Running UF-B underground

crash63 · 04-24-2008, 01:10 PM

Hi... I'm running a 160' leg of UF-B direct bury to a fire pit for a Laptop/2.1 system on a short post. I'm thinking of using 12/2 w/gnd and putting a 4 way outlet (outdoor of course) on this short post. The max current draw will be less than 10a. Will this 20amp set up hold up at 160'?? Thanks!!

ElectricianJeff · 04-24-2008, 08:05 PM

Quote:

Originally Posted by crash63

Hi... I'm running a 160' leg of UF-B direct bury to a fire pit for a Laptop/2.1 system on a short post. I'm thinking of using 12/2 w/gnd and putting a 4 way outlet (outdoor of course) on this short post. The max current draw will be less than 10a. Will this 20amp set up hold up at 160'?? Thanks!!

10/2 will get you closer to 3% considering a 10 amp load. You will need a GFCI also

jcalvin · 04-25-2008, 08:02 AM

I never load my breaker with more than about 80% max ampacity. This means that I will load a 20amp only to 16amp. I agree with ElectricianJeff with the 3% loss over the distance assuming optimum conditions and nothing on the wire that could raise its resistance. If you subtract 3% from the 16amp max, it will give you 15.52amps. I think this would be plenty to run most anything besides a power tool.

In this situation I wouldn't normally even think about the voltage drop. You may not believe it, but I have seen homerun inside homes exceed that length easily. In a simple 28x50 Cape Cod home, the length of the upstairs homerun is going to be around 110'.

BigJimmy · 04-25-2008, 10:21 AM

The OP's question was this:

Quote:

Originally Posted by crash63

The max current draw will be less than 10a. Will this 20amp set up hold up at 160'??

Part of Jcalvin's response is as follows:

Quote:

Originally Posted by jcalvin

I agree with ElectricianJeff with the 3% loss over the distance assuming optimum conditions and nothing on the wire that could raise its resistance. If you subtract 3% from the 16amp max, it will give you 15.52amps. I think this would be plenty to run most anything besides a power tool.

First of all, there is no problem whatsoever with 10A anticipated load on a 20A circuit. In fact, from an ampacity perspective, there's no reason that it cannot be protected by a 15A breaker. ElectricianJeff's concern is voltage drop at the receptacle outlet wherein he recommended 10 awg. conductors to keep the drop at or below 3% based on a 10A load. Remember, the concern is to keep the voltage drop at the load within 3% of the nominal voltage; you do not de-rate ampacity by 3%.

Consider the fine print note under 310.15(A)(1):

FPN No. 1: Ampacities provided by this section do not
take voltage drop into consideration. See 210.19(A), FPN
No. 4, for branch circuits and 215.2(A), FPN No. 2, for

feeders.

So, from the tables, you could conclude that a certain conductor has an ampacity that is acceptable for your particular application, OCPD. However, if the run was excessively long, it may no longer be applicable/acceptable from a voltage drop perspective and you'd need to go with a larger conductor for this reason.

Quote:

Originally Posted by jcalvin

I never load my breaker with more than about 80% max ampacity. This means that I will load a 20amp only to 16amp.

While there is nothing wrong with this, it is only required for continuous loads (see 210.19/.20).

Based on what the OP intends to use, I doubt that the load will actually approach 10A, especially at a sustained level. At the end of the day, I'm sure 12 awg. or even 14 would be fine but I'd run the 12 awg. just for the heck of it. Remember, voltage drop is a big concern with motor-driven equipment where low voltage at the motor leads will cause excessive heating of the windings.

Jim

jcalvin · 04-25-2008, 01:20 PM

Quote:

Originally Posted by BigJimmy

The OP's question was this:

Part of Jcalvin's response is as follows:

First of all, there is no problem whatsoever with 10A anticipated load on a 20A circuit. In fact, from an ampacity perspective, there's no reason that it cannot be protected by a 15A breaker. ElectricianJeff's concern is voltage drop at the receptacle outlet wherein he recommended 10 awg. conductors to keep the drop at or below 3% based on a 10A load. Remember, the concern is to keep the voltage drop at the load within 3% of the nominal voltage; you do not de-rate ampacity by 3%.

Consider the fine print note under 310.15(A)(1):

FPN No. 1: Ampacities provided by this section do not
take voltage drop into consideration. See 210.19(A), FPN
No. 4, for branch circuits and 215.2(A), FPN No. 2, for

feeders.

So, from the tables, you could conclude that a certain conductor has an ampacity that is acceptable for your particular application, OCPD. However, if the run was excessively long, it may no longer be applicable/acceptable from a voltage drop perspective and you'd need to go with a larger conductor for this reason.

While there is nothing wrong with this, it is only required for continuous loads (see 210.19/.20).

Based on what the OP intends to use, I doubt that the load will actually approach 10A, especially at a sustained level. At the end of the day, I'm sure 12 awg. or even 14 would be fine but I'd run the 12 awg. just for the heck of it. Remember, voltage drop is a big concern with motor-driven equipment where low voltage at the motor leads will cause excessive heating of the windings.

Jim

SHOW OFF!!!

BigJimmy · 04-25-2008, 01:28 PM

Quote:

Originally Posted by jcalvin

SHOW OFF!!!

Sorry, I wasn't trying to be. I guess I tend to get long-winded sometimes in my responses.

crash63 · 04-26-2008, 02:34 PM

Gentlemen (or ladies!),

Thanks for the help. If I remember right I calculated about 1.1vac drop running my laptop (or any MP3 player) and a 2.1 amplified speaker system which should keep me under 1%. 12/2 as you probably know is much cheaper than 10/2 in this type of cable. Especially with a 160' run! The ampacity of these 2 units is about 3.5a so I should be fine there. I was thinking of occasionally setting up my P.A. system out there to sing a few song around the fire... maybe some kamakazi karaoke! I'll have to be more careful there! And a GFCI duplex is indeed in my plan.

Anyway... thanks for the info and if your even in MN... come have a beer with me by the fire

... bring your singin' voice!

TT

nap · 04-26-2008, 03:31 PM

Quote:

1 conductor(s) per phase utilizing a 10 AWG Copper conductor will limit the voltage drop to 2.8% or less when supplying 10 amps for 160 feet on a 120 volt system.

Quote:

1 conductor(s) per phase utilizing a 14 AWG Copper conductor will limit the voltage drop to 2.5% or less when supplying 3.5 amps for 160 feet on a 120 volt system.

Quote:

1 conductor(s) per phase utilizing a 6 kcmil Copper conductor will limit the voltage drop to 2.36% or less when supplying 20 amps for 160 feet on a 120 volt system.

Quote:

1 conductor(s) per phase utilizing a 8 AWG Copper conductor will limit the voltage drop to 2.95% or less when supplying 16 amps for 160 feet on a 120 volt system.

just a few calculations so you can see what the difference a few amps would make. Personally, if it were me, I would run the #10, at least and would consider #8. 10 amps is not a lot of draw.

It is generally better to oversize a bit than regret lack of doing so later. Then when you upsize, you have essentially thrown away the money for the wire you originally used.