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Author Topic: 30A to 15A Pigtails?  (Read 1007 times)

Andrew Brubaker

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30A to 15A Pigtails?
« on: February 07, 2014, 12:48:46 am »

Quick electrical wiring question...

I have my power distro set up with several L14-30 twist lock receptacles that I run to amp racks, stage snakes, etc.
My question is when I am working a gig where I am unable to do a proper tie in would it be possible to make some adapters to "pool" power together for the amp racks? Aka have two 15A edison plugs that then run into an L14-30 receptacle and share a common neutral and ground but each keep their separate hot poles? The idea would be to pull power from two separate circuits with standard edison receptacles and "merge" them together to power the amp rack.

Is this possible?
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Mike Sokol

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Re: 30A to 15A Pigtails?
« Reply #1 on: February 07, 2014, 06:43:50 am »

Quick electrical wiring question...

I have my power distro set up with several L14-30 twist lock receptacles that I run to amp racks, stage snakes, etc.
My question is when I am working a gig where I am unable to do a proper tie in would it be possible to make some adapters to "pool" power together for the amp racks? Aka have two 15A edison plugs that then run into an L14-30 receptacle and share a common neutral and ground but each keep their separate hot poles? The idea would be to pull power from two separate circuits with standard edison receptacles and "merge" them together to power the amp rack.

Is this possible?

Possible, but a serious code violation to do so.

The first problem is that you can't guarantee that the two outlets you connect from will be on separate legs (phases) of the panel. If they're on the same phase, then instead of your neutral currents subtracting, they'll add together. So you can easily burn up a neutral wire. However, in your case the two 15-amp services could never draw more than the 30-amp capacity of your plug contacts and cable (assuming you're using 10 gauge wiring). 

The second problem is that any circuit breakers feeding such a receptacle must be ganged together. That is, if one of the legs trips from over-current, the other leg must also trip. This is to prevent 240-volt loads from cross-feeding power from the "on breaker" to the loads connected to the "off breaker". That would create very bad and unsafe conditions for both the gear and the user.

If you do build one of these gadgets and an inspector sees it, he will almost certainly shut down your show. And he'll likely want to put a magnifying glass on your entire rig. Worse still, if there's a fire at your gig, even if it's not your fault", you'll have every lawyer within a hundred miles after you for liability. "It's always the sound guy's fault".

So the answer is this. While it could works in theory, there's too many practical and legal reasons not to do it. Hate to say it, but in these cases running heavy Edison extension cords around is the only good option. And if you're worried about ground-loop hum from multiple outlets, that's the best reason to have 1:1 audio isolation transformers on the inputs of your power amps. 
« Last Edit: February 07, 2014, 09:04:36 am by Mike Sokol »
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TJ (Tom) Cornish

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Re: 30A to 15A Pigtails?
« Reply #2 on: February 07, 2014, 07:12:40 am »

Possible, but a serious code violation to do so.

The first problem is that you can't guarantee that the two outlets you connect from will be on separate legs (phases) of the panel. If they're on the same phase, then instead of your neutral currents subtracting, they'll add together. So you can easily burn up a neutral normally. However, in your case, the two 15-amp services could never draw more than the 30-amp capacity of your plug contacts and cable (assuming you're using 10 gauge wiring). 

The second problem is that any circuit breakers feeding such a receptacle must be ganged together. That is, if one of the legs trips from over-current, the other leg must also trip. This is to prevent 240-volt loads from cross-feeding power from the "on breaker" to the loads connected to the "off breaker". That creates very bad and unsafe conditions for both the gear and the user.

If you do build one of these gadgets and an inspector sees it, he certainly shut down your show. And he'll likely want to put a magnifying glass on your entire rig. Worst still, if there's a fire at your gig, even if it's not your fault"  you'll have every lawyer within a hundred miles after you for liability.

So the answer is this. While this could works in theory, there's too many practical and legal reasons not to do it. Hate to say it, but in these cases running heavy Edison extension cords around is the only good option. And if you're worried about ground-loop hum from multiple outlets, that's the best reason to have 1:1 audio isolation transformers on the inputs of your power amps.
In addition to your excellent list of reasons not to make a cheater, there is one more. If one of the receptacles is miswired with a H/N reversal, an adapter like this will short out the circuit. If both receptacles are H/N reversed, you will be shorting 240/208 volts together, which is a significantly bigger bang.
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Steve M Smith

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Re: 30A to 15A Pigtails?
« Reply #3 on: February 07, 2014, 07:31:10 am »

I have seen this done in the UK (only once and a long time ago) with two 13 amp plugs into both outlets of a double socket.

Obviously they will both be on the same circuit but will have double the capacity (13 amp fuse in each plug).

However, if one becomes unplugged, there is live 240 volts on the pin of the plug.


Steve.
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Keith Broughton

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Re: 30A to 15A Pigtails?
« Reply #4 on: February 07, 2014, 09:02:22 am »

Quick electrical wiring question...

I have my power distro set up with several L14-30 twist lock receptacles that I run to amp racks, stage snakes, etc.
My question is when I am working a gig where I am unable to do a proper tie in would it be possible to make some adapters to "pool" power together for the amp racks? Aka have two 15A edison plugs that then run into an L14-30 receptacle and share a common neutral and ground but each keep their separate hot poles? The idea would be to pull power from two separate circuits with standard edison receptacles and "merge" them together to power the amp rack.

Is this possible?
Technically...I could work.
However...BOBO cable BAD!
Certainly not legal.
Best to avoid.
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Kevin Graf

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Re: 30A to 15A Pigtails?
« Reply #5 on: February 07, 2014, 09:03:57 am »

In general, you can't parallel fuses like the way you parallel resistors to double their power handling capacity. With common thermal fuses what happens is one fuse carries slightly more current than the other. The hotter fuse then carries a little bit more current and so on. Now the fuses are not dividing the load evenly and the hot fuse will trip before the cooler fuse.

The exception being very large fuses (the size of a soap can) they may have 50 or 100 carefully matched parallel elements.
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Andrew Brubaker

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Re: 30A to 15A Pigtails?
« Reply #6 on: February 07, 2014, 09:10:11 am »

Thank you everyone for the responses. Heavy duty Edison only it is!
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Mike Sokol

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Re: 30A to 15A Pigtails?
« Reply #7 on: February 07, 2014, 09:36:28 am »

Thank you everyone for the responses. Heavy duty Edison only it is!

Here's your next challenge... How to determine if two outlets are on separate circuit breakers or wired on the same one. Clubs are notorious for daisy chaining a bunch of stage receptacles on a single 20-amp breaker.

http://forums.prosoundweb.com/index.php/topic,148095.0.html

Keith Broughton

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Re: 30A to 15A Pigtails?
« Reply #8 on: February 07, 2014, 01:08:07 pm »

Here's your next challenge... How to determine if two outlets are on separate circuit breakers or wired on the same one. Clubs are notorious for daisy chaining a bunch of stage receptacles on a single 20-amp breaker.

http://forums.prosoundweb.com/index.php/topic,148095.0.html
This is only a partial answer but if you measure 208 between the hots of 2 different extension cables then they are definately on separate breakers.
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Jonathan Johnson

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Re: 30A to 15A Pigtails?
« Reply #9 on: February 07, 2014, 01:49:27 pm »

One thing to note is that for the aforementioned cheater to provide 30A 120V from two 15A 120V circuits, both circuits MUST be on the same phase and leg of the electrical service.

Another problem with the cheater is that there is nearly 100% certainty that the wiring of the two circuits is different length and therefore different resistances. This means that more current will flow over the circuit with lower resistance; you will likely not get a full 30A before one of the breakers trips (then the other will trip immediately due to overload).

I believe the NEC does permit parallel conductors on one phase/leg (i.e., to achieve 30A service you can parallel two 14 AWG wires rated at 15A each; though it's usually used where larger size of wires are impractical to work with such as in 400A service) but there are specific methods of installation required.
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Stop confusing the issue with facts and logic!
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