My church’s band is getting some “on the road” gigs and our power set up has been pretty ad hoc: a collection of “medium” duty extensions cords of very suspect lineage!
I’m working up a system where we will bring 220 to a central distribution box near the stage and divide it up there with a load center “spider” type set up (with chained 20A GFI receptacles in 4-outlet boxes for each leg of the spider.) (We can’t afford a real ‘spider’)
In trying to figure out the power requirements, I figure we can’t “derate” circuits as in planning for a house because they’ll all be active at once.
Can anyone advise whether it makes sense to just add up the wattage requirements of the particular pieces of equipment and calculated the power needs at a straight 100% Or maybe work in a fudge factor?
Or can we derate some?
Also, one of our musicians has said that he has heard that we’re more likely to get dirty power if we work off of two legs of a 220 supply. Any thruth to that belief? (At the moment we also can’t afford a conditioner for the the whole setup.
Finally, the spider will be wired as a sub-panel with a floating neutral. We’ll have little control over the quality of the systems we connect to.
Will it make sense to have a “portable” ground round (or rods) driven at the spider location connected to the grounding bar in the spider box?
“It is as hard for the good to suspect evil, as it is for the bad to suspect good.”
— Marcus Tullius Cicero, statesman, orator, writer (106-43 BCE)
Replies
First off, you should consider whether you will be able to find a suitable 240V outlet in locations where you will be setting up. You're more likely to have to work off of several 120V outlets.
The best way to figure out your total load is to measure it with an ammeter. This should be done with the equipment silent, and then again with full guns blazing, to see how much of a surge there is on loud passages. Figure that the worst surge you will actually experience will be maybe twice what you can synthesize this way.
I suspect you'll find that most of the equipment doesn't draw anything near its nameplate amps.
I don't know that there would be any reason why 240V juice would be dirtier than 120.
Some folks will no doubt come up with scenarios that say it's a bad idea, but if it were my gig I'd add the "insurance" ground connection -- a driven rod or a hookup to an obviously well-grounded metal object. The odds of encountering wiring with an iffy ground are sufficiently high that this is warranted.
Thanks
The main place we'll be playing is going to wire in a [5]0A 220v receptacle for us, and we'll jury rig at other venues: I'll put together a collection of X0 Amp breakeres for differnt load centers and pop them in temporarily as needed.
I'll have a portable fused and surge protected main disconnect right at the power source for overall protection and to allow differnt "patch" cords for the main lead.
"It is as hard for the good to suspect evil, as it is for the bad to suspect good."
-- Marcus Tullius Cicero, statesman, orator, writer (106-43 BCE)
Also, one of our musicians has said that he has heard that we're more likely to get dirty power if we work off of two legs of a 220 supply. Any thruth to that belief? (At the moment we also can't afford a conditioner for the the whole setup
That seems to be a common "legend" among audiophile types. You'll often read posts in home theater fora that all equipment in a system should be run off the same leg.
This is supposed to help minimize hum pickup in the system. I've never heard a good explanation of how this is supposed to help. There's alot of utter B.S. in the audiophile crowd around cables, "conditioned" power, etc., so I take this suggestion with a grain of salt.
Actually, by reducing voltage drop on the neutral, using both sides of a 240v line might reduce hum/noise slightly. At least on some of the old tube equipment, the terminal of the power transformer chosen for the neutral connection was often chosen such that the effective ground plane provided by the neutral would shield electrostatic noise between primary and secondary. Other design decisions were also made based on the assumption that the neutral side was effectively grounded.