I recently purchased a PM66 (5hp, 230v, 1ph), and I’m a little confused about something – the owners manual states that you should use a 40amp breaker; the NEMA chart for 250v plugs/outlets shows 15, 30, 50 & 60 amp blade configurations, but no 40 amp. What gives?
Thanks –
Don
Replies
Don,
Not quite sure what you're asking here. Are you asking if 40amp breakers are available? (.jpg). If you can't find a 40 amp receptacle, just use one with a higher rating. Or did I not understand your question?
Jon
My question was - what plug/receptacle should be used?
I just find it odd that you can get 15 amp, 20 amp, 30 amp, 50 amp & higher - but NOT 40 amp.
Don,
I take it that I answered your original question then.
Jon
Yep, thanks! I just found it strange that the NEMA chart did not show a 40 amp plug & receptacle combo.
Don,
I just occurred to me, You said 5hp 230v motor. Since I'm not an electrician, I can't say this definitively, but a 40amp dedicated breaker seams high for that application. My 7.5hp 230v compressor only calls for a 30amp breaker. You might want to post on breaktime and see if you can get an electricians' [4LORN1]? attention.
Jon
Well, you know, that's what I thought - even though the manual says 40 amps, and the motor operates @ 24 amps, I was thinkin' 24 + 20% still gets you under 30 amps. So I called Powermatic and the rep said I should use a 40 amp breaker - but none of the local HDs or Lowes sell 40 amp plug & outlet combos.
Yeah, I bought one of those clearance PorterCable 7.5 hp compressors & it's manual says 20 amp is good and 30 amp is better.
Don,
Porter Cable makes a 7.5 hp compressor? Just out of curiosity is it a two stage and what is the CFM?
My thought with the breaker is that it is always better to go with the lowest rating you can get by with so it trips sooner. Certainly going with a higher rated receptacle and heavier gauge wires is a good thing, economics aside. But couldn't the motor toast itself before it trips the breaker? Granted the supply will be protected, but if this is a dedicated breaker I don't see any up side to going with something higher.
Jon
Jon-
There is a misconception that is used on this site often regarding breakers protecting equipment. Circuit breakers cannot protect any equipment, they only protect the wiring. If a circuit breaker trips because of the equipment you are running on that circuit you have either sized the breaker improperly or there is a problem with the equipment. The circuit breaker trips because too much current is flowing for the wire in the wall it has know way of knowing what the equipment needs or should use. It is almost always better to go with the larger size breaker than the lower rating but you must size the wire in the wall appropriately. Tom
Tom,
What if you sized the breaker for the load of the equipment? If the motor got bogged down, say hypothetically it was an air compressor, and the pump began to seize. Are you saying that if the motor normally drew say 13 amps, and you had a 15 amp breaker, that the motor would probably burn up before the breaker tripped? I'm asking this as a question, not because I think I know something you don't. If so, Why?
"Circuit breakers cannot protect any equipment, they only protect the wiring. If a circuit breaker trips because of the equipment you are running on that circuit "... "there is a problem with the equipment."
That is what I don't understand. You state that if there is trouble with the equipment, it may trip the breaker, but at the same time the rating has no effect. Does the amp rating of the breaker have absolutely no relationship when it trips as far as minimizing the damage to a machine?
Jon
Edited 4/15/2003 11:12:22 PM ET by WorkshopJon
Edited 4/15/2003 11:14:12 PM ET by WorkshopJon
Edited 4/15/2003 11:18:29 PM ET by WorkshopJon
That is what I don't understand. You state that if there is trouble with the equipment, it may trip the breaker, but at the same time the rating has no effect. Does the amp rating of the breaker have absolutely no relationship when it trips as far as minimizing the damage to a machine?
The breaker will only trip if there is too much current flow for the wire it is protecting. There is no way for it to know how much current flow is too much for the equipment that is plugged in to it. If you are running equipmeny that is rated for 15 amps on a fifteen amp circuit you will probably cause more problem with the machine than if it is on a 20 amp due to voltage drops from resistance losses in the smaller wire size. Like trying to drink a thick shake through a straw that is too small. You want to supply at least the amount of power required to operate the machine. More power being available is fine.
Over current protection for the motor itself should be part of the motor itself and many machines have thermal protection and circuit breakers built into them.Tom
Tom,
The scenario I was asking was if the amp rating on the breaker only were lowered ie. size the breaker to the dedicated application, and not the undersizing the entire circuit.
Back to the compressor example. Say that it is done in 10 gauge wire, 10' run from the main panel, the receptacle is rated at 25 amps. Are what you are getting at is that you have created a bottleneck in the circuit by putting that 15amp breaker inline? And the compressor would be much happier if the 15amp were replaced with a 25amp breaker? What if the compressor has a 14 gauge cord? Remember in this hypothetical, the only thing hooked up to this circuit is the compressor.
Jon
Edited 4/16/2003 9:30:12 AM ET by WorkshopJon
I can't think of any reason to undersize the breaker. An electrical device will draw whatever current it needs to operate and no more. Tom
A few considerations:
- The electric code requires that the circuit, including the breaker be sized for 125% of the rated motor current. In your case, a minimum 30 amp.
- Electric motors have what is called inrush current at start-up which is needed to overcome inertia and to accelerate the rotating components. Inrush can be several times the rated current. However, inrush is of relatively short duration for machines like a table saw so that the breaker should not normally trip.
- HP ratings on air compressors are notoriously overrated, typically stating "peak" HP since a lot of people by compressors based on HP, not the capacity of the compressor(as you should).
Don't have the NEC at hand, but when they talk about sizing a breaker to a motor, they are talking about a direct connection to the motor. Since the majority of our motors are on portable tools connected by plugs and cords, it doesn't really apply except as a "good suggestion" for sizing. There is also a maximum rating of 250% of the FLA, so you can't directly connect a motor that draws 5A to a circuit rated at 20A.
Compressor motors are like router motors and shop vac motors. Does anyone here believe their 16 gal shop vac draws twice the current as the 3HP cabinet saw, especially through that 14-16 ga cord. What you need to look at are the "full load amps" or FLA, then size the supply circuit to 125% of this, which conveniently menas that the motor will be running at a continuous load of 80% of the circuit size, which is the recommended maximum for circuits that are on for 3 hours or more.
Also keep in mind that wire has a different rating depending on the application. The rating for 14 ga wire in the wall vs 14 ga wire in a free standing cord is different.
For 220 circuits in the shop, it mostly comes down to 20 or 30A circuits. The components for a 20A are a bit cheaper in both wire and outlets, box size, etc, the breaker cost being nearly identical. Not sure if you can pull 4 10 ga conductors through 1/2" emt, so that cost and difficulty of working with materials is higher too. Also, all components of a circuit need to be at or higher rated than the breaker. So you can't put a 20A outlet on a circuit with a 30A breaker, but for a saw that draws 15A, you can put a 30A plug on the cable and use it on a 30A circuit.
Edited 4/17/2003 12:35:59 PM ET by Major Wood
Sorry, I was off by .5 hp - it's only 7. According to the specs it supplies 12.1 scfm at 40 psi and 9.7 scfm at 90 psi. w/ a 60gal. tank - model CPLC7060V. Got a pretty good deal @ Lowes - $359 I think. They said they were going to stop carrying anything over 25 gal., so they put them on clearance. Now, if I can just allocate enough time to wire the shop ...
Don,
Less than half the price of what I paid for mine, and in retrospect, mine is complete overkill for a one man shop. If I forget to turn it off for the night, and it kicks in at 2:00am, I get kicked out of bed, and it's in the basement and the bedroom is on the second floor. If you look back at the pic I posted, you will see red tape on the breaker below the 40. Now you know why, But mine is a two stage 24CFM 175PSI, completely different animal. Good luck on the wiring.
Jon
If the book says a 40 amp breaker, put in a 40 amp breaker and go with the 50 amp receptacle and plug. It is safe to use components on a circuit that have a higher rating than the breaker protecting the circuit, but normally it isn't done because of the increased cost.
In response to some of the other comments here, I think there is some misunderstood communication. You do not want to use a bigger breaker for a circuit dedicated to a single piece of equipement than what is recommended. The breaker protects all parts of the circuit, including the wire, the recepticles, and YES the motor. Should something become defective in the motor causing it to draw excessive current (which can happen) the proper breaker will hopefully throw before there is major damage to the motor. Should you use a breaker double what is recommended the motor may burn itself up before the breaker throws. Additionally if it is a warrented item and the company gets picky I imagine most of the time you'ld be voiding your warrenty if you use an over sized warrenty. With all this said it is becoming more common for equipment to have it's own overload equipment built in to protect the machines motor. My agazzani band saw it this way, but typically you don't see a lot of this type of thing on most consumer grade machines.
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