Extending existing electrical wiring

[machmeter62]

I am concerned about the ampacity of my #12 (solid copper) wires that is powering an electric motor (220V) greater than 100' away from the main sub panel on a 40 AMP CB. The existing wire is buried in metal conduit, and would be extremely difficult or impossible to pull new wire through it, for an additional electric motor. The total draw on the motors is three HP. (One is 2 hp, and the other is 1 HP.)

My question, I can increase the wire size in an electrical box at 60'....if I increase the additional 220V run with #10 stranded wire another 40'...will this be adequate to prevent a voltage drop harming the motors and wires? Or is it better to keep the entire length with the #12 wires? Thanks in advance for your advice!

 

[aczlan]

My question, I can increase the wire size in an electrical box at 60'....if I increase the additional 220V run with #10 stranded wire another 40'...will this be adequate to prevent a voltage drop harming the motors and wires? Or is it better to keep the entire length with the #12 wires? Thanks in advance for your advice!

As long as you keep the same amperage breaker, it will not hurt to up the wiresize for only part of the run.

Aaron Z

 

[dstig1]

You have a bigger problem. 12 ga is rated for 20A, not 40, and you have a 40A breaker. That is bad hoo-doo. Change the breaker to 20A, no matter what you do, unless you upsize the wire to 8ga like it needs to be for 40A.

Per typical NEC code:
14ga = 15A
12ga = 20A
10ga = 30A
8ga = 40A

 

[aczlan]

You have a bigger problem. 12 ga is rated for 20A, not 40, and you have a 40A breaker. That is bad hoo-doo. Change the breaker to 20A, no matter what you do, unless you upsize the wire to 8ga like it needs to be for 40A.

I missed that. Good catch.

Aaron Z

 

[MikeInEburg]

You have a bigger problem. 12 ga is rated for 20A, not 40, and you have a 40A breaker. That is bad hoo-doo. Change the breaker to 20A, no matter what you do, unless you upsize the wire to 8ga like it needs to be for 40A.

Per typical NEC code:
14ga = 15A
12ga = 20A
10ga = 30A
8ga = 40A

This is true for 120 volt circuits - but the OP stated that his was a 240V, essentially doubling the amperage capacity of the wire.
Mike

 

[aczlan]

This is true for 120 volt circuits - but the OP stated that his was a 240V, essentially doubling the amperage capacity of the wire.
Mike

However, for a 240v circuit over 12 ga wire, you would have a double 20 amp breaker, not a 40 amp breaker.
measuring from each leg to ground/neutral is still 120v, but from one leg to another is 240v.

Aaron Z

 

[MikeInEburg]

However, for a 240v circuit over 12 ga wire, you would have a double 20 amp breaker, not a 40 amp breaker.
measuring from each leg to ground/neutral is still 120v, but from one leg to another is 240v.

Aaron Z

Quite right you are...

 

[captaincrab55]

Keep in mind that it's legal to use a higher amp breaker on motor circuits in the USA when wiring to the NEC!!! Wire can be sized according to a Motor Name Plate and the Breaker sized according to Name Plate Start up amps, using the NEC... I'm not implying in any way that one can do this to a typical 15 or 20 amp branch circuit... Also when doing this a motor needs overload protection as outlined in the NEC...

 

[LD1]

Keep in mind that it's legal to use a higher amp breaker on motor circuits in the USA when wiring to the NEC!!! Wire can be sized according to a Motor Name Plate and the Breaker sized according to Name Plate Start up amps, using the NEC... I'm not implying in any way that one can do this to a typical 15 or 20 amp branch circuit... Also when doing this a motor needs overload protection as outlined in the NEC...

I am not up with the NEC so I cant say weather this is true or not, but it is not a good Idea.

You should always size the wire to the breaker. 110v and 220v are the same guages for the given amps as listed above.

Non capacitor type motors do draw about double the rated amps for start-up then when normally running. But you still must size the wire according to the breaker, because the breaker doesn't know the difference between start-up and too much load that may melt the wire and cause a fire.

40 amp draw at start-up wont hurt a 12ga wire, but 40 amps due to too much load or failing motor will burn it in two.

That is why most industrial electric motors have a combination of motor starters, thermals, slo-blo fuses, etc. But I cant stress enough the importance of sizing the wire to the breaker. If you dont want to change the wire, put a 20a breaker in there. If it works, no problem. If it trips @ start-up, go with time-delay fuses.

 

[rjkobbeman]

Keep in mind that it's legal to use a higher amp breaker on motor circuits in the USA when wiring to the NEC!!! Wire can be sized according to a Motor Name Plate and the Breaker sized according to Name Plate Start up amps, using the NEC... I'm not implying in any way that one can do this to a typical 15 or 20 amp branch circuit... Also when doing this a motor needs overload protection as outlined in the NEC...

I know this is true for compressors (like in HVAC) because they have internal breakers/thermal protectors. However for straight electric motors... I don't know if that is true or not. I would have to dig out my NEC book to make sure.

In the case of an HVAC system, there are several AMP listing. LRA is usally what the startup AMPS is. There is a MAX breaker size and a MAX circuit AMPS. The max circuit is usually what you size your wire to. The max breaker is usally bigger than what the wire is rated for. This is usually a no-no, but in the case of the HVAC compressor, it has a breaker internally that trips to protect the circuit size (wire).