Amps at 230' via 8/3

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rpdwyer

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I’ve got a run of 8/3 with ground direct burial cable to run to my work shop that is 230’ away. Using the following voltage drop calculator I get a voltage drop of 3.97% at 15 Amps:

https://www.calculator.net/voltage-drop-calculator.html

However, I have two hots...does that get me more usable amps at the end or does that simply allow me access to 220v at the 15 amp max?

I'll have an electrician do the hookups...just want to know what my limits will be in the workshop.


Thank you.

-Rick
 

Reach4

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However, I have two hots...does that get me more usable amps at the end or does that simply allow me access to 220v at the 15 amp max?
I ran your calculator and got
Voltage drop: 4.87
Voltage drop percentage: 4.06%
Voltage at the end: 115.13
with 40 amps and 250 ft!


So I would be thinking a 40 amp 2-pole breaker feeding the 8/3 to the subpanel.

At the other end I could use 80 amp loads of just 120 split up on the two legs. So yes, if using 120 circuits, you get twice the usable amps. The 120 breakers could add up to more than 80.

I am not a pro.
 

wwhitney

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So with 120V loads on a 120V/240V feeder, voltage drop behaves as follows (I used Southwire's calculator at https://www.southwire.com/calculator-vdrop )

If you have a single 11A 120V load with a 0.9 power factor, then your 230' feeder will drop 3.6V. That's 3% of 120V, a common target, that would work fine.

Say you now have balanced 11A 120V loads (one on each leg), or an 11A 2420V load. The voltage drop is still 3.6V, but now with respect to 240V, so only 1.5%. So with a 240V load, or balanced 120V loads, you can draw 22A with only 3% voltage drop.

Say you plug in a 120V, 1800W electric heater (power factor 1). Now the voltage drop is 5.4V (per the OP's calculator, as Southwire's assumes 0.9 power factor), or 4.5%. More than typical, but an electric heater doesn't care.

Due to a quirk of the NEC, it is more compliant to use a 40A breaker to supply the 8/3, just be aware that the voltage drop might start to be an issue (depending on the load, and depending on the voltage at the originating panel) if you go over 20A.

Your shop will need a main disconnect on the exterior or immediately where the feeder enters the shop, and it will need a grounding electrode system connected to the ground bar in your main disconnect. With an EGC in the feeder, neutral bars remain isolated from the EGC and cases.

Cheers, Wayne
 

rpdwyer

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Your shop will need a main disconnect on the exterior or immediately where the feeder enters the shop, and it will need a grounding electrode system connected to the ground bar in your main disconnect. With an EGC in the feeder, neutral bars remain isolated from the EGC and cases.

Cheers, Wayne

Does the small panel I have in the shed now not serve as a disconnect given it has a main breaker throw before the individual breaker feeds?
 

wwhitney

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Does the small panel I have in the shed now not serve as a disconnect given it has a main breaker throw before the individual breaker feeds?
No, that main breaker is a fine disconnect. The panel just needs to be on an exterior wall, with the feeder cable entering it directly through the wall, or at least in the same stud bay with just a vertical interior segment. The exact meaning of the code language is open to interpretation, but either of those should be fine.

And if you don't currently have any grounding electrodes, you need to drive two ground rods, at least 8' deep (so fully buried for an 8' ground rod), at least 6' apart, to make a grounding electrode system, and connect that to the panel ground bar.

Cheers, Wayne
 
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Reach4

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And if you don't currently have any grounding electrodes, you need to drive two ground rods, at least 8' deep (so fully buried for an 8' ground rod), at least 6' apart, to make a grounding electrode system, and connect that to the panel ground bar.
Do you think that this existing installation of the subpanel needs to get upgraded to current code if just adding a breaker or outlet?
 

wwhitney

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Do you think that this existing installation of the subpanel needs to get upgraded to current code if just adding a breaker or outlet?
It's unclear to me for the OP what is new and what is existing. Sounds to me like the feeder and panel are new, so certainly the grounding electrodes are required.

If you have an existing panel without the necessary grounding electrode(s), and you need to add a circuit to it, are you required to provide the missing ground rods? I don't know if it's required (that may vary by jurisdiction), but it would certainly be proper and a best practice.

Cheers, Wayne
 

rpdwyer

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The panel in the shed is existing and it doesn't have a ground rod as it was wired to connect to my generator and everything I found on grounding indicated that when a generator is used the genny itself provides the ground.

The feed to the shed from my house is replacing the generator feed permanently so I'm assuming I need to use two grounding rods at the shed panel.

This brings up the question of what role the ground wire in my 8/3 feed provides: first of all, it's much smaller in size than the 8 guage wires and secondly, wouldn't the grounding rods I'm going to install at the shed trump that ground?....or should I use both the grounding rods at the shed panel AND wire the ground within the feed to connect both the shed and house panels?
 

wwhitney

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The EGC in the 8/3 feeder is what ensures a circuit breaker trips when you have a "ground fault" at the shed. E.g. if you stuck a paper clip in a receptacle between the hot and ground pins. The ground rods have nothing to do with that, as any circuit that includes a path through the soil is going to be too high a resistance to trip a (normal) circuit breaker. Note that grounds and neutral are to be kept separate in your shed panel, with no bonding jumper (sometimes a green screw in the panel).

The utility of the ground rods at the shed is not clear to me, but they are required for a shed supplied by a feeder (i.e. that has a panel inside).

Cheers, Wayne
 

rpdwyer

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The EGC in the 8/3 feeder is what ensures a circuit breaker trips when you have a "ground fault" at the shed. E.g. if you stuck a paper clip in a receptacle between the hot and ground pins. The ground rods have nothing to do with that, as any circuit that includes a path through the soil is going to be too high a resistance to trip a (normal) circuit breaker. Note that grounds and neutral are to be kept separate in your shed panel, with no bonding jumper (sometimes a green screw in the panel).

The utility of the ground rods at the shed is not clear to me, but they are required for a shed supplied by a feeder (i.e. that has a panel inside).

Cheers, Wayne

Thanks for the clear explanation. Much appreciated.
 
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