Teach me about derating please.

[signcrafter]

Put this in the wrong forum if a mod could move it to electrical.

Running some circuits in the garage in 1/2" and 3/4" conduit. Almost all is 12 gauge THHN stranded wire. So hots and neutrals are considered current carrying conductors. What about switch legs going to lights, are they also CCC? So if I understand things right a 3/4" conduit can have 16 number 12 wires in it. But once you get over so many wires you have to derate the circuit? What exactly does derating mean and how do I figure out what size wire and breakers to use when I have to derate?

Say I want to run 6 twenty amp outlet circuits down a wall in one conduit from the panel. So out of the panel I pull 6 hots, 6 neutrals, and a ground. So 13 number 12 wires. Well within the 16 number 12s I'm allowed. But I'm over the CCC derating rule right?

So first, what is the CCC derating limit?

Second, what exactly would that mean in my example? Would I be limited to 15 amp breakers on those circuits now if I have to derate them? Or if I wanted to stay with 20 amp circuits would I have to step up to number 10 wires and then re evaluate my wire fill in the conduit since I stepped up a size of wire and you are only allowed 10 number 10 wires in a 3/4" conduit?

Just trying to learn and understand derating and the rules associated with it. Examples of situations would be helpful also. Thanks

 

[tfi racing]

Six twenty amp circuits? What kinda heavy machinery are you and your helpers running 24/7 to really need that kind of capacity?

 

[kaymccampbell]

Packing a conduit like that is never a good idea. Why not just put one or two circuits per conduit? You skip over the derating issue and vastly increase your maintainability.

 

[sberry]

No need for a pipe for each one and sometimes it is the place to use multi wire. Part of the whole reason to run pipe is for multiple circuits and switch wires. I am going to let others splain it better, not that I know it all that well but you could pull 8 circuits in a pipe without derate under some methods. Something to somewhat consider here is that the intent is to keep heat down, not all these going to run at once fully loaded.

 

[JordanOH]

You only need 3 neutrals, 2 circuits can share 1.

 

[signcrafter]

My numbers were an example. Not what I'm planning to do. Was only looking to learn about derating so I can apply it in my wiring planning.

 

[NUTTSGT]

Put this in the wrong forum if a mod could move it to electrical.

Moved it for you.


Next time just shoot me a PM and I'll get it moved for you as soon as I see the PM.

 

[sberry]

I was told there wouldn't be any math?
I can generalize, we got experts here for real technical but with 3/4 can run 9 current carrying and as you posted 16 wires sounds about right. For common 120V the neutrals are counted, as multi wire they are not. So you are fixing the shop and want to wire? I have read many of your posts and I know its basically general so what to do is different than the formula.
I saw one recently that could have been done with less. I aint scared to run another pipe in a small install so it can be all 3/4. I don't want to deal with a whole nuther level of fitting and nothing beats a single circuit in a pipe to a welding outlet and today it can be done 1/2 inch even. Handy box. Metal cable even, same holes, same wire, same fittings and boxes. If I am adding another circuit to a welder in a piped system ideally going to want to run a pipe to the panel vs daisey chain thru boxes, nothing beats direct. This is different than complicated long expensive runs/routes, expence with manlifts etc and anything a guy can get in the floor is good as over doors being slow.
If you can run a pipe to your welder recept, figure out what the true demand really is, run a pipe. If you are remotely short of panel spaces and this is a longer run would spring for some 6 in the same pipe and put another 8 space on it. Takes a lot of congestion off the main, gives great service local to the panel , lets a guy branch instead of pulling lots of wires.

 

[sberry]

Makes it super easy to tailor or run to the branches. Don't even got to add it all at once if you are on surface. Can add thru short runs of conduit easy. While its popular to theory you can disassemble 1/2 the building to add a new wire its a lot of work where a direct wire can be repulled, the plus of the pipe is that it can have the extra difficult wire pulled at the time even in another color,, ha
It cost me a breaker space oor 2 where I had plenty to tag the lights and switching plus gfci circuits, I never got to go back to add. The reality is its 1/2 wasted especially,,,,,,,,,,,,,,,,,,, and I wired when 500 watt work lights were common, never thought we could do it in 50 or 100. Run a 4 1/2 grinder and a battery charger on general circuits, could do the whole place on one or 2 easy. Makes for a welder recept, even air comp.

 

[sberry]

Kay hit it simply, no need to pack each one by virture of some simple good design. I did one a while back where I simply ran so much wire they cant overload it. The extra breakers were irrelevant and I bought 1 roll of wire and used it all. Its a fault we get when we don't do the same calculation every day. I could have saved a couple wires in hindsight, if I was doing the exact same job today would economize it some. At the time it was just as easy and the place was in service 10 years and had 40 open spaces.
It had 400Ax2x200's and they wired it like a house and even went out of the way to daisey chain where the diy guy would have looked and said,,, its cheaper and easier to home run. They had cords everywhere every event. In to other rooms.
No one even gave it any thought what the floorplan was gonna do when they wired it.
Anyway, if there is a single outlet to a circuit they cant overload it with a power strip. You would have to get a true 3 way tap. Now they aint got them and only place to get on it is with 1 plug. My neighbor has a bunch of stuff, does all the sawing from 1 circuit, pulls the tool out, once in a while unplugs from one to another but that keeps a bunch of shat out of the way.
I got a bud got a Bendpack hoist. Chopped off a 16 orange cord, strung it up just to "test" it. Plugged it in to the nearest outlet and its been like that for 35 years. Simply no other loads are on it while its in use. 2 guys use it, every day. They don't vacuum, chop saw, none of that while lifting.

 

[alfredeneuman]

First you have to derate the conductor for the ambient temperature. (Because a rooftop or attic at 120° and a walk in freezer at -10° wont be the same value. The default value is 86° F)

All derating after that is figured using the 90°C column.
If you use a separate neutral wire for every hot, it counts as a current carrying conductor. Multiwire neutrals aren't subject to this, unless non linear loads (computer power supplies, electronic ballasts etc) are involved.

In the case of #12 THHN/THWN for instance the 90° rating is 30A.

4-6 conductors require the rating to be reduced to 80%
7-9 @ 70%
10-20 @ 50%
21-30 @ 45%
31-40 @ 40%
41 and above @35%

In the case of #12,
30 X .70 = 21A (thus 9 cccs can go on a 20A breaker)
30 X .50 = 15A (and must go on a 15A breaker)
....and so on and so forth

 

[alfredeneuman]

Metal wireways aren't subject to derating for 30CCCs or less.
Once 31 is hit, they're subject to the regular rules of 40%.
Nonmetallic wireways are treated the same as conduit,

 

[sberry]

Good. So you could do this by multi wires. If you run separate neutrals then only 5 circuits + possibly a switch leg. The reality of this in a general shop is about moot. 1, maybe 2 of these ever see significant loading at a time unless you are constructing specific machinery or process.
I got one with 9 conductors for about 10 ft where its got some switch wires in it. Fixxed load of 200 watts, Grinder on another circuit on occasion of 6A, hoist/welder circuit, 2 number 10 in this deal Grinder doesn't run while either of the other operations go on with the wire being a size bigger than the welder needs and 2 for the hoist. Of the 9 conductors in reality only 6 real possibles could work at once and that is including welding. Even that wire only 2/3 loaded.
To the first j box pulled 8 current and 1 neutral, 2 20A 120 with own white to the fact they were gfci, neutral for 2 other lights. In my case had to count 2 neutrals as cc.
My lights would have the capacity for 3K watts or more, I got 300 in actual and could even cut that in half with led.

 

[Dh3256]

You only need 3 neutrals, 2 circuits can share 1.

Shared neutrals violate NEC in that application - so he cannot legally or safely do that.

 

[alfredeneuman]

Shared neutrals violate NEC in that application - so he cannot legally or safely do that.

Why does it violate the NEC?

 

[sberry]

so I can apply it in my wiring planning.

Figure out what you need then reverse engineer vs trying to apply derating first.

 

[MattT]

Multiwire neutrals aren't subject to this, unless non linear loads (computer power supplies, electronic ballasts etc) are involved.

A lot of traditionally linear loads are changing to non linear so it's safer to include neutrals IMO. Couple examples applicable to a shop are ECM "brushless" power tools and inverter window units. And inverter welders & plasma cutters.

Why does it violate the NEC?

I'm guessing either CAFI or GFCI protection is required which would make it difficult for all the neutrals to be non current carrying. The first GFCI receptacle will add two current carrying conductors unless the OP is hoe running every receptacle.

 

[sberry]

That is what my babble was about to some extent. I wanted a couple circuits like kitchen counter tops and didn't want the protection at the end, used common gfci units at the start.

 

[alfredeneuman]

.
I'm guessing either CAFI or GFCI protection is required which would make it difficult for all the neutrals to be non current carrying. The first GFCI receptacle will add two current carrying conductors unless the OP is hoe running every receptacle.

One neutral is not subject to mass classification one way or the other.
One neutral of a MWBC can be current carrying or not, depending on it's location in the circuit. When split into (2) 2wire circuits it would be current carrying.
When combined it may be non current carrying because the loads on the 2 legs cancel out each other.

I'll wait until Dh3256 returns to give me the answer

 

[sberry]

Yup, if the neutral is fully loaded means there is none on the other ungrounded conductor. Some can be designed so as to cut the current in half reducing v drop using multi wire.

 

[sberry]

So, double the voltage at the same amps and double the wattage. Double the voltage to get the same wattage and half the amps on the same wire. 2400 watts at 120v and both wires have 20A, at 240 each wire 10A.

 

[signcrafter]

Metal wireways aren't subject to derating for 30CCCs or less.
Once 31 is hit, they're subject to the regular rules of 40%.
Nonmetallic wireways are treated the same as conduit,

So since 3/4" conduit can only have 16 number 12 wires in it then I don't have to worry about derating at all? And when you say nonmetallic is that PVC?

Figure out what you need then reverse engineer vs trying to apply derating first.

Because I like to have a general concept of the rules. My example was just that, an example. Reverse engineering is good but having knowledge of the concepts is also good so you know how to apply them in different situations. But if I'm understanding alfredeneuman correctly I won't ever have to worry about derating because I will never run into a conduit with 31 wires in it. So hopefully I'm understanding that correctly.

 

[mm08822]

So since 3/4" conduit can only have 16 number 12 wires in it then I don't have to worry about derating at all? And when you say nonmetallic is that PVC?



Because I like to have a general concept of the rules. My example was just that, an example. Reverse engineering is good but having knowledge of the concepts is also good so you know how to apply them in different situations. But if I'm understanding alfredeneuman correctly I won't ever have to worry about derating because I will never run into a conduit with 31 wires in it. So hopefully I'm understanding that correctly.

You are not. A wireway is not a conduit.

CCC derating in conduit is subject to the details he provided in reply#11.

 

[signcrafter]

You are not. A wireway is not a conduit.

CCC derating in conduit is subject to the details he provided in reply#11.

Guess I misunderstood that one.

 

[sberry]

When I read all the rules first a lot of it didn't make sense. Nothing wrong with that but some specific applications to relate to helps. Even helps with the language sometimes, raceway vs conduit. The reason we spun off a little was to demonstrate methods of bringing it in compliance and reducing both the number of ccurrent conductors and number of wires.
Some of my babble was in general principle, one being to get a real grip on the load and demand. This would certainly be more critical say in a factory where it might be engineered and all running at max capacity. What part of my point was while the letter of the code is what it is in reality most of it moot without applied load.
While it is suggested to limit the number of recepts on a general circuit the reality is if there is a hundred it's irrelevent if there is one lamp on it.
Might have a 20A circuit, dedicated one with 5 amps load. Some of this comes with other factors, already using a derate for motor loads plus a 125%, could have a 12 with a 30 breaker, how does that effect derate? Etc.

 

[sberry]

Is there a senerio here we can alloy 180 of current thru this pipe?

 

[mm08822]

Is there a senerio here we can alloy 180 of current thru this pipe?

With 50% derating factor (used for 10-20 ccc), if eight 15a circuits were run through the conduit (16 #12's) and each circuit was loaded to 15a each, there would be 240a running through all of the copper at any cross-section.

In each linear foot of conduit, there would be 114w of heat generated.

P=I**2 * R = 240a**2 * (1.98ohms/1000ft)
*************
With 70% derating factor (used for 7-9 ccc) and when 20a were flowing through each of four #12 20a ckts, there would be 50w/ft of heat generated.

*************
If by error, there was no derating used at all and 20a were permitted to flow through each of the eight ckts, there would be 202w/ft of heat generated.

 

[signcrafter]

First you have to derate the conductor for the ambient temperature. (Because a rooftop or attic at 120° and a walk in freezer at -10° wont be the same value. The default value is 86° F)

All derating after that is figured using the 90°C column.
If you use a separate neutral wire for every hot, it counts as a current carrying conductor. Multiwire neutrals aren't subject to this, unless non linear loads (computer power supplies, electronic ballasts etc) are involved.

In the case of #12 THHN/THWN for instance the 90° rating is 30A.

4-6 conductors require the rating to be reduced to 80%
7-9 @ 70%
10-20 @ 50%
21-30 @ 45%
31-40 @ 40%
41 and above @35%

In the case of #12,
30 X .70 = 21A (thus 9 cccs can go on a 20A breaker)
30 X .50 = 15A (and must go on a 15A breaker)
....and so on and so forth

Are these numbers the same for all gauges of wires or just for the number 12 example?

Does the size of conduit matter or does it just go by the number of CCC no matter the size of conduit? For example, if you have 10 number 12 CCC in 3/4" vs in 2" conduit? Is it still 50% derating no matter the size of conduit?

 

[ard]

I was told there wouldn't be any math?.

lol

 

[alfredeneuman]

Does the size of conduit matter or does it just go by the number of CCC no matter the size of conduit?

It goes by the number of CCCs no matter the size of the conduit.
You could have (10) #10s in a 4" and they would still have to be derated to 50%

 

[sberry]

With 50% derating factor (used for 10-20 ccc), if eight 15a circuits were run through the conduit (16 #12's) and each circuit was loaded to 15a each, there would be 240a running through all of the copper at any cross-section.

In each linear foot of conduit, there would be 114w of heat generated.

P=I**2 * R = 240a**2 * (1.98ohms/1000ft)
*************
With 70% derating factor (used for 7-9 ccc) and when 20a were flowing through each of four #12 20a ckts, there would be 50w/ft of heat generated.

*************
If by error, there was no derating used at all and 20a were permitted to flow through each of the eight ckts, there would be 202w/ft of heat generated.

Right, I was just guessing at how much current could be running thru it and what the net heat would be,,, now what about 2 wires, one running a chop saw and a grinder for a minute and 3 amps of lights,,,

 

[Terry D]

Conduit size determines how many conductors can be installed in it. Derating determines how many amps can be ran through those conductors

 

[sberry]

As I recall Sign has an auto repair, general shop. I don't recall all the details. He said 1 wall in this thread. Something to remember is that not every 1 needs its own circuit. I am about sure he is not running air comp from 120V. Air is really the garage factor if its on automatic but even then a guy can stop welding or not start when its on.
I have 400A and rarely weld with air on, plasma a little. I have a couple other 3 hp loads that most don't have but it doesn't dent the demand,,, or the supply.
Could run darn near everything I own thru a couple conduits.

 

[Dh3256]

Why does it violate the NEC?

NEC prohibits shared neutrals.

The neutral is a current carrying conductor - conventional current flows out the hot and back through the neutral. If a neutral is shared, the neutral would have to be sized to accommodate twice the current of the hots.

The one allowable exception that has been grandfathered in is that in a two phase system (conventional home wiring) if the two hots are connected to opposite phases in some cases a shared neutral may be allowed. In this case, the current flow in the neutral is opposite for each hot and at full load will cancel out to zero.

 

[Terry D]

NEC prohibits shared neutrals.

The neutral is a current carrying conductor - conventional current flows out the hot and back through the neutral. If a neutral is shared, the neutral would have to be sized to accommodate twice the current of the hots.

The one allowable exception that has been grandfathered in is that in a two phase system (conventional home wiring) if the two hots are connected to opposite phases in some cases a shared neutral may be allowed. In this case, the current flow in the neutral is opposite for each hot and at full load will cancel out to zero.

No one meant sharing a neutral with two hots on the same leg. You definatly cant do that. It wouldn't even be called a neutral, it would be the grounded conductor. A neutral carries the unbalanced load of a multiwire circuit. Residential power is not 2 phase, it is single phase. I believe there was/is a 2 phase system. But I have never seen it. Single phase is generated with the legs 180 degrees apart, 3 phase is 120 degrees apart.

Came across this:

https://en.wikipedia.org/wiki/Two-phase_electric_power

 

[mm08822]

NEC prohibits shared neutrals.

The neutral is a current carrying conductor - conventional current flows out the hot and back through the neutral. If a neutral is shared, the neutral would have to be sized to accommodate twice the current of the hots.

The one allowable exception that has been grandfathered in is that in a two phase system (conventional home wiring) if the two hots are connected to opposite phases in some cases a shared neutral may be allowed. In this case, the current flow in the neutral is opposite for each hot and at full load will cancel out to zero.

We do not have two phase service in use today. Residential services are typically 120/240v single (split phase) phase.

I don’t believe the post you replied to was inferring to share a neutral from the same leg of the split phase service. Multi-wire branch circuits (MWBC) are permitted with a 2 pole cb used to ensure opposites legs of the split phase is used. This the electrical system where the neutral currents can cancel all or a portion out. Neutral does not need to be upsized.

If a common neutral was used on a true 2 phase system, the currents would not cancel and an over-sized neutral would be required.

 

[exranger06]

NEC prohibits shared neutrals.

The neutral is a current carrying conductor - conventional current flows out the hot and back through the neutral. If a neutral is shared, the neutral would have to be sized to accommodate twice the current of the hots.

The one allowable exception that has been grandfathered in is that in a two phase system (conventional home wiring) if the two hots are connected to opposite phases in some cases a shared neutral may be allowed. In this case, the current flow in the neutral is opposite for each hot and at full load will cancel out to zero.

You just described a multi-wire branch circuit (MWBC). MWBCs are not "grandfathered in," they're still a perfectly acceptable way of wiring circuits. Many electricians use them on new installs, and the 2020 NEC explicitly says they're still allowed (210.4). And as others have already said, houses don't have "two phase" systems.

 

[Dh3256]

No one meant sharing a neutral with two hots on the same leg. You definatly cant do that.

That seems to be what the posted I responded to did say. Maybe the terminology is unclear.

It wouldn't even be called a neutral, it would be the grounded conductor.

NO, the EGC is a completely different thing. We have to make certain no current flows in the EGC or it may not protect life in the event of a fault.

A neutral carries the unbalanced load of a multiwire circuit.

Which by definition is a neutral shared between two hot wires and is not permitted, in general, by NEC.

Residential power is not 2 phase, it is single phase.

Engineers typically refer to it as two phase, which it technically is, but some electricians prefer the term split phase. The 240 VAC (or 208 VAC) voltage provided to the residence has a reference neutral and each leg of the 240 VAC is 180 degrees out of phase. Typically it is generated from one pair of a three phase supply, and whether it is 240 VAC or 208 VAC depends on whether the 3 phase is wye or delta.

Hayt and Kemmerly is the classic text and treats the subject well if you want to learn more about this.

 

[alfredeneuman]

That seems to be what the posted I responded to did say. Maybe the terminology is unclear.

The neutral is typically connected to ground at the panel but is NOT a "grounded conductor".

Neutral IS the grounded conductor. That what the NEC calls it. Article 200 is titled > Use and Identification of Grounded Conductors.

EGCs are grounding conductors.

 

[alfredeneuman]

Engineers typically refer to it as two phase, which it technically is, but some electricians prefer the term split phase. The 240 VAC (or 208 VAC) voltage provided to the residence has a reference neutral and each leg of the 240 VAC is 180 degrees out of phase.

I know Electrical Engineers and they don't call it 2 phase. I've NEVER seen a set of plans with the term 2 phase. (after 45+ years)

208 VAC has phases 120° apart.