Derating more than 3 current carrying conductors in a conduit

[sharnrock]

I believe the article is 310.16. My wife took my code book with her this morning, sorry.

After being busted for this on a job I was on, there seemed to be much confusion about how to go about counting the number of current carrying conductors. The confusion being the exception about not having to count the neutral if it only carries the unbalanced load. When I was just starting out I had a journeyman explain that when you have a single split phase system the 2 hots go back and forth acting as returns to ground. That is why something like a water heater only needs two lines. The reason a neutral can be downsized on a residential service is because it only carries the unbalanced load. So, I figure that something like a receptacle circuit would have a hot and a ground. That would count as 2. Something like a range would have 2 hots and a neutral. That would also count as 2 because the neutral carries the unbalanced load.

Now, the inspector and 2 journeyman have explained to me that if you have a power circuit it counts as 1. Their reason being, if it's on the same circuit it doesn't count, but if you read a little more it says, "and it's only carrying the unbalanced load." If you have range it counts as 2 and if you have a shared neutral (such as dish/disposal) it counts as 3. This doesn't make any sense to me. It seems as though my perception as to how this all works is flawed.

I tried looking on the internet for a definition of unbalanced neutral or how to count this crap, but I didn't really turn up anything useful. Please set me straight, this has been bugging me for months.

 

[mcclary's electrical]

You're confusing yourself by making this too overly complicated. ALWAYS count a neutral as a CCC. EVERY INDIVIDUAL CONDUCTOR in a raceway counts as one CCC. EVERY NEUTRAL in a raceway counts as one CCC. Grounds HAVE NOTHING to do with this calculation. only worry about them when figuring for box fill,,,,,,in which ALL GROUNDS combined equals the same fill as ONE of the largest conductors in the box.

 

[sharnrock]

thank you. But then when is there ever an unbalanced neutral that doesn't need to be counted?

 

[augie47]

You have the basic concept. As a general rule, neutrals that only carry the unbalanced load are not counted. So a two wire circuit with a neutral is obviously counted. On a single phase system a 3 wire circuit with phase A, phase B and the neutral, the neutral is not a ccc as it is carrying the unbalanced load. On a three phase, A, B, C + neutral, again neutral doesn't count .
The exception is where, on a three phase system, the major portion of the load is non-linear (electronics, ballasts, etc) you must still count the neutral.
On a normal run with 20 amp circuits and #12 THHN you have to exceed 9 ccc to create a problem, so a good rule of thumb is just keep your conduit fill to 9 wires (+ equiop ground) and avoid the problem.

 

[480sparky]

........so a good rule of thumb is just keep your conduit fill to 9 wires (+ equiop ground) and avoid the problem.

Nine wires, or nine cccs?

 

[augie47]

some of the guys here just follow the "9 wire" rule... that normally gives them all they want and they don't have to try and make the determination. Not the most economical, of course, but no headaches.

 

[sharnrock]

so, you are saying that a range would still be counted because of the electronics used. What would be an example of something with an unbalanced neutral?

if my other post shows up, sorry, I believe the mod is on a coffee break or something.

 

[Dennis Alwon]

The reason a neutral can be downsized on a residential service is because it only carries the unbalanced load.

I don't believe that MWBC is the reason you can downsize a neutral and I also believe you can downsize a neutral on any service but it may not be advisable in commercial applications.

The reason to you may downsize is because the calculated load on a neutral is usually less because there are often many 240 volt (208V0, etc) circuits that do not need a neutral.

Let's look at a residence- you possibly have an electric hot water heater, heat pump/ ac , a blower with heat strips, well, etc. These circuits don't use neutral and thus would not be need in your calculation. The fact is I can downsize a neutral on a 200 amp service to the size of my GEC (#4 copper) if the calculations allow me. I have never seen that done but it is permitted. Usually I downsize to 1/0 copper on a 200 amp service in a residence and I never pull MWBC.

 

[Dennis Alwon]

thank you. But then when is there ever an unbalanced neutral that doesn't need to be counted?

On single phase if you have a neutral with 2 hots, on opposite phases, then it would not be considered current carrying.

For three phase the neutral would not count when you have a neutral with 3 hot wires from different phases.

If you have 2 hots and a neutral on a 3 phase system then it does count.

 

[iwire]

if my other post shows up, sorry, I believe the mod is on a coffee break or something.

Actually out shampooing the carpet in my truck. :roll: :grin:

 

[sharnrock]

On single phase if you have a neutral with 2 hots, on opposite phases, then it would not be considered current carrying.

For three phase the neutral would not count when you have a neutral with 3 hot wires from different phases.

If you have 2 hots and a neutral on a 3 phase system then it does count.

So a range would be 2 as well as a dish and disposal? It says, "on the same circuit." I thought that since it was 2 appliances it would be considered different circuits.

 

[Dennis Alwon]

So a range would be 2 as well as a dish and disposal? It says, "on the same circuit." I thought that since it was 2 appliances it would be considered different circuits.

You need to be clearer. I assume you are talking single phase. Now the range would count as 2 but are the dw and disposal on a MWBC, or are they separate circuits being fed with their own neutrals and hots, or are they on the same neutral and the same hot (same circuit). Don't mix phases and circuits.

 

[don_resqcapt19]

You're confusing yourself by making this too overly complicated. ALWAYS count a neutral as a CCC. EVERY INDIVIDUAL CONDUCTOR in a raceway counts as one CCC. EVERY NEUTRAL in a raceway counts as one CCC. Grounds HAVE NOTHING to do with this calculation. only worry about them when figuring for box fill,,,,,,in which ALL GROUNDS combined equals the same fill as ONE of the largest conductors in the box.

There is never an need to count the grounded conductor of a single phase multiwire circuit as a current carrying conductor for the purposes of derating.

 

[LarryFine]

... if you have a shared neutral (such as dish/disposal) it counts as 3.

If you're talking about a 3-wire MWBC, two hots (from opposite phases) sharing a neutral, this is incorrect. It still only need be counted as two CCC's.

 

[LarryFine]

ALWAYS count a neutral as a CCC. EVERY INDIVIDUAL CONDUCTOR in a raceway counts as one CCC. EVERY NEUTRAL in a raceway counts as one CCC.

Not correct. See above.

 

[erickench]

For a two wire circuit and a ground it would count as two because the neutral in this case is not carrying any unbalanced current and is therefore current carrying. For a three wire single phase circuit plus a ground it would still count as two because the neutral is carrying unbalanced current. But if the three wire circuit is derived from a four wire three phase system then it would count as three because the neutral in this case is current carrying. EGCs are never counted as current carrying conductors.

 

[zappy]

For a two wire circuit and a ground it would count as two because the neutral in this case is not carrying any unbalanced current and is therefore current carrying. For a three wire single phase circuit plus a ground it would still count as two because the neutral is carrying unbalanced current. But if the three wire circuit is derived from a four wire three phase system then it would count as three because the neutral in this case is current carrying. EGCs are never counted as current carrying conductors.

Hey I learned something. So why is a singe phase 240v. load neutral not counted, but a three phase 240v. load it is? Does it have something to do with the transformer?

 

[Smart $]

... But if the three wire circuit is derived from a four wire three phase system then it would count as three because the neutral in this case is current carrying. ...

Hey I learned something. So why is a singe phase 240v. load neutral not counted, but a three phase 240v. load it is? Does it have something to do with the transformer?

Actually Eric's statement is only correct regarding 3? 4W wye systems. For 1? 3-wire circuits (hi-leg not used) supplied by a 3? 4-wire delta system, the neutral would not count as a CCC.

 

[brother]

Ive always wanted to actually conduct an experiment on the shared neutral on a 3 phase system. I always just went off the code and what was taught to me, but I never actually put an amp meter on a circuit that was wired wrong (same phase) with the shared neutral.

Has anyone conducted this experiment/???

 

[LarryFine]

Has anyone conducted this experiment/???

No need. Picture a balanced 4-wire 3ph MWBC with, say, 10a per phase; no neutral current. Now, reduce the load on one phase to, say, 5a. The neutral current increases by the exact same amount, to 5a. Take that phase to zero, and the neutral current is now 10a.