Best way to run feeder 350 feet underground?

[wwhitney]

The calculator is increasing the resistance/kft with increasing insulation temperature. Which would be accurate if you were specifying operating temperature, but the actual operating temperature won't depend on the insulation temperature.

And so to be explicit, the Southwire calculator gives you different results for 120V 3% VD 30A 350' 1.0 PF if you choose Commercial 60C (#1 AWG Cu) vs Commercial 75C (#1/0 AWG Cu), which is not reasonable.

Cheers, Wayne

 

[kwired]

And so to be explicit, the Southwire calculator gives you different results for 120V 3% VD 30A 350' 1.0 PF if you choose Commercial 60C (#1 AWG Cu) vs Commercial 75C (#1/0 AWG Cu), which is not reasonable.

Cheers, Wayne

That doesn't make sense. Voltage drop doesn't change with change in insulation type when the conductor otherwise remains the same. Minimum safe ampacity can be different but this ampacity is more about maintaining insulation integrity.

 

[LarryFine]

I appreciate every post, including the side discussions.

Okay, I just spoke with the customer again. His RV has only a single A/C unit, which he does want to be able to use, and the TT30 plug, so he only needs the 120v.

I explained the options, and he only wants to accommodate the RV and is not concerned with the future, so the bottom line is what is the cheapest way to provide it.

He said that he "has backhoes" and can take care of the trenching and back-filling, which is fine with me. I really would like to avoid pulling wire into conduit, too.

It looks like the #2al MHF cable is the cheapest of any adequate wire for me to recommend, and he'll have to decide on the cost plus labor for running PVC conduit.

I am finding plug-in transformers for $100 to $200, and 'real' transformers for $500 to $750, with capacities of 3Kva to 5Kva to compare with running new wire.

Ongoing and new opinions, please.

 

[tortuga]

Did you find out what the existing wire size is?
Two things, when calculating RV loads I use the 551.73, even if its not a RV park, it gives you a commonly agreed upon load basis for CYA.
So for a site with a 30A 120V (TT30) and a 20A 120V GFCI the calculated load would be 3600VA.

Second thing is when you have a long feeder underground you may be able to update slightly based on the ambient temp at whatever depth the cable is.
For example K the standard value of 12.9 for copper is based on a conductor temperature of 75°C (167°F). In a direct-burial or underground conduit application you can adjust this.
In Virginia summers does ambient soil temperatures at 18" depth ever exceed 25°C (77°F)?
If not the actual resistance will be significantly lower NEC Chapter 9, Table 8, Note 2 shows how to adjust the k value.
I'd bet you end up with 25°C or 30°C, your K factor for aluminum drops from 21.2 down to roughly 18.
My *guess* is with temperature adjustment, #4 Aluminum would hit you 3% VD mark but I did not run any numbers.

 

[Little Bill]

i'm familiar with the situation. but ....... if it's as you say and just adding a second air conditioner, why isn't it just a 30a 240v receptacle? adding that second air conditioner on the second 120v leg, would fit just fine, right???

(some have electric hot water heaters)

I didn't mean to imply that the 2nd A/C was the only extra load. It's just the way I determine whether an RV needs 30A or 50A. I've been asked to put in an RV receptacle and the customer has no clue if they need a 30A or 50A. So my question to them is "How many A/C units does your RV have". Their answer tells me what to install.

 

[tortuga]

Here you go Larry I figured it at 15A and 20A scenarios , 18" depth soil temp = 25°C (77°F) or below. (Based on formula in Chapter 9 Table 8 Note 2)

​	​	constant α (alpha):​	T2:​	​	​
NEC CH 9 Table 8 note 2​	​	0.0033​	25​	​	​
K 75°C​	K 25°C​	A​	L​	V​	​
21.2​	17.702​	15​	350​	120​	​
2KIL=​	185871​	​	​	​	​
AWG​	Circular Mil Area​	Volts Drop​	Percent VD​	​	​
4​	41740​	4.45​	3.71%​	​	​
3​	52620​	3.53​	2.94%​	​	​
2​	66360​	2.8​	2.33%​	​	​
1​	83690​	2.22​	1.85%​	​	​

20A:

​	​	constant α (alpha):​	T2:​	​	​
NEC CH 9 Table 8 note 2​	​	0.0033​	25​	​	​
K 75°C​	K 25°C​	A​	L​	V​	​
21.2​	17.702​	20​	350​	120​	​
2KIL=​	247828​	​	​	​	​
AWG​	Circular Mil Area​	Volts Drop​	Percent VD​	​	​
4​	41740​	5.94​	4.95%​	​	​
3​	52620​	4.71​	3.93%​	​	​
2​	66360​	3.73​	3.11%​	​	​
1​	83690​	2.96​	2.47%​	​	​

 

[wwhitney]

Here you go Larry I figured it at 15A and 20A scenarios , 18" depth soil temp = 25°C (77°F) or below.

If the soil temperature is 25C, the conductor temperature will be higher than 25C, as the I²R heating needs to be offset by outward heat flow, which requires a temperature difference.

It seems to me a conservative methodology is for each conductor size, take the 90C ampacity table value, divide the actual current by that ampacity, take the square (the I² in I²R), and multiply by 60C to get a worst case temperature rise (as the 90C ampacity is based on 60C temperature rise). Add that to the ambient temperature (plausibly 25C if the soil temperature is never above 25C), and use that as your temperature to calculate K.

Of course, that means you are using different K values for each conductor size, but that's the reality of the second order effect you are trying to capture.

Cheers, Wayne

 

[tortuga]

If the soil temperature is 25C, the conductor temperature will be higher than 25C, as the I²R heating needs to be offset by outward heat flow, which requires a temperature difference.

It seems to me a conservative methodology is for each conductor size, take the 90C ampacity table value, divide the actual current by that ampacity, take the square (the I² in I²R), and multiply by 60C to get a worst case temperature rise (as the 90C ampacity is based on 60C temperature rise). Add that to the ambient temperature (plausibly 25C if the soil temperature is never above 25C), and use that as your temperature to calculate K.

Of course, that means you are using different K values for each conductor size, but that's the reality of the second order effect you are trying to capture.

Cheers, Wayne

I used the alpha value 0.0030 from Chapter 9 Table 8 Note 2, its the temperature coefficient (α) specifically for Aluminum when you are starting from a reference temperature (T1) of 75°C, so the wires already hot.

 

[wwhitney]

I used the alpha value 0.0030 from Chapter 9 Table 8 Note 2, its the temperature coefficient (α) specifically for Aluminum when you are starting from a reference temperature (T1) of 75°C

OK, so that equation tells you how to adjust the DC resistance based on the conductor's operating temperature. Not the ambient temperature around the conductor. The operating temperature will be higher than the ambient.

so the wires already hot.

That makes no sense. If you know a copper wire's temperature is 75C, then you use the table value for the DC resistance. If instead you know the wire's temperature is 35C, then that's 40C less, so for a copper wire, the DC resistance will be 40 * 0.323% less, or 12.9% less, or 87.1% of the 75C resistance, per footnote 2.

A #6 Cu conductor has a 90C ampacity of 75A. That means for some suitably worst case assumptions on how the environment around it will allow heat to dissipate, the conductor temperature could rise 60C over ambient when carrying 75A. If you only put 30A on that wire, the temperature rise will be (30/75)² = 16% as much as if it were carrying 75A, so it should be at worst 16% * 60C = 9.6C above ambient. So if you know ambient will be 25C, you would want the 35C DC resistance to calculate the DC voltage drop, and you could use 87.1% of the table 8 value of 0.491 ohms/kft for your voltage drop calculation.

Cheers, Wayne

 

[kwired]

I appreciate every post, including the side discussions.

Okay, I just spoke with the customer again. His RV has only a single A/C unit, which he does want to be able to use, and the TT30 plug, so he only needs the 120v.

I explained the options, and he only wants to accommodate the RV and is not concerned with the future, so the bottom line is what is the cheapest way to provide it.

He said that he "has backhoes" and can take care of the trenching and back-filling, which is fine with me. I really would like to avoid pulling wire into conduit, too.

It looks like the #2al MHF cable is the cheapest of any adequate wire for me to recommend, and he'll have to decide on the cost plus labor for running PVC conduit.

I am finding plug-in transformers for $100 to $200, and 'real' transformers for $500 to $750, with capacities of 3Kva to 5Kva to compare with running new wire.

Ongoing and new opinions, please.

Might only be common here and a few other limited areas and has been around for years. Non listed thin wall PVC pipe and pull direct burial rated conductors through it. Been particularly common on farms but you do find it in other longer run applications quite often. Supply house only stocks it in 2 inch and 3 inch. 2 inch is approximately same cost per foot as 1-1/4 schedule 40.

If I were doing your project I'd probably put in that 2 inch thin wall and run either 2 or 4 AWG URD cable, or the mobile home feeder cable you mentioned - is something I don't see much of around here unless you go to a big box store to get it. Is basically about same thing but with a 250.122 sized green where a URD quad would have three #2 and a #4 with a white stripe. If you are only feeding a single 120 volt receptacle you could get by with one less conductor and just go with a # 2 triplex. Then the 2 inch pipe you buried could someday have larger conductors pulled through without the need to excavate if load needs are increased.

 

[tortuga]

I don't know anybody who installs 2" PVC in the ground and regrets it later, or a @kwired suggests non NEC PVC 'duct'.
But I do know a fella that ran lots of URD and regretted it later.

 

[kwired]

I don't know anybody who installs 2" PVC in the ground and regrets it later, or a @kwired suggests non NEC PVC 'duct'.
But I do know a fella that ran lots of URD and regretted it later.

I forgot to mention in previous post a primary reason this thin walled PVC duct became pretty common here is for rodent protection - they still normally put direct burial rated conductors inside of it.

 

[roger]

I don't know anybody who installs 2" PVC in the ground and regrets it later, or a @kwired suggests non NEC PVC 'duct'.
But I do know a fella that ran lots of URD and regretted it later.

Exactly, even if the PVC was not used for the direct burial cable it's a small expense and will be there later.

Ten years ago I hand dug a 100' ditch from my house to a shed for a 100A sub panel and at the time was sure I would never need water which would have been very easy to throw in the open ditch, well years later I sure wish I would have.

 

[kwired]

Exactly, even if the PVC was not used for the direct burial cable it's a small expense and will be there later.

Ten years ago I hand dug a 100' ditch from my house to a shed for a 100A sub panel and at the time was sure I would never need water which would have been very easy to throw in the open ditch, well years later I sure wish I would have.

You can get away with burying something for water in FL. Up north you need to be at least 5 feet deep or it will freeze. Bet you wouldn't hand dig that.