Derating of Conductors: allowable # 10 wires in conduit EMT

Status
Not open for further replies.
F

Fordean

Senior Member
Location
New Jersey
Occupation
Electrical Contractor
1. Breaker Terminal Rating 75C Degree TYPICAL BREAKER QO CH QOB
2. # 10 Wire at 90C degree 40 amp
3. 1 Inch Emt.
4. Breaker 20 amp
5. Wire Thhn/thwn Solid
How many circuits can be in conduit

Derate
4-6 wires 80%
7-9 wire 70%
10-20 Wires 50%

I always have my neutral counted as current carrying.

How many? Breaker terminal confuses this. How does it affect. ?
 
The "standard" answer, without qualifications would be 20 conductors.
1" EMT will allow 20.
If you "always" have your neutral current-carrying then the neutrals must count in your ampacity adjustment
20 wires = 50% THHN = 40 amps = 20 amps.
110.14 allows you to terminate at the 75? if the terminals are so rated.

It get a bit "sticky" if we keep THWN and change to a wet location since the wet location rating of THWN is the 75? rating. Most conductors today are THWN-2 allowing you to continue to use the 90? rating for derating purposes.

Any other conditions that would effect the derating such as ambient have not been taken into
consideration.
 
Last edited:
For 20 amp, two wire circuits, without an EGC you could run 8 in a 1" EMT with #10 THHN.
 
I think the question may be trying to say how many conductors in the conduit without affecting the 20 amp capacity of the #10. The question is poor because it does not take into account the multiwire branch circuit where the neutral may not be counted as a current carrying conductor. Obviously 2 wire circuits would be at a lower limit then 3 or 4 wire circuits
 
Dennis Alwon said:
I think the question may be trying to say how many conductors in the conduit without affecting the 20 amp capacity of the #10. The question is poor because it does not take into account the multiwire branch circuit where the neutral may not be counted as a current carrying conductor. Obviously 2 wire circuits would be at a lower limit then 3 or 4 wire circuits
Click to expand...
I think Gus took the right approach, being 1" EMT is limited to 20 THHN conductors.

The max # of CCC's in any size conduit is higher. The minimum ampacity on a 20A breaker is 16A. THHN 90?C rating is 40A. Thus 16A/40A, or 40%, without consideration for possible ambient correction. So we could go as high 40 CCC's... as long as no load is greater than 16A. If we upped the load capacity to a full 20A, then we'd only be able to run 20 CCC's at 50% derating.
 
Smart $ said:
I think Gus took the right approach, being 1" EMT is limited to 20 THHN conductors.

The max # of CCC's in any size conduit is higher. The minimum ampacity on a 20A breaker is 16A. THHN 90?C rating is 40A. Thus 16A/40A, or 40%, without consideration for possible ambient correction. So we could go as high 40 CCC's... as long as no load is greater than 16A. If we upped the load capacity to a full 20A, then we'd only be able to run 20 CCC's at 50% derating.
Click to expand...


I know he did I was trying to explain to the op why the termination affects the install. I did not go into the details of it as Gus had done
 
infinity said:
Where did you find the reference that 1" EMT has a fill of 20-#10 THHN conductors?
Click to expand...

Obviously, out of thin air :D I actually went to the Tables but just as obviously need my money back on my recent cataract surgery... Table C1 1" 16 #10s so, as you stated (8) 2 wire branch circuits using the EMT as an equipment ground
for a total of 16 conductors.
(more circuits if you use MWBC)
 
iwire said:
Maybe, unless it is feeding receptacles. 240.4(B)
Click to expand...

Exactly. :)

The only limiting factor in this question is the number of #10 THHN conductors in the 1" EMT. The derating adjustment has no affect on the final ampacity of those conductors when using 20 amp OCPD. He stated the neutral is always current carrying so we can guess that maybe there is one per circuit or 8 circuits.
 
I didn't realize that neutrals weren't considered CCW if they were part of a MWBC. I was only made aware recently on one of the other threads. Just so I understand, any time the neutral is shared with two or more other phases on a MWBC it isn't considered a CCW, at least in terms of fill calculations.

Bob
 
iwire said:
Maybe, unless it is feeding receptacles. 240.4(B)
Click to expand...
Of course there are "unlesses". But it doesn't change the minimum ampacity possible... just under that condition. Same if a load exceeds 16A... it's a condition which would raise the lowest permitted conductor ampacity for that particular circuit. And the OP stated no particular load(s).
 
Smart $ said:
Of course there are "unlesses". But it doesn't change the minimum ampacity possible... just under that condition. Same if a load exceeds 16A... it's a condition which would raise the lowest permitted conductor ampacity for that particular circuit. And the OP stated no particular load(s).
Click to expand...

The "unlesses" are just as important as the rest, I found your statement about 16 amps misleading.
 
Dennis Alwon said:
I think the question may be trying to say how many conductors in the conduit without affecting the 20 amp capacity of the #10. The question is poor because it does not take into account the multiwire branch circuit where the neutral may not be counted as a current carrying conductor. Obviously 2 wire circuits would be at a lower limit then 3 or 4 wire circuits
Click to expand...

No multiwiring. Neutral Sharing. Sorry missed that.
 
rlundsrud said:
I didn't realize that neutrals weren't considered CCW if they were part of a MWBC. I was only made aware recently on one of the other threads. Just so I understand, any time the neutral is shared with two or more other phases on a MWBC it isn't considered a CCW, at least in terms of fill calculations.

Bob
Click to expand...

No multiwire. Most specs lately are against it anyways. Neutral Sharing
 
Fordean said:
No multiwire. Most specs lately are against it anyways. Neutral Sharing
Click to expand...

Also, I limit my circuits to about 8 to 9 amps max.


My question was.

How many circuits with each having their own neutral. Can be in a 1 inch EMT. With the Derating of 50% (#10 =40amps @ 90c).
And still legal for a 20 amp Breaker to be the ocpd.

I figured 8 about. But the Terminal Ratings of Breaker are only ( I don't have one in front of me 60c/75c). How bad does the Terminal Rating affect this. Hope I clarified this a little.
 
iwire said:
The "unlesses" are just as important as the rest, I found your statement about 16 amps misleading.
Click to expand...
Of course they are... and I'm not trying to make it appear otherwise. I used 16A because it is the extreme which yields the highest possible CCC count. Yes it will only apply under certain conditions. If you find it misleading, feel free to disregard... but that doesn't make it any less compliant under certain conditions.
 
rlundsrud said:
I didn't realize that neutrals weren't considered CCW if they were part of a MWBC. I was only made aware recently on one of the other threads. Just so I understand, any time the neutral is shared with two or more other phases on a MWBC it isn't considered a CCW, at least in terms of fill calculations.

Bob
Click to expand...

Not exactly, it depends on the system. Here's a little chart, if you notice in B) the neutral would be considered a CCC:


Neutral Conductors:
Here's some examples of when to count and not count the neutral as a current carrying conductor or CCC:

3?- 208Y/120 or 480Y/277 volt system-different circuit types:
A) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
B) 3 wire circuit w/ 2 ungrounded, 1 neutral = 3 CCC's
C) 4 wire circuit w/ 3 ungrounded, 1 neutral = 3 CCC's*

Notes:
A) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
B) In this circuit the neutral current will be nearly equal to the current in the ungrounded conductors so the neutral counts as a CCC
C) In this circuit the neutral will only carry the imbalance of the current between the three ungrounded conductors so it is not counted as a CCC, with an exception, *if the current is more than 50% nonlinear (see below for NEC article 100 definition) then the neutral would count as a CCC.

1?- 120/240 volt system-different circuit types:
D) 2 wire circuit w/ 1 ungrounded, 1 neutral = 2 CCC's
E) 3 wire circuit w/ 2 ungrounded, 1 neutral = 2 CCC's
Notes:
D) A normal 2 wire circuit has equal current flowing in each of the circuit conductors so they both count as CCC's.
E) In this circuit the neutral will only carry the imbalance between the two ungrounded conductors so the neutral is not counted as a CCC.
Nonlinear Load. A load where the wave shape of the steady-state current does not follow the wave shape of the applied voltage.
Informational Note: Electronic equipment, electronic/electric-discharge lighting, adjustable-speed drive systems, and similar equipment may be nonlinear loads.
Click to expand...
 
infinity said:
Not exactly, it depends on the system. Here's a little chart, if you notice in B) the neutral would be considered a CCC:
Click to expand...

Thanks Rob,
So pretty much the neutral should always be considered a CCW unless it is connected to task specific equipment where you will know what load types will be connected.
 
Status
Not open for further replies.
Top