Electrical question

[joeu235]

Anyone know why a 12 AWG is allowed for a 20A 120v single phase circuit but 10 AWG is required for a 30A 2 phase 240v circuit? The 30A 240v is actually carrying less amps per wire than the 20A 120v.

 

[mbohuntr]

Anyone know why a 12 AWG is allowed for a 20A 120v single phase circuit but 10 AWG is required for a 30A 2 phase 240v circuit? The 30A 240v is actually carrying less amps per wire than the 20A 120v.

You are only considering a single circuit, motors will reach 300% current when starting, and can frequently trip breakers once they load up. You don't want the wire glowing in the wall before the breaker takes it out...

 

[gary49]

Anyone know why a 12 AWG is allowed for a 20A 120v single phase circuit but 10 AWG is required for a 30A 2 phase 240v circuit? The 30A 240v is actually carrying less amps per wire than the 20A 120v.

Size of wire is determined by the rating of the overcurrent device (fuse or circuit breaker). A 30 amp 240 volt circuit can carry up to 30 amps per conductor. A 20 amp a 120 volt circuit can carry up to 20 amps per conductor. One is good for 7200 watts (240X30) the other is good for 2400 watts (120X20). 10 gauge would also be required for a 30 amp 120 volt circuit. This is before any de-rating that must occur for continuous or motor loads. Most circuit breakers will hold roughly 3 times their rating briefly in order to start motor or compressor loads, that's some of the reason you multiply motor loads by 1.25 (125%) to determine proper wiring ampacity.

 

[buickanddeere]

Anyone know why a 12 AWG is allowed for a 20A 120v single phase circuit but 10 AWG is required for a 30A 2 phase 240v circuit? The 30A 240v is actually carrying less amps per wire than the 20A 120v.

Not sure what you are asking.

 

[buickanddeere]

You are only considering a single circuit, motors will reach 300% current when starting, and can frequently trip breakers once they load up. You don't want the wire glowing in the wall before the breaker takes it out...

A 10HP 240V motor with 40.0 FLA uses #8 copper R75 rated for 50 amps on a 100amp breaker .

 

[Industrial Toys]

Just to be clear. 240 is still single phase. They (the two 110 legs) are just different windings of the transformer, with the neutral in the center.

 

[CobyRupert]

Not sure what you are asking.

I think the OP believes that a 30 amp load on a 240v circuit draws draws 15 amp through each conductor.

This is not true.

 

[Industrial Toys]

Just got me to thinking. You can draw 15 amp off each side of a split receptacle fed with 14/3, like plugging in a kettle and a toaster oven, for a total of 30 amps (well, in a way) but couldn't draw 30 amps between the two legs as you would exceed the capacity of the 14 AWG. That, can become confusing.

 

[mbohuntr]

A 10HP 240V motor with 40.0 FLA uses #8 copper R75 rated for 50 amps on a 100amp breaker .

Sizing branch-circuit conductors. Branch-circuit conductors that serve a single motor must have an ampacity of not less than 125% of the motor's FLC as listed in Tables 430.147 through 430.150 [430.6(A)]. You must select the conductor size from Table 310.16 according to the terminal temperature rating (60°C or 75°C) of the equipment [110.14(C)].
Not sure what your getting at, you might have a smart (adjustable) breaker, or engineered overloads or some kind of electronic circuit protection... If your everyday motor is FLA rated at 40 amps, and you run it on 12awg, and put a 60 amp breaker on it... you MIGHT have a problem.... I was only illustrating that a motor might only run at 8A, but is a dynamic load. No load amps, Full load amps, locked rotor amps, inrush current.... Single phasing not withstanding.

Your wire should never be the weak link.

 

[LD1]

I think the OP believes that a 30 amp load on a 240v circuit draws draws 15 amp through each conductor.

This is not true.

I think this is the answer. The OP doesnt understand how a 240v split-phase circuit works.

Quite simply a 240v DOUBLE pole 30 amp breaker....it wont trip until 30 amps are exceeded on ONE or BOTH legs. NOT 15 each totalling 30+

Just got me to thinking. You can draw 15 amp off each side of a split receptacle fed with 14/3, like plugging in a kettle and a toaster oven, for a total of 30 amps (well, in a way) but couldn't draw 30 amps between the two legs as you would exceed the capacity of the 14 AWG. That, can become confusing.

This is where people get into trouble with a MWBC. IF you wire the red and black in the panel to opposite legs....the amperage on the neutral will cancel.

But if someone does this, or in the future by re-arranging the panel, and end up with both feed wires of the MWBC on the SAME leg.....then if you tried to pull 15a on each....you are putting 30a back on that 14ga neutral.