Why do you need a 30 amp breaker on a planer with a 15 amp motor?

[Billbo]

I'm thinking about getting the Powermatic 15HH planer.

The online manual it says it has a 1 phase, 230 volt, 15 amp, 3 HP fan enclosed motor. But the recommended circuit breaker is 30 amps. I thought the standard formula for breakers was 1.25 X Load (1.25 x 15 amps) which would be 18.75 amps. If so, then a 20 amp breaker/circuit should be O.K.

I currently have a 20 amp 240 volt circuit available in my shop and although I can put in a 30 amp circuit, it would be a LOT of work I'd rather avoid if a 20 amp breaker would do the job.

Any advice/enlightenment on this subject would be appreciated.

Billbor
Smyrna, GA

[TDKPE]

The standard formula you're referring to is for the minimum sizing of conductors for a single-motor circuit. The higher ampacity circuit is to protect against nuisance trips on motor start-up, not because it's needed for normal running. Starting (locked-rotor) current is typically 4-8 times the full-load current (FLA) as indicated on the motor's nameplate, and can be determined from the "Code" or "kVA Code" letter on the nameplate - it translates to a multiplier on FLA at the voltage it's configured for, and is likely to be close to 100A.

Modern circuit breakers will trip on either a small overcurrent for a long period of time, or a larger overcurrent for a shorter period of time (inverse-time), and they will operate essentially instantaneously at some even higher current threshold, which is typically 6-10 times their rated ampacity, depending on ampacity, manufacturer, and even the individual breaker, as there is a bit of a tolerance on those parameters.

There's a good chance it will start just fine on a 20A circuit. If it nuisance trips, there are remedies, like a high-magnetic breaker (has a higher threshold for the instant-trip current), and others that aren't worth going into unless and until you can't make it work.

[Billbo]

Thanks Tom.

[BrentDH]

I have that planer on a 20A breaker - have not had any problems.

[BobW]

^^^ Me too!

[Aram]

BobW said:

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^^^ Me too!

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Same deal. No issues.

[Billbo]

BrentDH and Aram - thanks for the feedback/inputs.

We've got an anniversary coming up and the Mrs. will be speechless when I tell her I got her a new Powermatic 15HH for out anniversary!

Billbor
Smyrna, GA

[pprobus]

Billbo said:

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BrentDH and Aram - thanks for the feedback/inputs.

We've got an anniversary coming up and the Mrs. will be speechless when I tell her I got her a new Powermatic 15HH for out anniversary!

Billbor
Smyrna, GA

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In that case, you might want to still go ahead and put in that 30 amp line so that you can have heat while you're sleeping out in the shop.

Paul

[Curlycherry]

BobW said:

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^^^ Me too!

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Mine is a 3Hp Grizzly on a 20 Amp breaker and it too runs just fine.

[TDKPE]

Aram said:

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[blockquote]BobW said:

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^^^ Me too!

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Same deal. No issues.

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[/blockquote]I've noticed that Powermatic pretty much always recommends 30A 240V circuits for their 3 hp machines, and I've also noticed (and not just from this thread) that many folks just run them on 20A circuits without problem.

I've run my 3 hp Unisaw (Marathon motor; 12.4 FLA) on a 15A 240V circuit, because I retasked the single basement 15A 120V receptacle circuit to run my saw when I first moved here. Never had a problem, but it's a dual-capacitor motor (which makes a difference in both running and starting current), while PM machines use (or used to use) Baldor motors, which have a higher FLA rating for the same hp rating.

While technically not by the book (12A max for a motor on a 15A circuit), it never tripped. And it now runs on a 20A circuit. And still hasn't tripped on start-up.