Underground Box

A

Alwayslearningelec

Senior Member
Location
NJ
Occupation
Estimator
Anyone or familiar with underground boxes that have barriers? Is it uncommon or not practical to bring 120 and 277 into same box? I have light poles that the light is 277 and the receptacle is 120v.

Sorry..just reached out to my quazite guy and he said they do have barriers for them.
 
Alwayslearningelec said:
Because of the different voltages. Usually spec always says that 120/208 and 277/480v entering same box must have barrier. Obviously not a code thing??
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No they can occupy the same box without a barrier unless one is for emergency systems and the other is not. The neutral for each system would require a distinguishable type of identification like white for 120 volts and gray for 277 volts. They're both going in the same light pole which doesn't have a barrier?
 
infinity said:
No they can occupy the same box without a barrier unless one is for emergency systems and the other is not. The neutral for each system would require a distinguishable type of identification like white for 120 volts and gray for 277 volts. They're both going in the same light pole which doesn't have a barrier?
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yea so they want a separate conduit for the light pole light and the receptacle at the pole. So two conduits which is a bit ridiculous but no other way I guess.
 
So I need to run # 8's due to VD and the engineer confirmed this. I would not be able to terminate #8's on the receptacle correct? The junction box on the wall is where we are picking up the feed. Another contractor is running from panel to that foundation wall box. May a box where I peel off to the receptacles with #12?

1741035111875.png
 
That's where I get confused so why is it that at some point you can reduced the upsized wire down to say #12's to feed the load, in this case the receptacle? What dictates that " at this point" you can now go from #8's to #12? why is #12 all of a sudden ok when branching off.

Like this example below. We are picking up from the j-box on the wall from another subcontract who's binging the feed from a panel about 150-200' away. Can I just reduce at that box from #8's to #12'a and feed the receptacles?


1741035985665.png
 
Maybe I should put a box under each light pole. and run the lighting conduit and receptacle conduit to it??

Or maybe a box between the pole( purple dots) like below.

1741037053449.png
 
Alwayslearningelec said:
That's where I get confused so why is it that at some point you can reduced the upsized wire down to say #12's to feed the load, in this case the receptacle? What dictates that " at this point" you can now go from #8's to #12? why is #12 all of a sudden ok when branching off.
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Why are you upsizing in the first place? Voltage drop? Is #12 normally adequate for the load? If yes then if only concern is the voltage drop and to achieve a certain minimum VD then the ability to change back to the smaller conductor could be at any point the the voltage drop would no longer be effected. When you want to maintain a minimum voltage at a given amperage you could even start at a smaller size then go up to the larger one and reduce back down to land in a device.

Your scenario of a #12 assuming 15 or 20 A. Reducing to the #12 protected by appropriately sized OCP device could be done at any point that would no longer create excessive Voltage drop. Depending on your final amperage you need, Southwire app shows about 60ft of #12 without exceeding 3%VD and #8 could be 155ft for the same load requirement. Or a combination of both for lesser distance. but even if you're close the the max of #8 a few inches or ft won't make a significant difference.

I once needed 120V 8A at almost 1200ft out, A #2 can't be landed on a 15A breaker or receptacle so started with a #12 at breaker jumping to the #2 run the distance and drop back to the #12 about 5ft from the receptacle outlet. Final came back with only about 1.5 volts drop.
 
Fred B said:
Why are you upsizing in the first place? Voltage drop? Is #12 normally adequate for the load? If yes then if only concern is the voltage drop and to achieve a certain minimum VD then the ability to change back to the smaller conductor could be at any point the the voltage drop would no longer be effected. When you want to maintain a minimum voltage at a given amperage you could even start at a smaller size then go up to the larger one and reduce back down to land in a device.

Your scenario of a #12 assuming 15 or 20 A. Reducing to the #12 protected by appropriately sized OCP device could be done at any point that would no longer create excessive Voltage drop. Depending on your final amperage you need, Southwire app shows about 60ft of #12 without exceeding 3%VD and #8 could be 155ft for the same load requirement. Or a combination of both for lesser distance. but even if you're close the the max of #8 a few inches or ft won't make a significant difference.

I once needed 120V 8A at almost 1200ft out, A #2 can't be landed on a 15A breaker or receptacle so started with a #12 at breaker jumping to the #2 run the distance and drop back to the #12 about 5ft from the receptacle outlet. Final came back with only about 1.5 volts drop.
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Thanks very much. I' not getting the concept/parameters of dropping down to a smaller wire when needed larger for VD. In my case I need #8's for the recept but then obviously I can reduce to #12's. WHY AND WHEN CAN I REDUCE? You have to have #8's for a particular portion? EXACTLY HOW MUCH OF THE RUN DO YOU NEED THAT LARGER WIRE SIZE(#8's).
 
Alwayslearningelec said:
Thanks very much. I' not getting the concept/parameters of dropping down to a smaller wire when needed larger for VD. In my case I need #8's for the recept but then obviously I can reduce to #12's. WHY AND WHEN CAN I REDUCE? You have to have #8's for a particular portion? EXACTLY HOW MUCH OF THE RUN DO YOU NEED THAT LARGER WIRE SIZE(#8's).
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It's not a simple calculation. You could run the #8's all the way to the end and then just pigtail a #12 at each receptacle location. Personally with the infomation that you've given I would just use #10's for everything. There is no need to over complicate it.
 
Alwayslearningelec said:
Thanks very much. I' not getting the concept/parameters of dropping down to a smaller wire when needed larger for VD. In my case I need #8's for the recept but then obviously I can reduce to #12's. WHY AND WHEN CAN I REDUCE? You have to have #8's for a particular portion? EXACTLY HOW MUCH OF THE RUN DO YOU NEED THAT LARGER WIRE SIZE(#8's).
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Increase of wire size reduces resistance on the conductor, resistance causes the voltage to drop,
If over a specified needed distance the amount of voltage drop would either be contra indicated by a code requirement or for operation of a device or equipment, you can upsize a conductor, reducing resistance and increasing voltage (more accurately maintaining voltage).
The only limiting criteria for reduction of the conductor after determining the need to upsize the conductor for VD is that the reduced wire size is adequate for the amperage of the overcurrent device supplying the conductors. The length of a reduced conductor post increase for VD is only limited by the need for the maintenance of a specified voltage to the end device.

Alwayslearningelec said:
EXACTLY HOW MUCH OF THE RUN DO YOU NEED THAT LARGER WIRE SIZE(#8's).
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Without your voltage and amperage requirements can't answer specifically, as any change to either of these criteria will change the outcome.
That is a function of voltage demand and amperage requirements. Southwire has an app to plug in the various data to calculate the max length. What is the voltage requirement and amperage requirement, total length of the run?
With a manual calculation You can play with the numbers comparing various resistance and reactance of different conductors over a specified length and come up with a desire max length for the #8 and still maintain the minimum of voltage drop. Table 9 of the NEC has the the resistance and reactance of different conductors in different conditions, and the enhance content have some math examples for using specific criteria from the table.

The Amperage required for the calculation MAY NOT BE the breaker amperage but can be the operating amperage of the equipment, like in my example in previous post.

The reversing of this process is also useful to reduce AFC to a piece of equipment, Adding length or reduced size (last one limited by max amperage of the conductor) will add resistance and reduce voltage available. This is really a balancing act for maintaining a minimum voltage while reducing potential impact of available fault current, so not always the best way to meet the AFC to a piece of equipment's SCCR.
 
hillbilly1 said:
The barrier would be for low voltage things such as cameras or security feeds if present.
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There may be some misunderstanding because of the requirement for barriers between devices in the same box in some instances? That rule wouldn't apply to what OP has though.
 
Alwayslearningelec said:
Thanks very much. I' not getting the concept/parameters of dropping down to a smaller wire when needed larger for VD. In my case I need #8's for the recept but then obviously I can reduce to #12's. WHY AND WHEN CAN I REDUCE? You have to have #8's for a particular portion? EXACTLY HOW MUCH OF THE RUN DO YOU NEED THAT LARGER WIRE SIZE(#8's).
Click to expand...
Voltage drop is a percentage of the circuit. If you have 99% of the circuit on larger wire, the 1% of the circuit doesn’t matter if it’s smaller wire.
 
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