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Author Topic: Star ground.  (Read 5185 times)

Matthew Knischewsky

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Re: Star ground.
« Reply #10 on: March 23, 2017, 11:25:43 am »

The box itself is grounded via bus bar, no?  ...and therefore the rivets/flanges.  Why pull rivets to attach ground wires?
Add ground wires from additional inlets ground terminals to the bus bar would effectively turn it into a PMD, so ground wire size would be "handled" and there would be no reliance on that individual input ground system.   So, basically, all this "distro" would do is to get all the inlets and outlets at the same ground potential, correct?
frank

The flanged inlet/outlet connectors are non metallic, so they're not grounded. Even with connectors that have bonded metallic parts (duplex receptacle) there should still be a wire attached to the device's ground terminal.

Geoff's suggestion that it could be difficult to add the ground wires is a good one. Using pop rivets during assembly is convenient at the time but can make servicing a pain. Screws and nuts take longer to assemble but are easier to repair when the time comes. Eventually those terminals will need to be tightened up...

If the ground wires were installed to all of the inlet connectors it would basically be a twist lock PMD.
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Stephen Swaffer

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Re: Star ground.
« Reply #11 on: March 23, 2017, 12:38:50 pm »

I would agree that adding the grounds to all inlets would make this a reasonable solution.  Wired as is, it is a major safety hazard and should not be used-as others pointed out you are relying on one ground that may or may not get connected-and Murphy still lurks in my parts anyway.

The size of the EGC (grounds) is determined by the breaker supplying the system-so assuming that all of the 20 amp inlets are fed from circuits protected at 20 amps as they should be, then a #12 ground is sufficient.  Multiplying the grounds to get to 100 amps doesn't make any sense and is not found anywhere in the NEC.

Perhaps the biggest hazard is the one present with any PMD-if the ground potential of 2 sources is different, there could be significant current flowing in the ground wires which could present a problem.

The basic premise of grounding is that anything on the same premises should be bonded together-so some provision need to be made somehow to bond everything together if multiple sources are used.  Of course, arranging to use one source solves a host of problems and should always be the preferred method.
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Steve Swaffer

Jean-Pierre Coetzee

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Re: Star ground.
« Reply #12 on: March 23, 2017, 01:02:27 pm »

If I understand correctly they are saying you need 100A because all 5 connectors are linked to 1 ground connector. That means that should there be some significant failure there could be 100A shunted to ground. If that was only a 20A rated cable it could catch fire quite easily.

A lightning hit and some cheap badly designed surge protectors can very easily cause that situation if not worse...
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Stephen Swaffer

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Re: Star ground.
« Reply #13 on: March 24, 2017, 12:48:34 pm »

Wire ampacity is primarily determined by insulation and the effect heat has on it.  If I run a 100 amp circuit, I am required to use an 8 AWG ground which using the same temperature rating as that which gives #12 a 20 amp rating only gives #8 a 40 amp rating-and dumping 100 amps fault current onto the ground circuit with a 100 amp circuit is many thousands of times more likely than having 5 20 amp circuits simultaneously dump 20 amps to ground to add up to 100 amps-and even so, it would only have to carry that fault current for a few miliiseconds before the breakers started tripping.

FWIW, check out the ampacity of #12 TGGT wire-you will find it is rated at 55 amps-the copper is not the limitation on how many amps can flow through a given wire-ohms law will determine the maximum ampacity.  The only other limitation is temperature-TGGT at maximum ampacity will run 482 deg F, the copper won't melt until 1985 deg F.

I can run multiple circuits in a conduit-and share an EGC.  The size of the EGC is determined solely by the size of the largest breaker supplying those circuits-regardless of the number of circuits. 
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Steve Swaffer

TJ (Tom) Cornish

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Re: Star ground.
« Reply #14 on: March 24, 2017, 02:48:42 pm »

Wire ampacity is primarily determined by insulation and the effect heat has on it.  If I run a 100 amp circuit, I am required to use an 8 AWG ground which using the same temperature rating as that which gives #12 a 20 amp rating only gives #8 a 40 amp rating-and dumping 100 amps fault current onto the ground circuit with a 100 amp circuit is many thousands of times more likely than having 5 20 amp circuits simultaneously dump 20 amps to ground to add up to 100 amps-and even so, it would only have to carry that fault current for a few miliiseconds before the breakers started tripping.

FWIW, check out the ampacity of #12 TGGT wire-you will find it is rated at 55 amps-the copper is not the limitation on how many amps can flow through a given wire-ohms law will determine the maximum ampacity.  The only other limitation is temperature-TGGT at maximum ampacity will run 482 deg F, the copper won't melt until 1985 deg F.

I can run multiple circuits in a conduit-and share an EGC.  The size of the EGC is determined solely by the size of the largest breaker supplying those circuits-regardless of the number of circuits.
This is not directly related to the thread and not challenging your post in the least, but I think it's worth pointing out that the NEC is published by the NFPA - the National Fire Protection Association.  As such, they are concerned primarily with life safety - both fire and shock, and are much less concerned with functional best practices, such as how much power you lose heating up your wire to 75C or the 482F in your example above.  :)

I'm always amazed at how much money and effort people spend trying to get another couple dB out of their system (a 3dB increase in SPL requires doubling your electrical input, all else equal), but then hang their system off of undersized/overlength cords or other inadequate wiring.  QSC PLX series amps have been criticized because they shutdown at about 90 volts.  Seriously?  Maybe it's the 90 volts that's the problem, not the amp.
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Stephen Swaffer

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Re: Star ground.
« Reply #15 on: March 24, 2017, 05:27:03 pm »

I agree-and the number they should be looking at is the resistance of the wire, which is directly related to the voltage drop.  "Ampacity" is really only related to the fire hazard-which other than the distraction of a cord going up in smoke is minimal in many outdoor situations!  If you do the math, voltage drop is usually far more limiting than you think it is. It only takes 135 feet of #12 cord to equate to a 30 volt drop at 20 amps.

Inspectors and instructors often remind electricians that the code is a minimum safety standard, NOT an engineering guide-though far too often it is used that way.
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Steve Swaffer

David Buckley

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Re: Star ground.
« Reply #16 on: March 26, 2017, 04:51:47 pm »

How is this any different than the "poor mans distro" ?

If you go back in time far enough, this was exactly the way the original PMD worked.

It was subsequently updated to the way it is today with every ground wire being commoned.
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ProSoundWeb Community

Re: Star ground.
« Reply #16 on: March 26, 2017, 04:51:47 pm »


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