wire size vs. length of run

[Kenny hill]

I am looking to run a new electric line for a 7 1/2 hp 3ph 22amp resaw band saw and want to know what size wire i should be using? The total footage from breaker panel to machine is roughly 20'. I was figuring on 10 ga solid copper wire. Is there any benefit in using 8 ga wire over 10 ga. I know the advantages in using 10ga but do not want to under power the machine. This will have a 40 amp breaker per manufactures spec's.

thank you in advance.

Kenny

[John McClanahan]

With a 40 amp breaker you will need 8 ga wire. If the breaker was 30 amp, you could use 10 ga wire.

John

[Kenny hill]

thanks John!

[Anthony Whitesell]

The NEC has a note to try to stay above 3% voltage loss. At the maximum amperage per wire gauge (ie., 40 amps in 8 gauge wire), the 3% note is reached at a run just over 50 feet. For your setup, 20 feet, 22A, and 8g gauge; you have room to spare. Enjoy!

[Wade Lippman]

There is no harm to using #8 and a 40a breaker, and for 20' the cost doesn't much matter; but I am a little surprised you need a 40a breaker. You might try asking here what other people have used. I thought one of the big advantages of 3phase is that motors start easy.

My cyclone "requires" a 40a breaker according to Grizzly, but has never tripped my 30a. Yeah, it was only 30', but #8 just isn't fun to use.

[Joseph Tarantino]

10 gauge wire with a 30A breaker. nothing more is needed. anything more for the load featured is a waste on money and effort installing it.

[Rollie Meyers]

With a 40 amp breaker you will need 8 ga wire. If the breaker was 30 amp, you could use 10 ga wire.

John

The rules for motors are different assuming overload protection for the motor, 40A breaker is fine unless it's cord & plug connected, cannot use 40A breaker on a 30A receptacle.

[Matt Marsh]

I am looking to run a new electric line for a 7 1/2 hp 3ph 22amp resaw band saw and want to know what size wire i should be using? The total footage from breaker panel to machine is roughly 20'. I was figuring on 10 ga solid copper wire. Is there any benefit in using 8 ga wire over 10 ga. I know the advantages in using 10ga but do not want to under power the machine. This will have a 40 amp breaker per manufactures spec's.

thank you in advance.

Kenny

In order to calculate what you need, we also need to know the voltage that this motor will be wired for. 208?, 230? Also, does the starter that controls this motor contain overload heaters? Are they correctly sized? The wiring must be sized by calculating the FLA values listed in table 430.250 of the NEC, not the FLA values listed on the motor nameplate. Will this circuit be hard wired, and dedicated to this motor alone? The small conductor restriction listed in article 240.4(d) (i.e. #14 = 15 amp breaker, #12 = 20 amp breaker, #10 = 30 amp breaker), generally does not apply to dedicated motor branch circuits. The wiring method must also be known. Will NM, or any other type of cable or flexible cord be used? If not, what type of wire will be used? You mentioned a 40 amp circuit breaker, will this be a standard inverse time breaker installed in a panel? Or is it a instantaneous trip motor breaker?

[Matt Marsh]

Kenny, I threw my two cents worth in here because as yet, nobody has given you a thorough and difinitive answer. First off, at 20 feet voltage drop is not an issue. You were told "With a 40 amp breaker you will need 8 ga wire.". If this is a dedicated circuit to this motor, and the motor has correctly sized overload protection, this statement is generally not true. I say generally, because there are other factors that could further de-rate the ampacity of the circuit, such as high ambient temperature, and/or adjacent conductor fill, or if you choose to use NM as a wiring method. If all of the terminations involved in this installation are rated for at least 75 degree C. (which they probably are), a #10 75 or 90 degree C. rated copper conductor such as THWN, can be fed from your 40 amp breaker. When designing circuits for hard wired motors , the circuit breaker ampacity is not determined by the wire size, it is determined by calculating the FLA of your motor using the values listed in table 430.52 of the NEC. If your circuit breaker is a standard inverse time type, for most motors it's maximum size can be calculated at 250% of motor nameplate FLA. If your motor nameplate lists the FLA as 22 like you say, that's 55 amps. Since 55 amps is not a standard sized breaker, Table 430.52 exception #1 allows us to use the next higher standard sized breaker, which in this case is 60 amps. Further exceptions to this article go even further to say that if that maximum sized breaker is still insufficient for the starting of the motor, you can go even higher, but cannot exceed 400% if an inverse time breaker is used. Granted, these are maximum allowed values, and as you have already stated, the manufacturer recommends the use of a 40 amp breaker. If this is the case, then that is what you should use. Wire sizing is also a bit trickier when it comes to dedicated motor circuits. Article 430.22 is used for sizing the conductors to a single motor. Conductors are sized based on calculating 125% the FLA values in the tables at the end of article 430, not from the FLA values listed on the motor nameplate. For 3-phase motors, the values listed in table 430.250 are used. If your 7-1/2 HP motor will be wired using 208 volts (very common), that value is 24.2 amps. If it will be wired using 230 volts (less common these days), the value is indeed 22 amps. 24.2 X 125% = 30.25 amps. 22 X 125% = 27.5 amps. The ampacity taken from table 310.15 lists #10 copper conductors in the 75 degree C column at 35 amps. #10 - 75 degree C rated copper conductors are the minimum sized wires allowed by the NEC for this motor. The ampacity of a #10 THWN-2 90 degree C copper conductor is actually 40 amps, but it is the temperature listings of the terminations (normally lower than 90 degrees) that do not allow us to use the full 40 amp value.

[David Kumm]

Ken, due to the mass of the wheels and high tension of the wide blade, a resaw is a very hard starting machine and will draw start up amps for several seconds, maybe more. Probably why the larger breaker is recommended. Dave

[Jeff Duncan]

I think David has a good point, I have several 7-1/2 hp shapers and a 9 hp planer all run off of 30 amp circuits with #10 wire. I can't imagine you would need to go to a #8, but I'm not an electrician so

good luck,
JeffD

[Jim Neeley]

David,

You are right on the money. An induction motor has an inrush of 6-7X it's full load current for (typically) a few milliseconds at startup.

An standard inverse time circuit breaker can handle more than its rated capacity for a very short time; the shorter the time the more it can handle and the longer it takes to spin up the longer it takes for this current to drop. If you cross over the tripping threshold, the breaker does it's thing.

This short "surge" of power doesn't cause wire failure because the wire would typically fail due to getting to hot and the surge doesn't last that long. When it comes to the surge the other potential issue would be the voltage drop that occurs during this surge (higher current = more voltage loss in the wire); this is accounted for (conservatively) in the wire ampacity tables of the NEC for most cases combined with the design of the motor itself.

Jim

[John McClanahan]

Kenny, I threw my two cents worth in here because as yet, nobody has given you a thorough and difinitive answer. First off, at 20 feet voltage drop is not an issue. You were told "With a 40 amp breaker you will need 8 ga wire.". If this is a dedicated circuit to this motor, and the motor has correctly sized overload protection, this statement is generally not true. I say generally, because there are other factors that could further de-rate the ampacity of the circuit, such as high ambient temperature, and/or adjacent conductor fill, or if you choose to use NM as a wiring method. If all of the terminations involved in this installation are rated for at least 75 degree C. (which they probably are), a #10 75 or 90 degree C. rated copper conductor such as THWN, can be fed from your 40 amp breaker. When designing circuits for hard wired motors , the circuit breaker ampacity is not determined by the wire size, it is determined by calculating the FLA of your motor using the values listed in table 430.52 of the NEC. If your circuit breaker is a standard inverse time type, for most motors it's maximum size can be calculated at 250% of motor nameplate FLA. If your motor nameplate lists the FLA as 22 like you say, that's 55 amps. Since 55 amps is not a standard sized breaker, Table 430.52 exception #1 allows us to use the next higher standard sized breaker, which in this case is 60 amps. Further exceptions to this article go even further to say that if that maximum sized breaker is still insufficient for the starting of the motor, you can go even higher, but cannot exceed 400% if an inverse time breaker is used. Granted, these are maximum allowed values, and as you have already stated, the manufacturer recommends the use of a 40 amp breaker. If this is the case, then that is what you should use. Wire sizing is also a bit trickier when it comes to dedicated motor circuits. Article 430.22 is used for sizing the conductors to a single motor. Conductors are sized based on calculating 125% the FLA values in the tables at the end of article 430, not from the FLA values listed on the motor nameplate. For 3-phase motors, the values listed in table 430.250 are used. If your 7-1/2 HP motor will be wired using 208 volts (very common), that value is 24.2 amps. If it will be wired using 230 volts (less common these days), the value is indeed 22 amps. 24.2 X 125% = 30.25 amps. 22 X 125% = 27.5 amps. The ampacity taken from table 310.15 lists #10 copper conductors in the 75 degree C column at 35 amps. #10 - 75 degree C rated copper conductors are the minimum sized wires allowed by the NEC for this motor. The ampacity of a #10 THWN-2 90 degree C copper conductor is actually 40 amps, but it is the temperature listings of the terminations (normally lower than 90 degrees) that do not allow us to use the full 40 amp value.

So, what is the correct answer?

John

[Matt Marsh]

John, without the answers to the questions in my first post, a single correct answer cannot be given.