EMT, Trusses, and 358.30

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NeoAmish

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I will be placing 4" boxes on some wood trusses.

The trusses are spaced on 5' intervals with around 20 trusses total.

This work will be exposed. I will be using 3/4" EMT and will be keeping MC to a minimum.

It would be very convenient to place the boxes so that I simply run a 4'-8" piece of EMT between each one.

Initially this would seem to be allowed by 358.30(A) Exception 1. However, is this actually a violation of 358.30(C)?
 
pete m.

pete m.

Senior Member
Location
Ohio
358.30(C) wouldn't even come into play.

Exception 1 to 358.30(A) only allows you to increase the distance from the enclosure for your means of fastening.

The exception does not alleviate the requirement that the EMT still be securely fastened and supported.

Pete

I assume you are on the 2008 NEC.
 
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NeoAmish

Member
Location
United States
I need a new code book. :ashamed1:

I see that 358.30(C) has been removed from 2011.

So, is zero strapping over a 4'-8" span where structural members do not readily permit fastening (exception 1) a violation?
 
pete m.

pete m.

Senior Member
Location
Ohio
NeoAmish said:
I need a new code book. :ashamed1:

I see that 358.30(C) has been removed from 2011.

So, is zero strapping over a 4'-8" span where structural members do not readily permit fastening (exception 1) a violation?
Click to expand...

Yes, in your case, "zero strapping" is not permitted. As I said the exception only permits you to increase the strapping distance not eliminate it.

Pete
 
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NeoAmish

Member
Location
United States
Pete, we are under the 2011. I was looking at my 2008.

It sounds like I may have to run a 2x4 across the trusses the length of the building to provide support.
 
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petersonra

Senior Member
Location
Northern illinois
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engineer
Smart $ said:
True... but it's overkill for extremely short runs where the terminal enclosures are securely mounted.
Click to expand...

my argument would be that if a piece of emt goes from box to box it is very clearly secured fastened and supported at the box at both ends.

the code says it has to be fastened within 3 ft of a box.

if the piece of emt is less then 3 ft to the next box, it would appear to meet the requirements w/o anything other than the fittings at the box.
 
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petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
iwire said:
The box connectors do not count as support.

If they did I would never need a support within 3' of the box.
Click to expand...

It appears that is what is widely believed but the code does not say that. They clearly support, secure, and fasten the EMT.

My argument is that what the code actually says supports the idea that if the emt is < 3' long you do not need any other supports.
 
G

GoldDigger

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Location
Placerville, CA, USA
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Retired PV System Designer
petersonra said:
my argument would be that if a piece of emt goes from box to box it is very clearly secured fastened and supported at the box at both ends.

the code says it has to be fastened within 3 ft of a box.

if the piece of emt is less then 3 ft to the next box, it would appear to meet the requirements w/o anything other than the fittings at the box.
Click to expand...

Consider these two statements;
1. "There must be a fastener somewhere within 3' of the box."
2. "The closest fastener must not be more than 3' from the box."

Applying rigorous logic, the second statement does not require that there be any fasteners at all, but that is not the way the language of the code is written.

Sent from my XT1080 using Tapatalk
 
iwire

iwire

Moderator
Staff member
Location
Massachusetts
petersonra said:
It appears that is what is widely believed but the code does not say that. They clearly support, secure, and fasten the EMT.

My argument is that what the code actually says supports the idea that if the emt is < 3' long you do not need any other supports.
Click to expand...

Your argument is mistaken and the CMP have made that overwhelming clear over the past few code cycles.

Keep in mind I agree that in reality they do support the conduit but not per the NEC.

If I go to my desktop tonight I will post the changing requirements.
 
Smart $

Smart $

Esteemed Member
Location
Ohio
iwire said:
The box connectors do not count as support.

If they did I would never need a support within 3' of the box.
Click to expand...
Not debating that a box connector count with regards to the box it connects to.

What is debatable, IMO, is whether a box connector/box combination at the other end can count as the securing means... where there is 3' or less straight conduit between boxes.
 
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NeoAmish

Member
Location
United States
I think I can talk the customer into a box every other truss. That lets me mount a box on a truss, span 5' to the next truss, mount a strap, span 5' to the next truss, mount the next box, ad nauseam.

I believe this works with 358.30(A) Exception 1:

Exception No. 1: Fastening of unbroken lengths shall be permitted to be increased to a distance of 1.5 m (5 ft) where structural members do not readily permit fastening within 900 mm (3 ft).
Click to expand...

I will still have to bend some offsets or use a minnie, but that does away with playing carpenter. Those Caddy brackets look nice, but I really need to knock this out this afternoon.
 
pete m.

pete m.

Senior Member
Location
Ohio
Keep in mind that the exception only applys to unbroken lengths. If a coupling would have to come into play you're back to 3 feet.

Pete
 
renosteinke

renosteinke

Senior Member
Location
NE Arkansas
Let us, just for grins, consider this scenario:

Arrangement exactly as described as OP, except he bends a 90 into either end of the EMP, and enters the box parallel to the truss. The sparky then runs the pipe through a mini on the truss, right by the box.

Now we're 'legal.' Pipe is supported within 36" of the box. The support is before the bend, though - so does it really add anything? It seems to me that the 90's cancel out any support for the pipe. What have we accomplished?

What, pray tell, is 'support,' anyway? If the OP ran a piece of twine along with the pipe, and tied the pipe to it with zip ties, would not that be 'support?' I see no specifications or definition in the NEC for 'support.' You're certainly not required to use listed brackets, etc. Bands of 'plumbers' tape hanging from the roof deck would qualify as 'support.'

With the work exposed, I suspect these boxes will be facing downward, and not subject to being stepped on. 60" is a pretty long span between wood trusses, so I have to wonder what, exactly, sits atop the trusses. Perhaps there is enough substance to the decking to sink a lag anchor, then transition to threaded rod holding a mini.

What if the OP runs a 'dummy' pipe alongside the conduit? Would strapping the two together qualify as support?

Ultimately, it's an AHJ call. I can picture several situations where I would accept it.
 
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