Our small stadium is feed from a 3 phase 208 volt Y system. During our last football game the TV crew had a voltage drop down to 101 from 120. We have a meter logging voltage at the transformer and it show around 120 and 119 RMS, not issues shown there. The transformer feeds a MPD which then feeds a subpanel and from the subpanel it feeds another subpanel. The TV crew was coming off an outlet from the furthest subpanel. The problem does get worse the further you go out from the MDP. Example as follows (at the 2nd subpanel I may read 114, and from the outlet 110 and at the TV truck 103. I check the neutral connections at the second subpanel and it looks fine. I check the neutral connection at the first subpanel and it looks fine. I check the neutral connections at the MDP and it looks fine from what I can see, no sign of over heating. But, over half of the neutrals are connected to a split-bolt, from the split bolt one wire goes to a small bar that was added at some point, on that bar is where the main neutral lands. I am thinking this problem is a neutral problem and it resides in the split-bolts. Any help would be greatly appriciated. Thank you
Voltage Drop disrupting video during football game.
- Thread starter JdoubleU
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- Location
- Placerville, CA, USA
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- Retired PV System Designer
If you can put a switchable large load on the circuit, you can measure voltage to ground for both hot and neutral at different points on the way back and be sure where the different parts of the drop are happening.
If that small jumper is really just a bonding jumper or alternate path, it might not be a problem. But if it is the primary neutral path it needs to be sized for the total load current. A clamp ammeter on it might be useful.
The enclosure cannot be part of the neutral current path.
If that small jumper is really just a bonding jumper or alternate path, it might not be a problem. But if it is the primary neutral path it needs to be sized for the total load current. A clamp ammeter on it might be useful.
The enclosure cannot be part of the neutral current path.
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zbang
Senior Member
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- Roughly 5346 miles from Earls Court
So there are a lot of branch circuit (and feeder?) neutrals going into that split bolt, and then a single wire goes towards the service neutral. Unless that single wire is large enough, there'll be a lot of VD right there. Probably one of my first things would be to install a proper neutral bus and get rid of the split bolt.
(looks like GoldDigger beat me to it on using a test load.)
(looks like GoldDigger beat me to it on using a test load.)
Seems to me that the voltage drop is spread across the supply rather than at one point (split-bolt connection).
Assuming you have 120V at the MDP, there's a 6V drop to second panel... but you have another 4V drop to the outlet, and another 7V drop to the TV truck. The split bolt may affect the voltage drop to the first-reading location, but not additional drops after that.
It's hard to put a few voltage readings into context without knowing loads on at the time, wire sizes, distances, etc... anything pertinent to normal voltage drop.
Assuming you have 120V at the MDP, there's a 6V drop to second panel... but you have another 4V drop to the outlet, and another 7V drop to the TV truck. The split bolt may affect the voltage drop to the first-reading location, but not additional drops after that.
It's hard to put a few voltage readings into context without knowing loads on at the time, wire sizes, distances, etc... anything pertinent to normal voltage drop.
rlundsrud
Senior Member
- Location
- chicago, il, USA
Have you checked that the wires to each sub-panel and branch circuit were sized sufficiently to handle any voltage drop due to distance? I am bothered by the split bolt, I would prefer there be a bus, but I would still due the calculations for the VD.
- Location
- Illinois
- Occupation
- retired electrician
As far as I know there are no split bolts listed for use with more than 3 conductors, most are only listed for use with two conductors and a few for use with three conductors.
Actually, it's the other way around... voltage drop causes the potential between the N and G. The ground isn't carrying any current, so there's no voltage drop on it.
So if I measure between the neutral and ground I would see a voltage. Let say I read 3 volt on a multi meter what is going on here? I understand series and parallel circuits but I am having a hard time visualizing how this happens.
ptonsparky
Tom
- Occupation
- EC - retired
Try to visualize the EG as just an extension of one of your meter leads... all the way back to where the neutral and EG are tied together. The neutral has a current flow and consequently some voltage drop. In this case your 3 volts. You are measuring the VD across a resistor, it just happens to be _____ feet long and you have extended one of the meter leads via the EG.
ggunn
PE (Electrical), NABCEP certified
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- Austin, TX, USA
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- Consulting Electrical Engineer - Photovoltaic Systems
It means that the product of the resistance in the neutral and the current flowing through it are causing a 3 volt rise on the neutral at the point where you are measuring it.
V=IR
- Location
- Placerville, CA, USA
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- Retired PV System Designer
And the rise of voltage in the neutral is in series with the drop in voltage of the hot to provide the total voltage drop seen by the load.
grich
Senior Member
- Location
- MP89.5, Mason City Subdivision
- Occupation
- Broadcast Engineer
What kind of load is the TV truck presenting? Is this a local TV news van trying to yank 25A on a 120V circuit (most of that is the HVAC load), or is it a 53' trailer? The typical Game Creek Video semi calls for a 200A 3-phase service.
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