Splicing Different Wire Sizes for Voltage Drop

[theophilus88]

I have about a 1000 foot run for several light pole fixtures at 277V, 1PH (See attached VD calc below). Normally in this scenario, I would just take the voltage drop for the entire run and try to maintain my 3% VD for the branch circuit using one wire gauge along the entire run. However, I have seen multiple scenarios before where the engineer/ contractor has taken the first several hundred feet to the first light pole and used a higher wire size and then from there they drop their wire size to a smaller gauge. IE) segmented cable sizes. Is this code acceptable (If so where in the code does it allow this), and does it even save that much money? I have been asked to do this for one of my projects. Thanks in advance for any answers!

 

[david luchini]

Is this code acceptable (If so where in the code does it allow this)

Yes, it's code acceptable (240.4)

 

[infinity]

I have seen multiple scenarios before where the engineer/ contractor has taken the first several hundred feet to the first light pole and used a higher wire size and then from there they drop their wire size to a smaller gauge. IE) segmented cable sizes. Is this code acceptable

Yes it's acceptable, because VD compensation is not even required for your application by the NEC. But even if it were required the method of reducing conductor size somewhere is the run is fine.

 

[wwhitney]

Your table has an error, assuming the whole run is protected by a <= 30A breaker at the source (rather than having additional OCPD at the first light). The EGC in the 1st segment needs to be a #6, per 250.122(B).

Cheers, Wayne

 

[theophilus88]

Yes, it's code acceptable (240.4)

Where specifically in this code section does it say that you are allowed to splice different wire sizes together?

 

[wwhitney]

Where specifically in this code section does it say that you are allowed to splice different wire sizes together?

It's more that the practice is never explicitly disallowed, the controlling requirements are in 240.4, and the practice complies with all the 240.4 requirements. Codes are generally permissive, so if it's not disallowed, it's allowed.

Cheers, Wayne

 

[david luchini]

Where specifically in this code section does it say that you are allowed to splice different wire sizes together?

It doesn't specifically say that anywhere. It says conductors must be protected in accordance with their ampacity.

#10awg and #6awg would both be protected from overcurrent by a 30A c/b.

 

[theophilus88]

Your table has an error, assuming the whole run is protected by a <= 30A breaker at the source (rather than having additional OCPD at the first light). The EGC in the 1st segment needs to be a #6, per 250.122(B).

Cheers, Wayne

The entire run is protected by a 20A breaker.

It doesn't specifically say that anywhere. It says conductors must be protected in accordance with their ampacity.

#10awg and #6awg would both be protected from overcurrent by a 30A c/b.

Roger that! that makes sense to me. Are there any requirements when splicing these wires together. IE) Is this considered a tap of some sort? Do I need to have another form of OCPD when I splice these two?

 

[wwhitney]

The entire run is protected by a 20A breaker.

Right, so your first segment needs a #6 EGC per 250.122(B).

Cheers, Wayne

 

[david luchini]

Is this considered a tap of some sort? Do I need to have another form of OCPD when I splice these two?

No...and No.

 

[wwhitney]

Roger that! that makes sense to me. Are there any requirements when splicing these wires together. IE) Is this considered a tap of some sort? Do I need to have another form of OCPD when I splice these two?

The only requirement is a connection method rated to join a #10 and a #6. So no, and no.

An interesting question is for a layout such as described in the OP, whether it's easier/cheaper/better to run an MWBC with 4 (smaller) circuit conductors than to run a single 2 wire circuit. Splitting the load among 3 different line conductors reduces the current and hence voltage drop; and the currents in the neutral partially cancel, reducing the current and hence voltage drop.

Also, IIRC, when doing an MWBC and it's necessary to upsize for VD, it's economical (greater VD reduction per unit copper) to use a larger neutral than the line conductors. The basic idea being that the neutral upsizing helps reduce the voltage drop for all the loads, while upsizing a line conductor only helps for 1/3 of the loads. [Edit: in fact there's a post somewhere earlier where I did some of the math.]

Cheers, Wayne

 

[theophilus88]

The only requirement is a connection method rated to join a #10 and a #6. So no, and no.

An interesting question is for a layout such as described in the OP, whether it's easier/cheaper/better to run an MWBC with 4 (smaller) circuit conductors than to run a single 2 wire circuit. Splitting the load among 3 different line conductors reduces the current and hence voltage drop; and the currents in the neutral partially cancel, reducing the current and hence voltage drop.

Also, IIRC, when doing an MWBC and it's necessary to upsize for VD, it's economical (greater VD reduction per unit copper) to use a larger neutral than the line conductors. The basic idea being that the neutral upsizing helps reduce the voltage drop for all the loads, while upsizing a line conductor only helps for 1/3 of the loads.

Cheers, Wayne

 

[theophilus88]

Yes, agreed! Thank you for all of your input so far! Ultimately I am just trying to save money while still being code compliant. I do design build work for contractors. Some more questions I had are..

1) Is it difficult to splice a #6 with a #10

2) Is the scenario I have shown really saving that much money?

 

[wwhitney]

Others have more experience on pricing. But bear in mind that each light (except the last) is going to get a 3-way splice between incoming feed, outgoing feed, and side branch (presumably a minimum #12 given the 20A OCPD).

So if you compare the table to a solution that is all #6, you have savings of 38 * 7 * 2 feet of #10 instead of #6, and you have some savings on the splices: your table requires 1 splice of #6, #10, and #12, and 7 splices of #10, #10, #12, vs 8 splices of #6, #6, and #12.

Depending on the splice mention, the size might make no difference. E.g. if you use 3-way Polaris-type insulated splices, and the model that takes #10 and #12 also takes #6 (not sure on that). But for other methods the splices that only involve #10 and #12 may be easier/cheaper.

Cheers, Wayne

 

[tom baker]

Street lighting systems I have designed, based on State DOT standards have a fuse in the supply to each pole, The fuse rating is based on each luminaire, for example 2 amp. This prevents a faulted driver from taking out the entire circuit. Bussman is a great source for fuse and holders.
Also, I would suggest using XHHW-2 wire, it holds up much better in underground runs, esp at 277 V, than THHN/W.