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Author Topic: Multi-circuit stage stringer, best practice  (Read 3495 times)

Jeffery Foster

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Multi-circuit stage stringer, best practice
« on: October 20, 2015, 10:38:37 AM »

What is the preferred way to distribute multiple circuits in one stage stringer around your stage?
If served from a single-phase distro (with the two legs opposite in phase) would 4-conductor wire be acceptable, or can the two final circuits not share the neutral?
Thanks for any thoughts on this!
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Brian Adams

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Re: Multi-circuit stage stringer, best practice
« Reply #1 on: October 20, 2015, 11:20:55 AM »

What is the preferred way to distribute multiple circuits in one stage stringer around your stage?
If served from a single-phase distro (with the two legs opposite in phase) would 4-conductor wire be acceptable, or can the two final circuits not share the neutral?
Thanks for any thoughts on this!

If the 2 circuits are on separate phases, then there's no issue sharing the neutral. The same goes for a 3 phase stringer. It's OK as long as none of the circuits on a cable are sharing a phase. If you use an adapter to plug a 2 or 3 circuit stringer into a single 20A 120v receptacle, there's also no issue, since the total load will be limited to 20A. In fact, you'll probably want to have an adapter to go from standard edison to whatever plug you're using for your stringer, to allow you to use it in smaller venues or for testing purposes in the shop.

However, if you use an adapter to plug a 2 or 3 circuit stringer into 2 or 3 separate 20A 120v circuits, and some of those circuits are on the same phase as each other, then your neutral load will be high, and that could be a problem. Although, you could get away with it as long as the neutral load wasn't higher than the cable capacity, but it's tough to figure out exactly what the load is, so it's best to just avoid that scenario.

You can have several receptacles on one circuit. Say you have a 20A 125/250v stringer with 4 quad boxes, 2 on each circuit. The total draw of each pair of boxes can't exceed 20A or you'll start tripping breakers, but that's what the breakers are there for. If you put a 20A load on one box in the circuit, you're out of room and can't use the other box, since that'll put it over the max and trip the breaker. Hypothetically, if you had a 10A load on every box in the stringer, you'd be fine.

And actually, it's recommended not to exceed 80% of a breakers capacity, so you're really looking at a max of 16A per circuit. I was just using the 20A as an example, since that's likely what your breakers are labelled.

I'm assuming this stringer will be for backline though, in which case you're typically looking at fairly low current draws. Your stringer should be fine.
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Brian Adams
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Stephen Kirby

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Re: Multi-circuit stage stringer, best practice
« Reply #2 on: October 20, 2015, 05:44:07 PM »

I've seen several mentions of having GFCIs on the backline stringers and am thinking about converting mine.  If you have a shared neutral and a fault happens on one side, does that trip the GFCI on the other side too?
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Stephen Swaffer

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Re: Multi-circuit stage stringer, best practice
« Reply #3 on: October 20, 2015, 08:31:46 PM »

GFCIs are only concerned with the current flowing through.  While I am sure that false trips can happen, I would not expect that if you use a quality GFCI-I personally never saw any reason to shop safety devices primarily on price.
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Steve Swaffer

Geoff Doane

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Re: Multi-circuit stage stringer, best practice
« Reply #4 on: November 02, 2015, 11:29:52 AM »

I've seen several mentions of having GFCIs on the backline stringers and am thinking about converting mine.  If you have a shared neutral and a fault happens on one side, does that trip the GFCI on the other side too?

I haven't actually seen one, but I'm sure they do exist.  A 2-pole GFCI would vector sum the two hots and common neutral.  As long as the sum is < 5mA, it doesn't trip.  But a 2-pole GCFI has a common trip, so if one hot has a fault to ground, both circuits will go dead.

You can't use two single pole GFCIs with a common downstream neutral because the return current will divide through the two neutral connections on the GFCIs, causing a trip, probably on both of them.

If you want two circuits in your stringer, and GFCI protection, a better solution would be to have individual GFCIs at each outlet. Don't daisy-chain them.  That has the additional advantage that one crappy old guitar amplifier won't shut down the entire backline on the stage when it decides to try and electrocute somebody.

GTD
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Jonathan Johnson

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Re: Multi-circuit stage stringer, best practice
« Reply #5 on: November 02, 2015, 03:11:46 PM »

The neutral can be shared UPSTREAM of the GFCIs. Once the neutral passes through the GFCI, it can't be shared with any other circuit.

GFCIs should not be daisy chained (GFCI installed downstream of another GFCI) UNLESS the upstream ones are set to a higher fault current than the downstream ones. Otherwise, you may experience instability. Since all consumer GFCI are factory-set to the same fault current and non-adjustable, they are usually the last ones in the chain. Practically speaking, upstream GFCIs protect the equipment, endpoint or branch circuit GFCIs protect the personnel.

Take a look at this thread: http://forums.prosoundweb.com/index.php/topic,154557.0.html -- Page 2 is where the discussion turns academic.  ;)
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Stephen Swaffer

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Re: Multi-circuit stage stringer, best practice
« Reply #6 on: November 03, 2015, 11:33:00 AM »

I haven't actually seen one, but I'm sure they do exist.  A 2-pole GFCI would vector sum the two hots and common neutral.  As long as the sum is < 5mA, it doesn't trip.  But a 2-pole GCFI has a common trip, so if one hot has a fault to ground, both circuits will go dead.


Two pole GFCIs are commonly used for things like hot tubs (good idea for pools and church baptisteries, too).  I would not recommend using one on a stage.

As Mike and others have pointed out, UL allows equipment to have some small level of ground leakage current.  By themselves the leakage currents are harmless, when you start plugging multiple devices into a circuit, those currents add up, at some point the aggregate leakage will be high enough to create nuisance trips.  Even though the cost is higher, individual GFCIs offer protection while minimizing nuisance trips.
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Steve Swaffer

ProSoundWeb Community

Re: Multi-circuit stage stringer, best practice
« Reply #6 on: November 03, 2015, 11:33:00 AM »


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