310.15(C) Ampacity Adjustment Factors (2023)


Explanation of 310.15(C): How and why we adjust ampacities when we have more than three current carrying conductors sharing a raceway or cable.


Remember that the currents on our opacity tables limit us to three current carrying conductors.

So this video is about 310.15c, ampacity adjustment factors.

How do we adjust these ampacities in order to legally carry more than three current carrying conductors in a raceway or a cable here? Are some related sections? 3.17 is a foundational section.

It tells us that I cannot put more conductors in a raceway then can adequately dissipate their heat it's like putting too many people on a train car.

Each of our body temperatures is going to start heating up everybody else around us.

And before we get to our destination, someone faints wouldn't.

It be nice.

If before we got on the train, we could all have a little ice pack something that would lower our individual temperatures.

So when we get on the train we're all safe by the time, we get to the conductors destination similar.

What we do is we restrict their ampacities.

So each conductor is starting off from a lower temperature that way when we put them in a raceway together, they still heat each other up a little bit, but not to the extent that they do damage to one.

Another 110.14c also restricts us to the opacities in the column with the lowest temperature rating of any connected equipment.

So we know we're often restricted to these amperages or these.

But a lot of our conductors are rated for 90 degrees celsius.

Now, it's the second sentence here or the second paragraph here that allow us to use the conductor's own ampacity based on its insulation rating.

So if I have a 90 degree rated insulation, I can use these opacities to start my calculation.

75 degree rated conductor.

These ampacities to start the calculation as long as I don't end up higher than what my restrictions are for the connected equipment.

These two sections both deal with short raceways.

If my raceways are no longer than 24 inches.

It doesn't matter how many conductors I have in there.

I don't need to restrict the opacity it's, a short enough raceway.

So that that heat can be dissipated through the other length of the copper or aluminum.

And this final one item, two deals with conductors who might have a different calculated ampacity along their length let's say, you calculate, 100 amps at the beginning.

It goes through a j box and leaving there it's calculated at 120 amps.

And then drops down further down item.

Two will show you how to deal with that and item three has some good informational notes below it.

So the basic process deals with my ampacity tables.

And also this table that you'll find a page or two before these tables, as we can tell the more conductors we have current carrying conductors the more we have the lower the percentage of my original ampacity that I'm allowed to use.

So the first step is to find out how many current carrying conductors I'm dealing with let's clarify.

A key point all of these conductors need to be counted when we're doing conductor fill how big of a conduit do.

I need to safely fit all the conductors.

However, 310.15c is just concerned with the conductors that will be adding heat.

So in the set, the written section 310.15 c1, it tells me I only need to consider power and lighting conductors in my diagram.

I drew some signal conductors they would not need to be counted, because they don't have an appreciable current.

They wouldn't be adding heat, they're just sending a signal back and forward, or maybe some small controls.

So for this calculation, the signal or control wires would not count zero wires.

However, there is an asterisk right beside the number of conductors on the table.

And it refers to three sentences written below the table.

The first sentence there tells me that if I have spare wires, I need to count all of them.

So here I have four spare wires, I'm, not using them now, they're, not adding heat now.

But when we do connect them in the future, I don't know what configuration they'll be used in, but they will start adding heat.


So I need to account for them in at this stage of the process.

The second sentence tells me to look at 310.15, e and f.

Now, e answers the question, do, I count the neutral or not, I have another video on that to talk about the specifics of that section.

But for now let's just say here, if I have two phases of a three-phase system with a shared neutral, I would need to count all of them the neutral included.

So here I have three current carrying conductors f deals with the ground.

And so I ask, you does the ground carry current.

It shouldn't, therefore, will it add heat to the bundle of wires? No.

So I do not need to count the ground.

The third sentence below the table talks about conductors that cannot simultaneously be energized.

I picked travelers, because if we think back to our three-way and four-way switches can the two travelers, both carry current at the same time, no it's either flowing on one or the other one.

So only one of the conductors is adding to the heat load at any particular time.

So this rule allows me to count both travelers, just as one conductor.

Now, the other conductor with them, depending where it is whether it's a switch leg neutral or or a hot it, um, it will be carrying the full load.

So the true travelers count as one.

But then this other conductor counts as another one two conductors for those three wires.

So here I have two three, four, add up to nine current carrying conductors as calculated in this section let's start by assuming all these wires are the same size.

If they're all number four th ends.

The basic process is to take the conductor's initial opacity from my ampacity tables.

Number four wire thn being a 90 degree celsius.

Insulation is good for 95 amps.

But because my conditions of use have changed, I now have nine current carrying conductors.

I need to use this table here.

And each of these conductors is now limited to 70 percent or 0.7 of their original opacity, giving a new ampacity of 66.5 amps for each of those conductors that's their max load under these conditions of use.

And those conductors would need to be protected accordingly by the rules back in 240.4, but let's say, not all the wires were the same size.

What if some of them were number fours? And some of them were number twelves, we would still have nine current carrying conductors according to our rules.

And therefore we would still be limited to 70 of any of the conductor's original ampacities.

So the number fours would still have a new opacity of 66 and a half amps.

And the number 12s let's double check on our table.

Number 12, wires, still, th, hhn.

So 90 degree weighting can start their calculation at 30 amps, 30 amps times 0.7 again.

The factor used for the total number of current carrying conductors 0.7 is 21 amps.

Each of their new ampacities would be 21 amps.

But I also picked number 12 just to remind us of the asterisk here that explains where to go at the bottom of the table, always double check your small conductor rules.

If you're using number 10s, 12s, 14s or smaller, okay.

The next item I'm looking at is this.

This basic process is to take the conductor's opacity times.

The factor is the new ampacity.

But what if when I designed this, I said, wait a minute, I want these conductors to each be able to carry 80 amps 66.

And a half is not enough.

Do I just have to keep trying with new opacities and see when the math works out? I could redraw the equation with 80 amps as the answer.

I still know what my factor will be, because I know how many current carrying conductors I'm going to have in the raceway so let's say, it's, still, 9 still 0.7 or 70 percent.

The unknown is, where do I start? How big of a conductor do? I need to end up with 80 ants after my d, rating process, add a little bit of algebra here by dividing each side by the same number 0.7.

And on the left side, they cancel out, leaving only the unknown on this side of the equation.

So whereas the original way we took the conductor's opacity times the factor from the table here.

We take our desired outcome, divided by the factor from the table to get 114.3 amps that's.

The starting point I need a conductor that can carry 114.3 amps at least before the d rating.

So number four is not big enough.

If I have a 90 degree conductor, I go down this column, that'll work.

A number three would be adequate, or if I had a 75 degree rated conductor, I would have to go up to a number two in order to have a large enough conductor.

So you can choose which method suits your circumstances.

The next thing I want to talk about is a very common branch circuit use of this.

But first a couple of rules on gutters and wire ways.

This is a common scenario in electric rooms is to have several panels nippled together to a common gutter or wire way and then going out through conduits into the field.

Now, the rules for our wire ways and gutters are at the end of chapter three.

But what we're interested in is when do we have to de-rate.

And if you look in those articles you'll find that if the gutter or wire way is non-metallic made of plastic or some other material, you have to de-rate anytime, you have more than three current carrying conductors as you did in our previous examples.

However, if these gutters or wire ways are made out of metal, you don't need to de-rate until you have more than 30 current carrying conductors.

And the way we measure them inside the the wire ways are cross-sectional area.

So if we have a bunch of wires coming through like this, we would take a cross-sectional area.

How many wires are going back and forward at this section or at this section or some other, and as long as it's less than 30 in a metal gutter, then I don't need to de-rate.

But once I go over 30 it's punitive look here, 40 percent or even less of my original ampacities.

What that means is you've got to be careful when you design a system like this, the circuits coming out of this panel.

You want to go out into the field through these conduits on this panel through these conduits.

And likewise down the wave, because if these conduits are all coming in willy-nilly and circuits here are going this way.

And this way I'm going to quickly get more than 30 current carrying conductors in some cross-sectional area and that's going to create problems.

There is one way around it.

If you've got a few extra conductors in there, look at 312.8, a lot of folks don't like to do this and for good reason, but if you meet the conditions there, you can sometimes take a conductor through this nipple through this enclosure.

And then this nipple to its breaker over here.

One of the requirements, however, is I need a label on here stating, where are the disconnecting means for any circuits that pass through this enclosure? So you got to be real careful with that planning is always key.

Another situation we run into here is to have a 20 amp breaker with a number 10, leaving it going out into the field, but isn't a number 12, even considering small conductor rules adequate to be protected by a 20 amp breaker.

Yes, it could be sized larger because of a spec it could be all home runs leaving 20 amp breakers got to be number 10 or for voltage drop.

We need to size up one size, but let's.

Consider it from this angle at least in our area.

A lot of our branch.

Conduits are one inch.

And we put we fill them up how full can you make a one inch conduit with number 10? Th, hhn thwn-2.

Our wire is often multiple rated.

Well, if I look in annex c, I can get in a 1 inch conduit 13 to 17 of these, depending on which type of conduit.

It is that puts me right here in the teens.

And in that case, I'm limited to 50 percent of my conductor's original ampacity, a number 10, 40 amps for 90 degree, rated installations.

And that works out perfect to 20 amps, 20, amp breaker.

So I'm, fine.

If I tried that with the number 12 I'm starting at 30 amps and half of that or even less, because I can fit more of them in a 1 inch, puts me at 15 amps or less half of this.

So 15 am breaker could work.

But in the commercial industrial world, we like 20 amp breakers.

We can get more receptacles on a circuit more lights on a circuit more loads, whatever they are on a 20 amp over a 15 amp circuit.

So you'll find them to be more prevalent, 20, amp circuits.

One final comment, I want to make here.

This video has been about 310.15c opacity adjustment.

But if we look at b, that's, another video, it's on temperature correction.

How do we adjust these amps? If the ambient temperature is different than that that the table is rated for? And if we need to do both of these opacity adjustment from c and correction from b, the process would be take my table amps, multiply by my factor for the adjustment.

And then multiply again by the factor from the tables for temperature correction, and that would equal the new amps.

So if you have to employ both of them that's the process, so I hope that's helped with ampacity adjustment and understanding various aspects of this.

Thank you.


What is an ampacity adjustment? ›

Conductor ampacity adjustment (reduction) is required when four or more current-carrying conductors are bundled together because heat generated by current flow is not able to dissipate as quickly as when there are fewer current-carrying conductors.

What table in the NEC gives the adjustment factors for your ampacity? ›

Table 310.15(B)(3)(a) provides adjustment factors when installing more than three current-carrying conductors (CCC).

How do you calculate allowable ampacity? ›

Ampacity can be calculated by dividing the Wattage by the rated voltage. The quotient is the expected Amperage of the circuit.

What are the factors of ampacity? ›

For cables in air the effect on cable ampacity of the following parameters is studied: conductor size, intensity of solar radiation, distance to the wall and cable grouping.


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