Reducing conductor sizes towards end of circuit.

[new_ee]

This doesnt apply to a specific design I'm working on. But I've always wondered about it.

Say you have a circuit where you need to increase conductor size to account for voltage drop (length of run) and also for more than 4 current carrying conductors in a single raceway.

You start out with many conductors in the raceway serving separate loads. The loads are spread out so as you moved toward the end of the circuit you are dropping conductors out of the conduit to feed their loads.

As you start to reduce the number of conductors in the conduit can you also reduce the conductor size? Is this considered a "tap"?

 

[cpal]

If you review the definition of a tap conductor in 240.2 I believe you will see that a reduced conductor as you refer is not necessarily a tap. That is, if the smaller conductor has an ampacity (310.16T) that coincides with the overcurrent protection, it would not be considered a tap.

 

[charlie b]

I agree. So long as the "smaller conductors at the end of the run" will be protected by the overcurrent device at the begining of the run, it is not a tap. You are free to do this, if you wish.

 

[steve66]

I agree with Charlie, but I don't think it is often very practical. If you are running large wires, it usually saves more money to limit them to 4 conductors per conduit just by starting the runs with separate conduits.

And with smaller conductors, to reduce the size usually takes a splice. Most people don't like adding additional splices.

However, one case in which I think it does make sense is often parking lot lights. As the wire runs from light to light, the current drops on the portion of the wire that doesn't feed as many lights. So you can reduce the size of the wire without too much effect on voltage drop. And you can reduce the size at a point where there will be a splice anyway.

I've shown that on a plan before, but for all I know the electrican may have just decided it was easier to run the full size wire the entire length of the circuit.

 

[infinity]

Actually there is no requirement for the smaller conductors to be at the end of the run. They could also be in the beginning or the middle.

 

[LarryFine]

Actually there is no requirement for the smaller conductors to be at the end of the run. They could also be in the beginning or the middle.

Beep!

The location of the smaller conductors will indeed matter if the loads are spread along the circuit and not all at the end.

 

[infinity]

I have a 20 amp circuit, I run #12 AWG about 10' to the first light and then #10 AWG to the next light 200' feet away. How is this not code compliant?

 

[bkludecke]

I too agree that reducing wire size is ok. I do have some heartburn about doing this in residential construction. There is one EC that I know of that ran #12 homeruns for 15amp circuits if the homerun was over 50ft, then change to #14 for the rest of the circuit. Circuit breaker trips years later due to overload and an unsuspecting EC comes in and replaces the 15 with a 20. Now the #14 wire isn't properly protected.

 

[infinity]

I too agree that reducing wire size is ok. I do have some heartburn about doing this in residential construction. There is one EC that I know of that ran #12 homeruns for 15amp circuits if the homerun was over 50ft, then change to #14 for the rest of the circuit. Circuit breaker trips years later due to overload and an unsuspecting EC comes in and replaces the 15 with a 20. Now the #14 wire isn't properly protected.

Anyone who just changes a 15 amp CB to a 20 shouldn't be called an electrician in my book. And even though this is possible it still is code compliant to use different size conductors in the circuit.

 

[winnie]

I have a 20 amp circuit, I run #12 AWG about 10' to the first light and then #10 AWG to the next light 200' feet away. How is this not code compliant?

This is totally code compliant.

Also for a single load on a single circuit, it doesn't matter what order the thicker versus thinner wires take.

I believe that the point that Larry was making was that in a circuit with multiple loads distributed across the circuit, that it is _good design_ to put the thicker wire where the current is highest.

-Jon

 

[LarryFine]

This is totally code compliant.

Also for a single load on a single circuit, it doesn't matter what order the thicker versus thinner wires take.

I believe that the point that Larry was making was that in a circuit with multiple loads distributed across the circuit, that it is _good design_ to put the thicker wire where the current is highest.

Yeah, what he said. So there. :wink:

 

[iwire]

A common installation with mixed conductor sizing is site lighting.

We might use 10 AWG from the panel through the building to the point the conductors go underground, underground we might use 4 AWG and once at the pole we may switch to 10 or 12 AWG to get up the pole from the base.

 

[infinity]

A common installation with mixed conductor sizing is site lighting.

We might use 10 AWG from the panel through the building to the point the conductors go underground, underground we might use 4 AWG and once at the pole we may switch to 10 or 12 AWG to get up the pole from the base.

Yes this is a rather common installation practice. This is what I was thinking of when I said that the larger conductors don't need to be at the beginning of the circuit.

 

[new_ee]

Yes this is a rather common installation practice. This is what I was thinking of when I said that the larger conductors don't need to be at the beginning of the circuit.

I'm not sure that applies to the case I'm describing here. If there are more than 4 current carrying conductors in a conduit the size is increased to to heating effects, correct? If the beginning conduits have the higher # of conductors they they need to have the larger conductor sizes.

 

[ramdiesel3500]

Very interesting discussion. I just finished a design for a large parking lot at a shopping mall. They wanted 1000 Watt Metal Halide fixtures on 35' poles. I fed them with 480V circuits. It was a design/build bid situation so I had to come up with the most cost efficient design possible in order to be competitive on price. I was able to reduce conduit and conductor sizes at then ends of the circuit runs saving lots of $$ and labor. Usually I avoid having different sizes of conduit and wire on these jobs for the sake of simplicity. But in this case, better than half of the raceway/wire runs were able to be reduced in size without violating the voltage drop specs! Quotations are being reviewed so I don't know if we got the job yet!
I think Infinity was referring to the issue of OCP vs. conductor size in a circuit. You can jump all over the place on conductor size throughout a circuit so long as the smallest conductor in the circuit still has an ampacity equal to or greater than the overcurrent protection device protecting the circuit. Same is true for voltage drop so long as the calculated drop at the end of the circuit does not exceed the specified limits. But when it comes to "conduit fill", we must take into account rule 310.15(B)(2) for this issue as well. Gits kinder compillykated!!!

 

[tallgirl]

I'm not sure that applies to the case I'm describing here. If there are more than 4 current carrying conductors in a conduit the size is increased to to heating effects, correct? If the beginning conduits have the higher # of conductors they they need to have the larger conductor sizes.

Of course it doesn't apply -- like all good threads, this has now been hijacked and is a discussion about voltage drop

Given that copper is more pricey than steel, why are you upsizing your conductors and not adding a second piece of conduit? That's my only question.

 

[new_ee]

Of course it doesn't apply -- like all good threads, this has now been hijacked and is a discussion about voltage drop

Given that copper is more pricey than steel, why are you upsizing your conductors and not adding a second piece of conduit? That's my only question.

That's a good question. This is just a theoretical example.

 

[iwire]

I'm not sure that applies to the case I'm describing here. If there are more than 4 current carrying conductors in a conduit the size is increased to to heating effects, correct? If the beginning conduits have the higher # of conductors they they need to have the larger conductor sizes.

You are of course correct.

This is also a common installation.

Quite often I will run two or three larger conduits out from the panels in an electric room to a large junction box outside the electric room above the suspended ceiling so I can change to MC cable in the concealed space.

In the raceways I will run 10 AWG and once I get to the cables I may, or may not reduce to 12 AWG depending on the distance.

 

[peter d]

Quite often I will run two or three larger conduits out from the panels in an electric room to a large junction box outside the electric room above the suspended ceiling so I can change to MC cable in the concealed space..

Sometimes the original installer will run a few 1" conduits out of 42 circuit panel into an undersized can and then jam it with #12. I haven't seen that lately.