[beachcable]

Hello!

Quick electrical question about voltage drop:

From the meter which is directly conneced to the electrical main from the pole,

I am running 8/3 OR 6/3 Tray Cable 550 feet to the cabin.

The supply through this cable will be 240v.

The electrical needs at this cabin will not exceed 6000 watts at any one time (50 amps at 120 volts).

So, the maximum current in the cable will not exceed 25 amps.

To allow for voltage drop over this distance, do I need to use 6/3 or can I get away with 8/3?

Thanks!

[donf]

Let me begin by saying that I am not an electrical engineer.

Are you connecting this supply feed into a panelboard?

First: Tray cable may not be listed for outdoor use.

Second: A 50 amp load 500' with a 240 V supply you will need at least a three # 1/0 copper conductors (2 hots, 1 neutral and one ground).

This is for direct burial copper conductor cable. . If you want to use Aluminum you would need a larger cable. Likewise if you are planning to use conduit or run the cable above ground (open air) the cable sizes may vary.

[stanfortyman]

A) What does an electrical engineer have to do with it?
B) Why tray cable?
C) What do you mean by "will not exceed 6000 watts at any one time"? Did you do a load calculation, or just add up everything in the cabin?

Even #6cu is way too small for even a 25A load @ 240V @ 550'.
This is assuming a perfectly balanced load @ 240V.

[stanfortyman]

Second: A 50 amp load 500' with a 240 V supply you will need at least a three # 1/0 copper conductors (2 hots, 1 neutral and one ground).

Don, where do you get 1/0?

[donf]

Don, where do you get 1/0?

I used the Southwire calculator.

I selected: Single phase, copper, direct burial. 240 V, 3% drop, 500' and a 50 amp load.

[beachcable]

First, THANK YOU for your responses! I do appreciate it, and am thankful that you are sharing your knowledge.

I'll answer all questions asked here:

1) Yes, the supply feed goes into a panelboard (a circuit panel is what I mean, I assume a panelboard is the same thing).

2) I have triple-checked and "tray cable" or at least the type I am ordering is listed for everything: exposed, UV, direct burial, wet locations, etc. The sales guy told me I could use UF-B but tray cable costs just a little more and is far better in quality.

3) I used the Southwire calculator and came up with a 8% drop for 240v, 550 feet, 8/3, 25 amps I understand 8% is high but probably acceptable for hosehold use (especially since iI have built so much play in it) This may not be true. Also, I may have used the calculator wrong.

4) I don't need an "electrical engineer" I suppose but that's the category they put me in so I left it there.

5) Why tray cable? See #2 above.

6) "Will not exceed 6000 watts"- I just added up uses and doubled it (no hot water, no range, no dryer, no heat, mini-split 1200 v AC only, yes refrigerator, LED lights, fans, misc.) But it seems from your question that I should have done a "load calc" which is probably a different procedure from adding 'em up. 99% of the time the use will not exceed 2000 watts.

The critical thing here is acceptable voltage drop. Wikipedia has a great article about this. I would love to use 4/3 or better but 4/3 would be $2500, and you can add 50% for each increase in size from there down. (ouch)

THANKS!!

[donf]

The total recommended Voltage drop is a maximum of 5%. Be aware that this "recommended", not code enforceable.

Personally I don't believe your sales guy when he says that Tray cable is listed for direct burial.

I will check the NEC book for verification that it is listed for that use. However, tray cable may be listed for a wet location but not listed for underground burial.

[donf]

Chapter 3, article 392.20 of the NEC covers the uses of tray cable. Basically, tray cable is listed for use in industrial sites, not residential sites.

Just curious, where does this salesman of yours work?

You might want to go to http:/www.copperindustries.com and search for their manual on "Tray Cable" or call your local city, state or county electrical inspector to see if they would allow the use of a "Tray Cable" as an underground feeder cable.

[beachcable]

Donf, Thanks for your response!

It's in the Bahamas in a remote area and code is not enforced so I can do it any way I feel comfortable with that works.

The tray cable specs are listed as follows (for 8/3, other sizes have similar properties:
◦Stranded bare copper
◦3 #8 Conductors and 1 #10 Uninsulated Ground Wire
◦.603" Diameter
◦292 lbs per 1000 feet
◦600 Volts - Direct Burial Underground Cable
◦Black Jacket and all black numbered conductors
◦PVC with Nylon Insulation on conductors
◦Sunlight resistant PVC Jacket
◦Temperature 90°C or 75°C Wet

I chose tray cable because other than UF-B I was not aware of any other cable that is OK for direct burial, which is mandatory in this application. I understand it is an industrial product but considering that it will be hot, wet, and direct buried, I don't know of any other choice. Again I could use UF-B but industrial tray cable only costs about 20% more. The salesman works for the company that sells the tray cable but even ignoring his comfirmation, it seemed like the appropriate product to me.

Here is my mindset: The wikipedia article says up to 8% voltage loss in private applications for non-lighting use is the maximum acceptable. 6/3 tray cable 25 amps 240v 550 feet puts me under that, per Southwire (supposedly taking into consideration both resistance and reactance). Voltage loss in the cabin itself should be below average (short runs, low loads, 12 gauge romex.)

So I'm hoping I'm OK?

Thanks! Don/beachcable