Stubups into a Gutter

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jap

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Electrician
Is there a good way to determing the number of, or size of ,stubups needed out of a panel that wouldnt be an over or underkill?
a job we're on has 2 loadcenters side by side and we were thinking of utilizing larger nipples into the gutter to get away from the random knockouts that are in them. both are 200 amp 42 circuit. Also if the nippples happen to be longer than 24 inches, do you derate for the amperage of the circuit or the actual load that its feeding?

If you had a 2" nipple that was 4' long jammed full of #12's on 20 amp breakers, what amperage would they derate down to?
 
jap said:
Is there a good way to determing the number of, or size of ,stubups needed out of a panel that wouldnt be an over or underkill?
a job we're on has 2 loadcenters side by side and we were thinking of utilizing larger nipples into the gutter to get away from the random knockouts that are in them. both are 200 amp 42 circuit. Also if the nippples happen to be longer than 24 inches, do you derate for the amperage of the circuit or the actual load that its feeding?
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Skip the gutter. Use 1/2" emt up to the first j box on each run... can't use more than nine current carrying conductors that way; still leaves room for a green if you are one of those types.
If you had a 2" nipple that was 4' long jammed full of #12's on 20 amp breakers, what amperage would they derate down to?
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Less than would be allowed on a 20A breaker.
 
A 4'-2" EMT nipple would allow 101, #12 THHN conductors. Maximum derating for above 40 CCC's is 35%.

30 amps * 35% = 10.5 amps

There are other variables like whether or not you can apply 310.15(A)(2)Exception.
 
I pity the fool who puts 1/2" runs into a panel. 3/4 minimum for me. :)

I try to throw a couple spares, I usually wind up using them anyway. The biggest thing is leaving a path (via strut) and knockout space for future runs. A few 12x12 junction boxes outside the electrical room can clean up mistakes and keep panels pristine.
 
infinity said:
A 4'-2" EMT nipple would allow 101, #12 THHN conductors. Maximum derating for above 40 CCC's is 35%.

30 amps * 35% = 10.5 amps

There are other variables like whether or not you can apply 310.15(A)(2)Exception.
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Decrease the nipple/run length to 2' and no derating for number of conductors required.
 
As long as all your runs were 40' or more, 310.15(A)2 Exc. would allow you to run those #12's at full ampacity through a 4' nipple.
 
Cow said:
As long as all your runs were 40' or more, 310.15(A)2 Exc. would allow you to run those #12's at full ampacity through a 4' nipple.
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dont have my codebook, not sure I understand this.
the 2 panels are actually loadcenters. Some of the runs will be over 100' bringing #10s into play in some places, therefore 1/2" is out.
need to pick up as many circuits I can.
The reason for the gutter Idea is because the panels are below a hard decked ceiling and it would be much easier to start the pipe rack out on the mezanine above, but the nipples would be 4'. if they were 2' or less I wouldnt have the issue.
 
jap said:
dont have my codebook, not sure I understand this.
the 2 panels are actually loadcenters. Some of the runs will be over 100' bringing #10s into play in some places, therefore 1/2" is out.
need to pick up as many circuits I can.
The reason for the gutter Idea is because the panels are below a hard decked ceiling and it would be much easier to start the pipe rack out on the mezanine above, but the nipples would be 4'. if they were 2' or less I wouldnt have the issue.
Click to expand...

If they're 4' you wouldn't have an issue either as long as each run is at least 44' in length. Here's the exception:

310.15 Ampacities for Conductors Rated 0?2000 Volts.
(A) General.
(1) Tables or Engineering Supervision. Ampacities for
conductors shall be permitted to be determined by tables as
provided in 310.15(B) or under engineering supervision, as
provided in 310.15(C).
Informational Note No. 1: Ampacities provided by this
section do not take voltage drop into consideration. See
210.19(A), Informational Note No. 4, for branch circuits
and 215.2(A), Informational Note No. 2, for feeders.
Informational Note No. 2: For the allowable ampacities of
Type MTW wire, see Table 13.5.1 in NFPA 79-2007, Elec-
trical Standard for Industrial Machinery.
(2) Selection of Ampacity. Where more than one ampacity
applies for a given circuit length, the lowest value shall be
used.
Exception: Where two different ampacities apply to adjacent
portions of a circuit, the higher ampacity shall be permitted to
be used beyond the point of transition, a distance equal to 3.0
m (10 ft) or 10 percent of the circuit length figured at the
higher ampacity, whichever is less.
Click to expand...
 
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Cow said:
As long as all your runs were 40' or more, 310.15(A)2 Exc. would allow you to run those #12's at full ampacity through a 4' nipple.
Click to expand...

infinity said:
If they're 4' you wouldn't have an issue either as long as each run is at least 40' in length. Here's the exception:
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Actually the runs would have to be at least 44' long. The adjacent portion at the higher ampacity must be 40' long, so the 4' is not more than 10%.
 
infinity said:
I agree, but he did ask about a 4' nipple. :D
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I was aware of that but brought up 2' because many just say 4' to be certain the 'gutter' will be above the ceiling. With 2' being the cutoff, where (A)(2) exception does not apply, sometimes installing the panel a little bit higher and using 2' nipples will get the 'gutter' above the ceiling.
 
Doesn't look to bad if the top of the 'gutter' is right at ceiling height for suspended ceilings either... giving a little extra distance.
 
Smart $ said:
I was aware of that but brought up 2' because many just say 4' to be certain the 'gutter' will be above the ceiling. With 2' being the cutoff, where (A)(2) exception does not apply, sometimes installing the panel a little bit higher and using 2' nipples will get the 'gutter' above the ceiling.
Click to expand...

And we were aware of that. :D
 
Smart $ said:
Actually the runs would have to be at least 44' long. The adjacent portion at the higher ampacity must be 40' long, so the 4' is not more than 10%.
Click to expand...

Oops, yes you're correct my math was a little fuzzy so I made a correction. Here's a graphic from Mike Holt:

808ecmCBfig3.jpg
 
Smart $ said:
I was aware of that but brought up 2' because many just say 4' to be certain the 'gutter' will be above the ceiling. With 2' being the cutoff, where (A)(2) exception does not apply, sometimes installing the panel a little bit higher and using 2' nipples will get the 'gutter' above the ceiling.
Click to expand...

This is exactly what I'm up against. 2' nipples or less above the panels would put me right in the ceiling joist area.
It's going to take 4' min. from the top of the panel to the bottom of the gutter, maybe even a little more.
I'll have to studu up on the 44' rule,,,, all my runs are well over that.

In a nutshell then the 2" emt stubups 4' long from the top of the panel to the bottom of the gutter,,,you don't have to derate if the branch circuits are at least 44' long?
 
jap said:
This is exactly what I'm up against. 2' nipples or less above the panels would put me right in the ceiling joist area.
It's going to take 4' min. from the top of the panel to the bottom of the gutter, maybe even a little more.
I'll have to studu up on the 44' rule,,,, all my runs are well over that.

In a nutshell then the 2" emt stubups 4' long from the top of the panel to the bottom of the gutter,,,you don't have to derate if the branch circuits are at least 44' long?
Click to expand...
Leaving the exact numbers out of it, in case you end up with slightly different dimensions, what you have is this:

If the lower calculated ampacity portion of the circuit is less than ten feet and less than 10% of the higher ampacity portion's length, then you can disregard the lower ampacity segment when sizing the wires.

Now if the combination of adjustment and correction worked out to something like a factor of .2, I would not be particularly comfortable applying this exception. But the code would be satisfied with it.
 
Smart $ said:
Actually the runs would have to be at least 44' long. The adjacent portion at the higher ampacity must be 40' long, so the 4' is not more than 10%.
Click to expand...

I'm sorry I'm still trying to wrap my head around this.
can you show me the math as to why the run would have to be at least 44' long to be able to use the 4' nipple?
I'm trying to figure out why 40 * .10 = 4 doesn't work.

or are we saying we include the 4' nipple in the equation and figure 36' of higher Amperage * .10 = 3'6" and that's not greater than 10% ?
 
jap said:
I'm sorry I'm still trying to wrap my head around this.
can you show me the math as to why the run would have to be at least 44' long to be able to use the 4' nipple?
I'm trying to figure out why 40 * .10 = 4 doesn't work.

or are we saying we include the 4' nipple in the equation and figure 36' of higher Amperage * .10 = 3'6" and that's not greater than 10% ?
Click to expand...
The part in the nipple cannot be more than 10% of the part outside the nipple.
4+40=44.
 
jap said:
...
In a nutshell then the 2" emt stubups 4' long from the top of the panel to the bottom of the gutter,,,you don't have to derate if the branch circuits are at least 44' long?
Click to expand...

jap said:
I'm sorry I'm still trying to wrap my head around this.
can you show me the math as to why the run would have to be at least 44' long to be able to use the 4' nipple?
I'm trying to figure out why 40 * .10 = 4 doesn't work.

or are we saying we include the 4' nipple in the equation and figure 36' of higher Amperage * .10 = 3'6" and that's not greater than 10% ?
Click to expand...

GoldDigger said:
The part in the nipple cannot be more than 10% of the part outside the nipple.
4+40=44.
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To be more explicit, the higher-ampacity portion adjacent to the 4' in the nipple must be 40' in length. This is typically stated as the run must be 44' in length... but technically it is the adjacent portion, not the rest of the circuit.

Then there's some technicalities which are seldom considered in the application of this exception. First, there is no established means to determine ampacity of conductors in an enclosure or j-box. So the wire length in enclosures appears to not count in circuit length. And with no means for j-boxes, that would mean the adjacent portion ends at the next j-box. Then there's situations such as this case where the 'gutter', which is actually a wireway in this use, may have a different ampacity than the continuation of the circuit after it exits the wireway. That would nullify exception applicability as written. Perhaps I'll write up a proposal...
 
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