Spacers?

lady sparks lover · Aug 23, 2004

An inspector told the contractor on one of my jobs that he must install spacers for the branch panel feeders or we must have a letter stating that the engineer says it ok to not to install the spacers? Do you think it's ok to not install the spacers for the branch panel feeders that are going underground?

ron · Aug 23, 2004

Re: Spacers?

What do you mean by spacers?
Expansion joints for the raceway comming out?

lady sparks lover · Aug 23, 2004

Re: Spacers?

Originally posted by ron:
What do you mean by spacers?
Expansion joints for the raceway comming out?

Spacer between the conduits that are run underground.

Lady

steve66 · Aug 23, 2004

Re: Spacers?

I believe you mean spacers to keep the conduits a certain distance from each other? I guess the inspector is worried about heat dissapation like in a duct bank?

I doubt you have enough feeders or enough load on them to worry about this. I might be tempted to call the couplings "spacers". But that will probably just give you a mad inspector to deal with.

Steve

lady sparks lover · Aug 23, 2004

Re: Spacers?

Originally posted by steve66:
I believe you mean spacers to keep the conduits a certain distance from each other? I guess the inspector is worried about heat dissapation like in a duct bank?

I doubt you have enough feeders or enough load on them to worry about this. I might be tempted to call the couplings "spacers". But that will probably just give you a mad inspector to deal with.

Steve

Yes, that's what I mean. He doesn't think that we should run conduit underground without spacers. We are talking about a 400A panel tops, but you never know the inspector might know something I don't!

lle · Aug 23, 2004

Re: Spacers?

I think your inspector must've just read that the night before he inspected you.

Does he want concrete as well? Maybe with some red dye in it?

Good Luck!

iwire · Aug 23, 2004

Re: Spacers?

310.15(B)(2)(b) More Than One Conduit, Tube, or Raceway. Spacing between conduits, tubing, or raceways shall be maintained.

Even though it gets ignored many times it is in the code.

It is hard to run raceways above ground without spacing because of the supports themselves taking up space.

It all changes when you throw half a dozen PVCs in a common trench.

websparky · Aug 23, 2004

Re: Spacers?

310.15 Ampacities for Conductors Rated 0?2000 Volts.
(B) Tables.
(2) Adjustment Factors.
(b) More Than One Conduit, Tube, or Raceway. Spacing between conduits, tubing, or raceways shall be maintained.

It would be a bit of a stretch to say this would apply to the question asked! The spacing here is in reference to the Adjustment Factors for ampacity in the table.

For the inspector to simply say that spacing is required, period, would be incorrect. The most he could ask for is a load calc based on the conductors just as you would for bundling.

iwire · Aug 23, 2004

Re: Spacers?

Dave what would you say 310.15(B)(2)(b) is telling us?

Under 310.15(B)(2)(a) we need to apply the derating table'

Under 310.15(B)(2)(b) we must maintain spacing, period, no table referenced.

hurk27 · Aug 24, 2004

Re: Spacers?

I think 310.15(B)(2) exception #4 should void 310.15(B)(2)(b) Unless it is ran in PVC over 10'
I think this exception allows this because the thermo properties of the soil will remove heat from the wires in metal conduit much faster than if in PVC which would insulate the wires from the soil some what. But then again I still learning. Let it rip.

Exception No. 4: Derating factors shall not apply to underground conductors entering or leaving an outdoor trench if those conductors have physical protection in the form of rigid metal conduit, intermediate metal conduit, or rigid nonmetallic conduit having a length not exceeding 3.05 m (10 ft) and if the number of conductors does not exceed four.

iwire · Aug 24, 2004

Re: Spacers?

Originally posted by hurk27:
I think 310.15(B)(2) exception #4 should void 310.15(B)(2)(b)

I do not agree, IMO that exception is applicable only to 310.15(B)(2)(a) that exception is above 310.15(B)(2)(b)

Originally posted by hurk27:
the thermo properties of the soil will remove heat from the wires in metal conduit much faster than if in PVC which would insulate the wires from the soil some what.

I do not know which raceway sheds heat better but I do agree that the soil will remove the heat from the raceways if the raceway is in contact with the soil.

Picture if you throw 7 raceways in a common trench you could end up with them in a bundle with the middle raceway surrounded not by soil but by other warm raceways.

IMO you could meet the code by simple placing them side by side with some space in between them.

Here is what the handbook says after 310.15(B)(2)(b)

2002 NEC Handbook

Spacing is normally maintained between individual conduits in groups of conduit runs from junction box to junction box because of the need to separate the conduits where they enter the junction box, to allow room for locknuts and bushings. Field experience has indicated this degree of spacing between runs has not caused any problems.

I think that confirms what I said early that this is a non issue above ground but it can be an issue underground.

Originally posted by hurk27:
But then again I still learning. Let it rip. I am never going to know the whole code. If I start thinking that I do know it all somebody whack me in the head with piece of 750 kcmil cu.

Let it rip.

Bob

[ August 24, 2004, 04:21 AM: Message edited by: iwire ]

hurk27 · Aug 24, 2004

Re: Spacers?

Ouch LOL

steve66 · Aug 25, 2004

Re: Spacers?

I assumed this is a row of conduits, and not a rectangular pattern where one conduit would be completely surrounded. I think the couplings would probably provide enough spacing for a single row or layer.

Steve

nvcape · Aug 25, 2004

Re: Spacers?

At one time, I was asked to help argue with the inspector when 12 larger PVC conduits were placed side by side (without spacers) under a drive that could even have trucks! Sometimes it's more than heating.

macmikeman · Aug 25, 2004

Re: Spacers?

For myself at least, I sure would hate to hear how that some bundleing of underground conduits caused the dirt to burn down. Lets be sure to get those conduits seperated with the proper expensive plastic ul listed spacing devices, and hear no more of it.

iwire · Aug 26, 2004

Re: Spacers?

Originally posted by macmikeman:
Lets be sure to get those conduits separated with the proper expensive plastic ul listed spacing devices, and hear no more of it.

Glad to hear that as I actually sell these spacers and will cut you a deal.

Really we use the spacers quite often, but then again we use a lot of products. Considering we charge more for products then we buy them for it usually works out pretty well.

charlie b · Aug 26, 2004

Re: Spacers?

I?m a bit late getting into this one, but that has never stopped me from engaging in a discussion of physics. I have always believed that the purpose of maintaining spacing in underground runs was to maintain a constant impedance throughout the length of a run, and thereby to prevent the occurrence of hot spots. Here is what I mean:

Any single conductor in isolation has a value of resistance and a value of self-inductance. If it is close to no other conductor, and is far enough from planet Earth, it will not possess the property of capacitance.

Any set of two or more conductors in proximity to each other or in proximity to planet Earth will possess the following properties: Each conductor has resistance and self-inductance, and any combination of two conductors will have mutual inductance and capacitance.

All of these properties can be measured on a ?per unit of distance? basis.

If the spacing between conductors in held constant, then the values of all impedances (resistance, inductance, and capacitance) will maintain a constant value on that ?per unit of distance? basis. Therefore, any heating that takes place along the length of the run will have the same impact on each linear foot of conductor along the entire run. There will be no hot spots. Please note that this will be true whether the conduits are one millimeter apart for the whole run, or 30 feet apart for the whole run. Distance of separation is not relevant here, only that the distance remain constant.

Now suppose you have two conduits in a long run. Suppose they are nearly in contact with each other for some distance, then they are separated by a foot or so for some distance, then they are close to each other for some more distance, and throughout the run the distance between the conduits varies randomly.

In this case, the values of impedances (inductance, and capacitance, but not resistance), as measured on a ?per unit of distance? basis, will vary throughout the run. That is because the values of mutual inductance and capacitance depend on the distance between the two conductors. The current flowing will be based on the supplied voltage and the total effective impedance. But the heating that takes place along the length of the run will depend on the impedance in the local area. As a result, some portions of the run will experience a greater amount of heating then other portions.

The danger here is that a run with properly selected overcurrent protection can still be at risk of hot spots that lead to insulation breakdown, eventually resulting in a ground fault, without the total current ever exceeding the breaker's trip point.

I think that this discussion does apply to any installation, but the actual differences in heating effect will depend on the voltage and current levels. It is probably insignificant below 4160 volts, and would probably not the installation that Lady Sparks Lover has described. That notwithstanding, I believe that this is the reason for the code requirements for maintaining spacing.

lady sparks lover · Aug 26, 2004

Re: Spacers?

Originally posted by charlie b:
I?m a bit late getting into this one, but that has never stopped me from engaging in a discussion of physics.

Charlie b,

You are such a gem! Your explanations are golden, and I also like the theory end of things in your responses.

charlie b · Aug 26, 2004

Re: Spacers?

I don't know how to go about maintaining spacing without a set of spacers. But if you can do it, I think you would be in compliance with the NEC.

Spacers?

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