New service or sub panel advice needed

[ururk]

I'm getting ready to order my wire (XHHW, Cu). I've decided to go with 3AWG CU. The max length of the run is 110', so I'm estimating 120':

#3 CU - 120' - 100A = 2.5%

In reality, when using the system I doubt I'll go above 60A:

#3 CU - 120' - 60A = 1.5%

Am I missing anything? Switching to a #2 will be very costly, and only nets me .5%

#2 CU - 120' - 100A = 2%
#2 CU - 120' - 60A = 1.2%

The only thing I can think of would be starting up a motor for a tool, it could draw a large number of amps (inrush current), but I don't know whether in practice this matters.

 

[mike93lx]

WY are you trying to get further below 3%? Why copper anyway? If you are worrying about cost, that is the place to save money

 

[ururk]

WY are you trying to get further below 3%? Why copper anyway? If you are worrying about cost, that is the place to save money

At this point it comes down to my preferred conductor and whether a 2.5% voltage drop is acceptable.

If I switch to aluminum I need to size up the conductor to get the drop below 3% - I'm not set up for the larger conductor with my mogul body/conduit (though I could change them out). I'm OK with the cost I've been quoted for a #3 Cu, and if needed could go up to #2 Cu, but would prefer not to (due to cost).

I know what the NEC allows (up to 3% on a feeder, up to 5% on a branch). I guess what I'm wondering is, is 2.5% @ 100A acceptable? Again, if I'm pulling 100A, something is seriously wrong. I estimate at most - at most - at any one time I'll be using 60A.

With my insulation, the barn is mildly uncomfortable on the hottest days we've had so far - I may add a small ductless mini-split - which would draw up to 20A (I might be able to get away with a 15A unit). Lighting will barely make a dent (all LED). That means I would have to draw 40A just to get to 60A.

 

[mike93lx]

You said 3% is acceptable and then question if 2.5% is good enough...


Yes, it is plenty. Install the wire and move to the next thing. You could probably run #6 copper and never have a problem

 

[wyliesdiesels]

@ 120' and a small one man shop, I would do #2 al MHF and not worry about it anymore.

Doubt youll come close to any significant amount of load that will cause voltage drop to go above 5% at the end of a circuit.

 

[ururk]

One quick question somewhat unrelated. When I had the house's load center open, I noticed a darkening of the ground/neutral bars in the bottom right corner. Should this concern me? It's just the section I took a picture of - the rest of the bars are bright and shiny. The copper case grounding "leg" has a patina of green...

Anyhow - the darker color on the bar looks more like heat damage than moisture issues. We had a weird power event a few years back - lost two power strips (MOV's exploded) and several breakers tripped. It effected our neighbors too. The poco denied any problems on their lines, but there was no lightning or storm at the time.

 

[mike93lx]

Wow, that is freaky. Have a temp gun? I'd check it. Seems odd that it is so even. For the steel to get to that color, something would have needed to get pretty damn hot

 

[ururk]

I don’t have a temp gun - but I’m pretty sure it isn’t hot right now.

 

[mike93lx]

If the wiring got that hot, insulation would have melted.

 

[ururk]

Good point! I’m not too worried about this, just kind of curious.

 

[ururk]

Stupid question... which ground is the primary ground? From the house's panel, or the ground rods?

 

[wyliesdiesels]

Stupid question... which ground is the primary ground? From the house's panel, or the ground rods?

what do you mean by primary ground?

there is no such thing

The ground wire in the feeder is a low impedance pathway for fault currents allowing the breaker to trip open upon fault and the ground wire going to the rods is for shunting lightning, limiting voltage potential to earth and shunting primary high voltage when lines contact secondary lines.

they are 2 completely different animals that do not share the same functions.

 

[ururk]

what do you mean by primary ground?

there is no such thing

The ground wire in the feeder is a low impedance pathway for fault currents allowing the breaker to trip open upon fault and the ground wire going to the rods is for shunting lightning, limiting voltage potential to earth and shunting primary high voltage when lines contact secondary lines.

they are 2 completely different animals that do not share the same functions.

I'll explain where my confusion came from - and ask that you ignore my original question

Looking over the instructions for the panel, it calls the conductor going to the ground bar the "Main Ground". The other lugs are also designated "Main Neutral", etc... However the big clue to me should have been which wire sizes it allows. Ground rods are usually connected with #6 wire, whereas grounds from the source could be much larger depending on how the primary/neutral are sized.

So... I connect the ground wire from the house's panel to the "Main Ground" lug, and the ground wire from the ground rods to one on the ground bus.

 

[wyliesdiesels]

I'll explain where my confusion came from - and ask that you ignore my original question

Looking over the instructions for the panel, it calls the conductor going to the ground bar the "Main Ground". The other lugs are also designated "Main Neutral", etc... However the big clue to me should have been which wire sizes it allows. Ground rods are usually connected with #6 wire, whereas grounds from the source could be much larger depending on how the primary/neutral are sized.

So... I connect the ground wire from the house's panel to the "Main Ground" lug, and the ground wire from the ground rods to one on the ground bus.

Those both go to the same bar- the ground bar.

Make sure your neutral bar is isolated and does not have any ground wires on it.

 

[ururk]

Those both go to the same bar- the ground bar.

Make sure your neutral bar is isolated and does not have any ground wires on it.

Yes - it is isolated. And yes, the "Main Ground" lug is directly attached to the ground bar.

 

[ururk]

Two questions:

1) I used a box extension for my GFCI outlets below the load center. Do I need to wrap this in metal tape? There are large gaps at the corners.

2) I bought a RF light switch from Amazon. I was a bit leery of this device, but whatever. It's rated for 10A, and I only have 15A breakers (can't get any smaller). The wire it has, is of course, sized for a 10A circuit. It also claims to have a built-in 10A fuse.

Anyhow, I ordered an inline resettable 10A breaker, and am wondering if I should go through and install it in the junction box with the RF relay - will an inspector be puzzled by this sort of thing?

FYI - the relay supports up to 1000W of power, my LED lights on this circuit only draw 20W each.

 

[mike93lx]

Metal tape is for HVAC. Why are you concerned about the box? Tape it up if you want, I guess

The breaker protects the wire, not the device. Why be concerned about your switch when you regularly plug in other stuff into 15 and 20a circuits that never draw that much?

 

[wyliesdiesels]

dont bother with the gaps on the box. As said above, that tape is for HVAC and isnt UL listed for electrical

Dont bother with the fuse either. The device and wire is U/L listed for the load it was tested under.

And keep this in mind. Most lamps have 16ga or 16ga wire. You dont go putting inline fuses on those despite them being plugged into 15a and 20a circuits.

You are putting too much worry into this stuff....

 

[ururk]

Thanks - just wanted to be sure about things.

 

[ururk]

One last question - would it be foolish for me to power the system up prior to the inspector coming over? Since I'm not doing a rough, and don't expect him to do much (ie, I'd be surprised if he wanted a panel opened up), it seems like he'll just give me the go ahead to flip the circuit breaker.

 

[ururk]

Electrical was approved!

Just want to thank everyone here for the advice and suggestions/encouragement/etc...

He basically wanted to see where the power came in, was OK that I told him it was 18" to top of conduit (didn't need to see pictures).

Was in the barn for at most 10 minutes, asked some questions (wire gauge, # of conductors, separate ground/neutral), and that was it.

After which he asked me if I was an electrician, and I said no - just that I did a lot of research, asked questions on the internet, and even went as far as purchasing a torque wrench and screwdriver.

When he left he said, "A lot of electricians don't know or understand everything like you did".

So I turned power on when he left, outlets worked!


The only thing he had any concern over was the 200A breaker downstream of the 100A breaker. He said he had seen something like this a while back, and did a lot of research. He agreed it was allowed, but he thought the main problem was someone loading the panel up with circuits, and then using too much power. I said yes, but the breaker at the house would trip as it's protecting the downstream conductors. The switch at the sub panel is just a main disconnect.


Next up is finishing the gas line and getting that inspected. Long-term I need to run conduit for dedicated lighting circuits, side outlets, etc.... However I'm not yet sure how those need to be run so I'll be revising my current extension cord setup.