# need help on polarity of three phase

#### obener

Joined Jul 27, 2009
4
hi all,

I know how to check the polarity of a single phase extension cable. I have never checked the polarity of a three phase extension cable. Is it possible someone can help me on this?

thanks

#### mik3

Joined Feb 4, 2008
4,843
AC has not a polarity actually. Maybe you mean to distinguish between the live and the neutral wires. In a three phase system you have only three phases (live wires) and not neutral.

#### obener

Joined Jul 27, 2009
4
Yes I mean the L1,L2,L3 live wires in a 4 lead 3 phase extension cable. Also L1,L2,L3 live wires and N wire in a 5 lead 3 phase extension cable.

I tested today a 15A single phase extension cable and I got 0.2ohms for Active-Active. I also got the same value for Neutral-Neutral.

I also tested a 10A 3 phase (4 lead) extension cable and for L1-L1,L2-L2 and L3-L3 i got 1.6ohms and I wanted to know if this was acceptable values for a 3 phase extension cable?

#### obener

Joined Jul 27, 2009
4
It depends on how long is the extention cable.
If the 10 Amps extention is 20 times longer, then value could be acceptable; If the two extention cables have the same lenght then the 10 Amps one must be checked because resistance is too high.

Check for loose screws ; oxidated or burned temination.

If you don't find the reason for such a high resistence than discard the extention because could be dangerous.

Alberto.
Is there any book or site where I can read more up on this?

#### KL7AJ

Joined Nov 4, 2008
2,225
The only sure way I know of doing this is seeing if any motors run the right direction. If a motor runs backwards, reverse ANY two lines.

Eric

#### Toughtool

Joined Aug 11, 2008
63
hi all,

I know how to check the polarity of a single phase extension cable. I have never checked the polarity of a three phase extension cable.
I think the OP is meaning to check the resistance of each conductor, not polarity, since AC power presents an alternating polarity between phase to phase and phase to neutral.
Polarized single phase extension cables usually refers to the physical wire connection that has the neutral (longer slot) identified and thus prevents connecting L1 (hot) to the chassis in devices like cheap transformer-less TV sets that ground one side of their chassis to the power neutral. In other words, the polarized plugs mechanically protects you from touching a hot chassis when you are grounded. Of course it only works if the cable is unmodified and you use polarized plugs.
As Alberto mentioned, the longer the cable, the higher the resistance and voltage drop. Here is a table that may help.
"Copper wire resistance table

AWG Feet/Ohm Ohms/100ft
10 490.2 .204
12 308.7 .324
14 193.8 .516
16 122.3 .818
18 76.8 1.30
20 48.1 2.08
22 30.3 3.30
24 19.1 5.24
26 12.0 8.32
28 7.55 13.2
These Ohms / Distance figures are for a round trip circuit. Specifications are for copper wire at 77 degrees Fahrenheit or 25 degrees Celsius. "
Joe

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#### alim

Joined Dec 27, 2005
113
AC has not a polarity actually. Maybe you mean to distinguish between the live and the neutral wires. In a three phase system you have only three phases (live wires) and not neutral.
This is not accurate . In 3 phase wye systems it may or may not carry a neutral, that is you will get phase voltage --line to neutral and line voltage --line to line . line voltage is 1.732 * phase voltage.

#### n1ist

Joined Mar 8, 2009
186
If a motor runs backwards, reverse ANY two lines.
Swap any two HOT lines (L1, L2, or L3). Leave ground and neutral (if any) where they are. I know it's obvious, but I have seen people do it wrong...

#### Toughtool

Joined Aug 11, 2008
63
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#### alim

Joined Dec 27, 2005
113
Hi Toughtool I was respoonding to mik3's post that I quoted which said "there are only three phases and NOT neutral."