ECN Electrical Forum - Discussion Forums for Electricians, Inspectors and Related Professionals
ECN Shout Chat
ShoutChat Box
Recent Posts
Lock-down Thread
by Bill Addiss - 02/27/21 01:16 PM
Northern Tool Recalls Powerhorse Generators
by Admin - 02/25/21 09:49 PM
You will never guess
by gfretwell - 02/25/21 07:48 PM
New tool
by SMOKEYBOB - 02/15/21 04:59 PM
New in the Gallery:
Facebook follies, bad wiring
FPE in Germany pt.2
Who's Online Now
1 registered members (Scott35), 21 guests, and 20 spiders.
Key: Admin, Global Mod, Mod
Previous Thread
Next Thread
Print Thread
Rate Thread
Page 1 of 2 1 2
Voltage dop #217017 04/01/16 08:42 PM
Joined: Oct 2002
Posts: 830
sparkync Offline OP
I have a friend who teaches guitar lessons. A while back he built a small building and had it wired with a #4 USE aluminum feeder. After about a year he's decided to move the building another 220 ft. From where it was, making the total distance around 380 ft. from the breaker to the subpanel in his building. Total load 30 amps.???
Ran out of room on my phone

Work Gear for Electricians and the Trades
Re: Voltage dop [Re: sparkync] #217018 04/01/16 08:58 PM
Joined: Apr 2002
Posts: 7,233
HotLine1 Offline
Should be under 3% at the subpanel.

A quick Vd calculator is here...

You can put this calc in most smartphones.

Give it a try.

Re: Voltage dop [Re: sparkync] #217019 04/01/16 08:59 PM
Joined: Apr 2002
Posts: 7,233
HotLine1 Offline
Member did he move the building??

Re: Voltage dop [Re: sparkync] #217020 04/01/16 11:14 PM
Joined: Oct 2002
Posts: 830
sparkync Offline OP
Don't know how he moved it. It wasn't a large building. Probably a 12x12 or something like that. I'm on my computer now. I tried to type the question above in on my phone and it didn't give me enough room to finish. There is a 60 amp breaker ahead of this feeder. Like I said the actual load is 30 amps or less. The biggest load he will have is a window air condition ( 20 amp. ) and I'm assuming it is a 120 volt a/c. I figure without even without any demand factors the #4 USE should be a large enough wire to carry the load, even at that length. Just double checking myself on the figures. He wants me to splice the wire for him underground.
I figure if the load was even 4000 watts./240 = 16.6 amps, the wire is big enough, if I am figuring it right, and according to what he told me he had in the building, the total don't even come up to that if he runs everything at one time, which he won't.
Thanks again

Re: Voltage dop [Re: sparkync] #217021 04/02/16 07:16 AM
Joined: Dec 2001
Posts: 2,491
Texas_Ranger Offline
Well I suppose in theory you still have to consider the breaker rating rather than the actual estimated load.

Re: Voltage dop [Re: sparkync] #217022 04/02/16 12:14 PM
Joined: Apr 2002
Posts: 7,233
HotLine1 Offline

Did you try the calculator in the link I posted above?

Using that calc app

the Vd would be 2.79% with a 30 amp load.

6.696 volts, so 233volts approx. at load, IF the source voltage is 240 volts.

Should be no problem.

Re: Voltage dop [Re: sparkync] #217023 04/02/16 02:26 PM
Joined: Jul 2004
Posts: 9,569
gfretwell Offline
I doubt that AC is anywhere close to 20 amps. The start up may spike up to a big number but running, these things are not that big a load. A 144 sq/ft building should be fine with half a ton. A 6k BTU window shaker pulls about 5a.
The playhouse we set up for the kids has a big old CRT TV in it, lights and a 1/2 ton AC. It runs fine on a 15a circuit.
I think the TV draws as much as the AC.

Greg Fretwell
Re: Voltage dop [Re: sparkync] #217024 04/03/16 05:34 PM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
And now, for a different perspective ....

I think we are setting ourselves up for confusion when we focus on extraneous things like the "percentage" of voltage drop, or the running amps of the appliance.

Instead ... what's your EXACT voltage at the end of the line, when the circuit is under load? I'm looking at one air conditioner right now, whose nameplate is marked "115." There's my target; I need to deliver 115 volts to the air conditioner while it is running.

If your voltage at the house is, for example, 118.2 volts, you can afford to lose about three amps.; yet, there's a joker in the deck.

The 'joker' helps you, in that the manufacturer has designed the air conditioner to tolerate slightly lower voltages; you have a 10% margin from the nameplate marking. That is, you "can" use a supply voltage as low as 106 volts.
It's best not to use this "extra" margin, as you have to allow for drops in what the power company supplies you. That will depend partly on how many of your neighbors are running their air conditioners at the time.

Now, let's look at the issue of amperage. The same unit I looked at before says it needs a 15 amp circuit and draws 4.9 amps.

Why the difference? Because the motor needs more than 4.9 amps to start up; the '4.9' figure is just to keep it running. I need my wires to be big enough to deliver 15 amps, or the air conditioner won't start.

OK, so you have a 60-amp breaker feeding the smallest possible wires to the shed. That means there's no way you can supply the shed with 60 amps of power. Your panel should have a smaller 'main' breaker, perhaps a 40-amp, or you'll be setting yourself up for lower voltages when the panel is fully loaded. Lower voltages burn out motors and give electronics nervous breakdowns.

With the panel IN the shed, you can power the air conditioner in my example with an ordinary 15-amp circuit.

You're not really allowed to simply run a 15-amp circuit from the house, for the air conditioner in my example. Since there WILL be voltage drop, especially at start-up, you'll need at least a 20-amp circuit .... and you really ought to have that on a 15-amp breaker at the shed.

Bigger wire means less voltage drop. Higher supply voltages allow for a greater drop.

Your wire is plenty large enough, for even a 30-amp panel at the shed. This would allow you to power the air conditioner, and have a lighting circuit as well as two receptacle circuits.

Re: Voltage dop [Re: sparkync] #217025 04/03/16 05:40 PM
Joined: Jan 2005
Posts: 5,316
renosteinke Offline
Cat Servant
As for your splice ....

Yes, there are those 'direct burial' wire nuts. I'm paranoid, though, and assume a splice is a weak point that will later need to be checked. I don't like buried splices.

Here's what I do:

I place a fiberglass 'handhole' in the ground. About the size of a 5-gallon pail, these have a top you can readily remove. The bottom remains wide open; I usually set a small amount of gravel there, just so I can 'fine tune' the way the box sits in the hole. I set the box to match the slope and level of the ground around it.

I leave enough loose wire to come out of the ground about a foot; this makes it much easier to work with. I connect the wires with ordinary wire nuts, then dip the connections in Scotch-Kote to waterproof them. Then, I fold the wires into the handhole, placing the nuts so they open downwards.

Re: Voltage dop [Re: sparkync] #217026 04/04/16 03:39 PM
Joined: Apr 2002
Posts: 7,233
HotLine1 Offline
The 60 amp cb in the source panel is protecting the feeder conductors, and possibly the buss in the subpanel.

And knowing the exact voltage available at the source panel will weigh on the exact voltage at the subpanel. Hence that is why my comments say "IF".

The common trade practice of using a percentage for Vd is how I was taught, and what is 'suggested' within the NEC as a FPN.

And, yes there are voltage tolerances that appliances and a lot of other equipment have.

Page 1 of 2 1 2


2020 National Electrical Code
2020 National Electrical
Code (NEC)

* * * * * * *

2020 Master Electrician Exam Preparation Combos
2020 NEC Electrician
Exam Prep Combos:
Master / Journeyman


Member Spotlight
Minneapolis, MN, USA
Posts: 21
Joined: September 2019
Show All Member Profiles 
Top Posters(30 Days)
Popular Topics(Views)
275,482 Are you busy
209,385 Re: Forum
196,771 Need opinion
Powered by UBB.threads™ PHP Forum Software 7.7.3