12/5 flexible cord and neutral ampacity adjustment factors

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DerrickM

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Billings, Montana, USA
Hello,

I have some portable training electrical equipment that has a 20 amp, 120/ 208V, 3 phase, 5 wire, twist lock plug (L21-20P), on the end. When I first looked at this to size the flexible cord and receptacle that will be hanging down from the strain relief and junction box, I thought-- 20 amps, #12 AWG, with a 20 amp, 3 pole breaker. I purchased some 12/5 SOOW, about 20 feet.

My issue if this:
Table 400.5 (A) (1) for #12 flexible cord Col. A ( 3 conductors ) = 20 amps.
But the plug requires a 5 wire supply as the plug is a L-21-20P. ( 3 ungrounded, 1 neutral grounded, and 1 EGC )

This equipment is 3 phase, using all three ungrounded conductors, so when referring to what the neutral conductor is doing in this circuit I referred to 310.15 (B) (5). Even though I am sure that all 3 loads are NOT balanced, and at some point when this equipment is running there will be some neutral current (incandesent bulbs, signals etc.) can I still treat the neutral as a conductor carrying only unbalanced current, therefore not needing to use the adjustment factors of Table 310.15 (B) (3) (a)- 2011 NEC.
I realize this could all be fine if I just used 10/ 5 SOOW cord, and ran #10 conductors from the panel, but like I said, I already have the cord purchased.

Any thoughts on this?
If I know that this is a 3 phase, 4 wire, wye connected load, am I safe to NOT count the neutral for adjustment even though the loads are NOT balanced. Code article 310.15 (B) (5) says -- carries only unbalanced current, it doesn't say anything about the circuit needing to be balanced in order to not count the neutral as current carrying.

I am starting to confuse myself.
Thanks,
Derrick
 
The table for rubber cord is based on 3ccc so if you had 4 current carrying conductor then you would need to derate however, I don't believe you need to count the neutral as a current carrying conductor unless the circuits are feeding a lot of linear loads.
 
DerrickM said:
This equipment is 3 phase, using all three ungrounded conductors, so when referring to what the neutral conductor is doing in this circuit I referred to 310.15 (B) (5). Even though I am sure that all 3 loads are NOT balanced, and at some point when this equipment is running there will be some neutral current (incandesent bulbs, signals etc.) can I still treat the neutral as a conductor carrying only unbalanced current, therefore not needing to use the adjustment factors of Table 310.15 (B) (3) (a)- 2011 NEC.
I realize this could all be fine if I just used 10/ 5 SOOW cord, and ran #10 conductors from the panel, but like I said, I already have the cord purchased.

Any thoughts on this?
If I know that this is a 3 phase, 4 wire, wye connected load, am I safe to NOT count the neutral for adjustment even though the loads are NOT balanced. Code article 310.15 (B) (5) says -- carries only unbalanced current, it doesn't say anything about the circuit needing to be balanced in order to not count the neutral as current carrying.

I am starting to confuse myself.
Thanks,
Derrick
Click to expand...

Your confusion comes from the fact which the NEC recognizes, namely that if the neutral carries a large unbalanced current it can only be because one or more of the hot conductors is not carrying full rated current. So the maximum heating effect will still correspond to the smaller number of conductors.
 
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