Lead length for SPD

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Shujinko

Shujinko

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What is the maximum lead length on an SPD (surge protection device) between the SPD and the circuit breaker? The manufacturer's installation manual says "Wire length should be left as short as possible..." and "For wire lengths longer than 4 inches, Phase wires should be twisted once for each 4 inches of wire length to maximize SPD performance."

What is the longest lead length that is acceptable for correct SPD performance? What are the max number of wire turns? What is the reasoning for twisting the phase wires every 4"?

I have an installation where the lead length between the SPD and CB is 24" and has two (2) 90 degree turns in it. Existing conditions in the Elec Room made this the shortest lead length possible.

I want to make sure that the SPD won't have any significant performance issues based on the installation.
 
Shorter is better. It's hard to say what a maximum length is. The longer, the higher surge voltage you will get at the switchboard or panel.

Does the surge suppressor come with factory leads, and if so, how long are they?

I would say that 24" doesn't sound excessive - any large switchboard with a separate surge suppressor is probably going to have that much lead length.

Twisting the wires will help lower the wire impedance since the magnetic fields will cancel each other out better.
 
steve66 said:
Shorter is better. It's hard to say what a maximum length is. The longer, the higher surge voltage you will get at the switchboard or panel.

Does the surge suppressor come with factory leads, and if so, how long are they?

I would say that 24" doesn't sound excessive - any large switchboard with a separate surge suppressor is probably going to have that much lead length.

Twisting the wires will help lower the wire impedance since the magnetic fields will cancel each other out better.
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Lead length is 48", so based on this I assume 24" isn't the worst case. What are the negative effects of performance for the sharp turns, In this case (2) 90 deg turns?
 
As steve66 said, more length means more let through voltage. Approximately 15-25V/inch of length.

SPDs are tested with just 6 inches of lead length. This is likely why recommendations have twisting after 4 inches. Twisting helps avoid cross coupling of transient energy to adjacent conductors.

I can't speak to the bends. It is just a common recommendation.

Good installation practice: Twist and be less than 14 inches - max 36 inches. Arrange breakers to help with shorter lengths if possible. Use low impedance conductors when longer than 36 inches. Minimal bends.

https://ewh.ieee.org/r3/nashville/events/2011/Surge Protection Device Standards - IEEE Meeting.pdf
 
Sharp bends add more impedance, just like adding length does. Also, if there are high voltage surges, sharp bends tend to concentrate the charge, making it more likely the high voltage could arc through the wire insulation at a sharp bend.

Try to keep the bends gradual (without adding more length.) A conduit elbow would be better than one of those small pull fittings.

I remember one surge suppressor company doing a show and tell, and they showed a photo of the inside of a competitor's surge suppressor. The wiring had lots of sharp, tight bends. It looked really nice, but it wasn't helping to reduce the surges.
 
realistically, it probably does not matter as much as people think or claim. the terminations are probably as big a problem as anything else as are any bends.

Keep them as short as possible consistent with not having sharp bends and don't worry about it. I would not be worried about leaving a few extra inches in either. It just is not going to make enough difference to matter and you might need that few extra inches some time down the road.

I wonder about the twisting idea. I would think that might introduce some inductance into the system.
 
petersonra said:
realistically, it probably does not matter as much as people think or claim. the terminations are probably as big a problem as anything else as are any bends.

Keep them as short as possible consistent with not having sharp bends and don't worry about it. I would not be worried about leaving a few extra inches in either. It just is not going to make enough difference to matter and you might need that few extra inches some time down the road.

I wonder about the twisting idea. I would think that might introduce some inductance into the system.
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Twisting the wires will reduce the inductance. Keeping the wires twisted will help make sure their magnetic fields cancel each other out, and help reduce the radiated magnetic field. That in turn will help reduce the choke effect of any metal conduits the wires are running through.

Less inductance means less voltage drop between the panel and the suppressor, and therefore the suppressor will work better.
 
steve66 said:
Twisting the wires will reduce the inductance. Keeping the wires twisted will help make sure their magnetic fields cancel each other out, and help reduce the radiated magnetic field. That in turn will help reduce the choke effect of any metal conduits the wires are running through.

Less inductance means less voltage drop between the panel and the suppressor, and therefore the suppressor will work better.
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maybe.

one would think that if it mattered that it would come twisted from the factory.
 
petersonra said:
maybe.

one would think that if it mattered that it would come twisted from the factory.
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Maybe. But if its fairly large wire, they probably don't stay twisted until they are terminated. And there is no point in twisting 4' of wire if most people are just going to cut 3' off.

Page 27 says twisting the wires gives a 23% reduction in the let through voltage:

http://www.ewh.ieee.org/r3/nashville/events/2006/TVSS.pdf

But looking at the chart, and with only 2' of leads, I wouldn't loose any sleep over un-twisted wires either.
 
steve66 said:
Maybe. But if its fairly large wire, they probably don't stay twisted until they are terminated. And there is no point in twisting 4' of wire if most people are just going to cut 3' off.

Page 27 says twisting the wires gives a 23% reduction in the let through voltage:

http://www.ewh.ieee.org/r3/nashville/events/2006/TVSS.pdf

But looking at the chart, and with only 2' of leads, I wouldn't loose any sleep over un-twisted wires either.
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one would think that if it actually does perform better that if nothing else one of the manufacturers would supply twisted leads just as a marketing ploy.

as for size, I can't say I have ever seen any of them with really large wires on them.
 
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