it that feed the 2 amp racks (picture x 2).  The amp racks were not wired by an electrician, but by a person certified to teach electricity in a classroom setting (high school). So im pretty sure he knew what was up.  The 208 out of the wall is 2 legs out of a three phase panel.  

Any ideas of what might be up? Thank you
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Simon-ETEC

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Re: 208 Amperage issues
« Reply #1 on: December 30, 2004, 10:43:58 am »

It think its working just as it should be, as you should have the same ampage going into the distro on the live (15A+15A) as coming out on the neutral (30A), which is why you can take an amp reading with a clamp meter arround all the cable as the reding should always be 0 ((15A+15A)-30A). Now if you had 30A going down the earth you would have problems!!!

Hope this helps!
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David Buckley

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Re: 208 Amperage issues
« Reply #2 on: January 02, 2005, 09:18:44 pm »

So let me get this straight.

You have your distro wired across two phases of a three phase system.  You have half your amps on each phases.  And you are seeing 15A on each phase, and 30A on the neutral, as measured heading back to the plug on the wall.

And those amps are mostly QSC PLX1202, which are lightweight (switch mode) amps?

If all the above is true, then yes, I'm not surprised, and it is probably all about to catch fire, and the licenced electrician probably did a reasonable job, 'cos this is a very poorly understood area of electrical engineering.  People who build computer rooms know about it though Smile

I'll leave this as a teaser, to ensure I've got my assumptions correct, and then come back and explain whats going on...

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Dave Sturzenbecher

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Re: 208 Amperage issues
« Reply #3 on: January 03, 2005, 02:33:54 am »

Pretty much correct.  The outlet is a standard 208 dryer outlet, that I traced back to a distrobution panel that had three phase going into it.  

This certified electrician is the same guy that has accedently forgoton to tie in a nuetral when other sound companies have come into our house.
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David Buckley

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Re: 208 Amperage issues
« Reply #4 on: January 03, 2005, 06:20:35 am »

Dsturzy wrote on Mon, 03 January 2005 20:33

This certified electrician is the same guy that has accedently forgoton to tie in a nuetral when other sound companies have come into our house.


Oooh, thats not good - that could seriously ruin your day, not to mention the rig.

Anyway, there are two facets to your problem.

a) You have an unbalanced three phase system.  Although you are only using two phases, its still a three phase system, just with zero amps in the third phase.

So, if you just had lamps instead of amplifiers, you would have phase currents of P1=15, P2=15, P3=0.  Plug those numbers into a formula for calculating neutral currents in a three phase resistive system, you you get a neutral current of about 15A.

See the math http://www.electrician.com/electa1/electa5htm.htm

You were hoping for zero amps.  You dont get that - if you had a perfectly balanced three phase resistive load, you can get zero amps, but you aren't running three phases, nor a resistive load, so there will be a return current through the neutral.

Thats all stright textbook three phase, nothing contoversial here.

b)  Now it gets worse.  I'm doing some extrapolating from things I know to things I haven't measured, so its a bit speculative.

The big issue is harmonic currents and non-linear loads.

I'm going to make an unproven assumption here that a switchmode PA amp works - in practice - much the same way as any switchmode power supply, for example, the one in your PC.  Switchmode power supplies (SMPS for short) have a particular characteristic, in that they dont draw current linearly, but as a sharp edged pulse.

Have a look at this link, and check out the graph with the thick red line:  http://www.marcspages.co.uk/pq/3230.htm

Now this nasty current waveform generates harmonics, and the real sod is that if you overlay the harmonics waveforms on each of the phases, particularly the third harmonic at 180Hz (assuming USA 60Hz) the currents in the neutral add, not subtract.  Again, to save me typoing and drawing, heres another link:

http://www.marcspages.co.uk/pq/3240.htm

Thus in a power system with an unbalanced load and non-linear current draw, you get neutral currents that frequently exceed the phase currents.  Despite what Marc (whose pages I link to above) says, and I dont disbelieve him, and his logic appears sound to my non-mathematical mind, I've never seen N current greater than twice the biggest phase current.

I can also say that it is standard computer room practice to build power distribution with 200% N current capability.  So if you have a 200A per phase service, your neutral capability is uprated to 400A.  Otherwise, you can get overloaded neutrals, and thats when the fires start.  That 200% neutral has to go right back to the transformer, and the transformer will dissapate the excess N current as heat.  To do this you need specially designed transformers, known as K rated transformers.  Otherwise, the transformer get overheated.

So, getting back to rock and roll.

a) If you can, run your system single phase.  Then you cant get all this bad stuff happening.

b) If you must go multiphase, use all three phases.  Derate your power system by 50%, ie if you intend to connect to a 50A supply, draw no more than 25A per phase.  Its no use putting 200% neutrals in your distro, and then plugging into the house power, which only has 100% neutral, and running full current from the house supply, its neutral will be overloaded.  Remember what will happen if the neutral melts....

So, thats my theory.  I haven't found any data on switchmode PA amps causing non-linear harmonic current problems, its just my suspicion, but it fits the facts you've measured.

I'd be interested in comments, particularly from anyone who has a really big PA with switchmode amps on three phase, who has a clampmeter...

(and yes, lighting rigs suffer from the same phenomenon)


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Dave Sturzenbecher

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Re: 208 Amperage issues
« Reply #5 on: January 03, 2005, 08:17:07 pm »

By saying "run your system single phase"
Are you refering to axing the distrobution compleatly and run each amprack off a 20 amp edison outlet?  

You say to go three phase and use all the phases.  The amp rack (and distrobution unit) was made to run of a singe (split) phase 208 outlet.  That was what it was plugged into at the time.  It was only plugged into this outlet (that was connected to the 3 phase panel) for testing purposes.  Am I to imagin that when this unit is plugged into an outlet connected to a true split phase 208 panel that all the problems will be eliminated?

When I say three phase, im not sure if its a true three 3 phase system.  Their are 3 120 hots when measured to the nuetral or ground. 208 when measured between any two hots.
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David Buckley

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Re: 208 Amperage issues
« Reply #6 on: January 03, 2005, 11:22:03 pm »

First, some definitions.

Single phase is 110V hot to neutral, just two wires, white (neutral) and black (hot).  (plus ground, but lets ignore that).  Like you get out of a single socket at home.

Split phase is what you have at home for the whole house, 110V between hot and neutral, 220V between hot and hot.  The two phases are 180 degrees out of phase.  You can consider this as single phase twice over(!)

Three phase is what you find backstage, three hots, a neutral, 110V hot to neutral, 208V hot to hot, 120 degrees between phases.

If you use purely a single phase feed for all your rig then the hot and neutral current will be balanced.  of course, you will need lots of current Smile  A big three pin plug.

Either split phase or three phase will have neutral current, and in either case the neutral current can exceed the phase current.  So in my opinion you should derate a feed by 50%.

I've now found some stuff with a google search that supports what I've suggested, do a search for "undersized neutral", with the quotes.

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Dave Sturzenbecher

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Re: 208 Amperage issues
« Reply #7 on: January 18, 2005, 02:28:48 am »

Update.

I checked the rig on a true split phase 208 grid, and the double nuetral amperage was still there, and then i proceded to check another employers rig which consists of 2 3002's, and 2 3402's and the same results were obtained, even with a compleatly different distro.  I called up QSC technical support, and they explained that the cancilation of nuetral amperage is only true with resistive loads, and that when you throw electronics into the mix, it doesnt apply.  I asked about the switching power supply, and he said that it really didnt have anything to do with it.
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