Another wiring in a compressor question

[usmc_noma]

I have a compressor I'm going to wire up within the next week or two. I just want to make sure I understand the wiring concept. I've wired my garage with your standard outlets, so I think I'm going in the right direction.

So the compressor motor is a USA made Century from back in the 80's. I think the tank was stamped 1983. The motor is rated at 3hp, 15 amps, and 230 volts continuous. The plug is NEMA 6-20P. I'll have to buy the 6-20R outlet.

Will a 20a double pole breaker and 12-2 wire be sufficient for this compressor? The main thing I'm worried about is the start up, but do I really need to bump up to 30a breaker and 10-2 wire? The run on wire will be less than 10 feet total.

I've done a lot of reading to cover my bases, so I hope I'm on the right track.

 

[2ManyProjects]

So the compressor motor is a USA made Century from back in the 80's. I think the tank was stamped 1983. The motor is rated at 3hp, 15 amps, and 230 volts continuous. The plug is NEMA 6-20P. I'll have to buy the 6-20R outlet.

Will a 20a double pole breaker and 12-2 wire be sufficient for this compressor?

IF all the numbers you quoted above are accurate, then yes, a 20A breaker and 12/2 wiring will be adequate.

HOWEVER... I somewhat mistrust those numbers. For example, per code (cf. http://www.buildmyowncabin.com/nec/nec2005_table430.248.html), you have to allow AT LEAST 17 amps for a 3HP motor; and there are further requirements with respect to utilization percentage and breaker capacity that get into the act. Others can supply the gory details; but the long and short of it is, at least MOST 3HP compressors require at least a 25-30A circuit.

HOWEVER (again)... The fact that your compressor was originally supplied with a NEMA 6-20P plug implies that it CAN live (happily?) on a 20A circuit, which further implies that it's not REALLY a 3HP motor.

Bottom Line: You need to determine what you REALLY have before you can make a definitive judgement.

The main thing I'm worried about is the start up, but do I really need to bump up to 30a breaker and 10-2 wire? The run on wire will be less than 10 feet total.

Were it me, for the downright trivial cost difference, I'd use AWG 10 in any case.

 

[Norcal]

Since it came w/ a 20A attachment plug, that's how I would wire it. My 3 HP Unisaw has a 3 HP motor & it does fine w/ a 20A circuit, certainly not a hard starting load like a compressor, but the existing 20A circuit was one reason I went with a 3 HP over a 4 HP motor when it was being replaced, & was too lazy to pull 15' of wire out of the conduit & replace the breaker & receptacle.

 

[Charles (in GA)]

1980's portable compressor almost certainly has "fake" hp rating. If it says 15 amp I would believe that way before a bogus hp rating.

 

[G_P]

Since it is only a 10' run of wire I would go with the 10ga. It wont cost much more and then if you get a larger compressor in the future you wont have to run all new wire again.

 

[usmc_noma]

Here's the label from the motor. In all honesty it looks like the plug is after market and not the original especially for being over 20 years old. All of my info in the original post was based off of the label and not necessarily the plug.

If the hp is false, then what would you estimate the actual hp is based on what the label reads?

 

[Norcal]

"SPL" is used on lower end compressors, lower end meaning lower priced big box store type.

 

[G_P]

That's the first one I've seen with a number and "SPL" in the HP rating.

Usually the really cheap compressors just say "HP:SPL" and don't give you any numbers.

 

[2ManyProjects]

Here's the label from the motor. In all honesty it looks like the plug is after market and not the original especially for being over 20 years old.

In which case, you probably ought to ask yourself just how long you expect that compressor to last, and what you would most likely replace it with when (not "if") it does buy the farm. Then wire your circuit/outlet to support AT LEAST that much.

All of my info in the original post was based off of the label and not necessarily the plug.

That was the right approach; but you apparently got tripped up by bogus labeling.

If the hp is false, then what would you estimate the actual hp is based on what the label reads?

It's anybody's guess, since the most useful translation of "SPL" in this context is "We're lying through our teeth, Suckah!"

My guess is, certainly no more than 1.5-2HP, probably more like 1 HP (if that).

"SPL" is used on lower end compressors, lower end meaning lower priced big box store type.

That's the first one I've seen with a number and "SPL" in the HP rating.

Usually the really cheap compressors just say "HP:SPL" and don't give you any numbers.

FWIW, I've seen a number of explanations for this nonsense; but this one is particularly cogent:

http://www.wisedan.com/aircomp

 

[Kevin C]

My 80's vintage Sears / Devilibis compressor has a similar label, 15 amps at 230 V, 3 hp but no SPL suffix.

It also has a standard 20 amp, 240 V plug and the owners manual calls out a 20 amp circuit. Since I have owned the compressors since it was new, I know that's the plug it came with.

On my compressor, the 3 HP rating is realistic. When running it draws about 11 to 12 amps at 240 V, about 2760 watts. 745.7 watts per hp at 80% efficiency gets you just about 3 hp.

Yours is pretty close to mine, if the 15 amps at 230 V is accurate, it sure looks like a 3 hp motor to me.

On a single stage compressor its takes about that much HP to put out 8-9 SCFM at 90 PSI. I have been able to do a lot with that output.

In short (no pun intended) .... 20 amps is fine for the unit you have.

 

[JoeFin]

3hp, 15 amps, and 230 volts continuous

What - everyone missed the 230 volt rating ??

You just want to find a cord-cap and receptacle combination that will prevent anyone from mistakenly plugging a 120volt device into the 230volt receptacle

The 12 awg circuit your planning on using will have to be dedicated and will handle the load just fine. If your worried about motor surge / start-up currents then go to a HVCR rated breaker which will give you a little more in rush current capabilities

Best to make sure the appliance cord (for lack of a better name) coming off the compressor is at least 12 awg wire too. Wouldn't want to bottle neck your in-rush current at the cord cap where the receptacle won't appreciate the added heat. A 10' piece of SO cord will do just fine

 

[usmc_noma]

Thinking ahead of myself, I think I'm going to go with the 20a breaker and run the 10/2 in the event I upgrade at a later date. It seems trivial, as previously mentioned, not to at this point since ill have to run wire anyway.

Now if I wanted to run a switch in would I just run the 10/3?

 

[JoeFin]

Now if I wanted to run a switch in would I just run the 10/3?

Nope

But you will need a 3 Hp or better, rated, 2 pole motor switch

 

[2ManyProjects]

My 80's vintage Sears / Devilibis compressor has a similar label, 15 amps at 230 V, 3 hp but no SPL suffix.

It also has a standard 20 amp, 240 V plug and the owners manual calls out a 20 amp circuit. Since I have owned the compressors since it was new, I know that's the plug it came with.

I'm guessing that the age has something to do with that. When exactly did the current iteration of NEC-mandated motor-circuit rules come into effect? As of today, given that "3 HP" on the data plate, it would require a circuit rated for a MINIMUM of 21.5 Amps (effectively, 25 Amps). That in turn would seemingly rule out the 20A plug.

On my compressor, the 3 HP rating is realistic. When running it draws about 11 to 12 amps at 240 V, about 2760 watts. 745.7 watts per hp at 80% efficiency gets you just about 3 hp.

It may be a somewhat "wimpy" 3HP. Per the NEC table, 3HP = 17 running Amps. So if that motor was REALLY about 2.65 HP, it could "slide by" at 15A, yet still semi-legitimately claim (by "rounding up") a nominal 3 HP.

In short (no pun intended) .... 20 amps is fine for the unit you have.

Maybe. But given the age of his compressor, the real question is what will he need for whatever he replaces it with (probably sooner than later)? If it were me, I'd go with AT LEAST a 30A circuit, on general principles. The marginal cost difference is not large enough to justify scrimping here.

What - everyone missed the 230 volt rating ??

No.

Did you perhaps miss that, per the NEC, a 3 HP motor effectively REQUIRES a 25-Amp (or larger) circuit?

The 12 awg circuit your planning on using will have to be dedicated and will handle the load just fine.

Not if he wants to meet code with a (nominal) 3 HP motor. Now granted, given that "SPL" designation on his motor plate, he near-certainly does NOT really have a 3 HP motor. But I would not put it past an inspector to ignore that distinction.

And besides, that's "now"; but it's "later" which is the larger concern.

Thinking ahead of myself, I think I'm going to go with the 20a breaker and run the 10/2 in the event I upgrade at a later date. It seems trivial, as previously mentioned, not to at this point since ill have to run wire anyway.

If you're going with AWG 10, might was well also use a 30A breaker. Then you're really set for a new (and really 3 HP) compressor.

Now if I wanted to run a switch in would I just run the 10/3?

Nope

But you will need a 3 Hp or better, rated, 2 pole motor switch

And good luck finding that, at least at an "affordable" price.

 

[usmc_noma]

In going with the 30a breaker, wouldn't I have to change the plug and receptacle? Or could I stick with the 6-20p and r?

 

[2ManyProjects]

In going with the 30a breaker, wouldn't I have to change the plug and receptacle? Or could I stick with the 6-20p and r?

Well, the RIGHT way to do it would be to install a 6-30R or L6-30R receptacle, then use an adapter with a 6-30P/L6-30R plug and a 6-20R socket, such as:

http://www.lockingpowercords.com/Products/144-l6-30p-to-6-1520r-power-cord-adapter.aspx

to connect your existing compressor.

However, if this is strictly a DIY job, AND you know going in that the outlet will be dedicated to THAT compressor, you could "fudge it" with a 6-20R receptacle, and it would work. (Not strictly code-compliant; but not a catastrophe in waiting, either.) When the time comes to replace/upgrade the compressor, swap out the receptacle for a 6-30R.

 

[Kevin C]

It may be a somewhat "wimpy" 3HP. Per the NEC table, 3HP = 17 running Amps. So if that motor was REALLY about 2.65 HP, it could "slide by" at 15A, yet still semi-legitimately claim (by "rounding up") a nominal 3 HP.

Your misunderstanding the purpose of the NEC table. Its not the actual running draw of the motor, just what you use for circuit calculations.

Most motors will draw less current, 3hp at 80% efficiency and 230 V, requires about 12 amps of current. Most 3hp motors draw less then 15 amps.

http://www.grainger.com/product/MARATHON-MOTORS-Air-Compr-Motor-3K788 Link to a 3 hp motor.

I have used a 20 amp breaker on my 3hp compressor as well as for my monster 3HP belt sander for 25 plus years. Never had a single nuisance trip.

I attached a pic of a typical plug on a 15 amp draw compressor. Yup, its a 20 amp. This is one the compressors from the era of when HP rating were inflated (pun intended). 6 HP rating , but a current draw of 15 amps. The SCFM is typical for a 3hp compressor.

 

[Norcal]

Just wire to match the factory plug, there is no need to over think this.

 

[JoeFin]

I'm guessing that the age has something to do with that. When exactly did the current iteration of NEC-mandated motor-circuit rules come into effect? As of today, given that "3 HP" on the data plate, it would require a circuit rated for a MINIMUM of 21.5 Amps (effectively, 25 Amps). That in turn would seemingly rule out the 20A plug.

For the last time 2manyprojects "Get a Gawd Damm Code Book" and at least attempt to learn what you are talking about before you go making yourself look like a fool again

 

[JoeFin]

In going with the 30a breaker, wouldn't I have to change the plug and receptacle? Or could I stick with the 6-20p and r?

USMC - you would do better on ignoring 2Manyprojects as the conversation has exceeding his level of understanding.

The 3Hp motor will do just fine and be code compliant on the 240v 20a circuit as described above.

I will admit I am a little out of the loop and a bit rusty on my residential wiring methods as I make my money designing and commissioning a little bit larger scale

and I pretty much specialized in Instrument and Control

But every so often - I do have to work on a breaker - control side mostly

but I still remember my horse power formula

Hp x 746
------------ = Amps
Voltage

 

[DeadSock]

USMC,
I would upsize tho wire to awg 10 for future possibilities but simply use a 20a breaker and 6-20r.
In the future if 30a is necessary, you can switch out the breaker and receptacle.

 

[2ManyProjects]

Your misunderstanding the purpose of the NEC table. Its not the actual running draw of the motor, just what you use for circuit calculations.

On the contrary. I wasn't addressing the "actual running draw"; I specifically and explicitly stated that the code would require A CIRCUIT RATED FOR 25-30A. [To be pedantic about it, 21.5A would suffice; but there's no such "rating" for either wire or (AFAIK) breakers in real life, so we round up.] Silly as it might seem, what the motor actually draws is irrelevant to this.

For the last time 2manyprojects "Get a Gawd Damm Code Book" and at least attempt to learn what you are talking about before you go making yourself look like a fool again

Since when does asking a question ("When exactly did the current iteration of NEC-mandated motor-circuit rules come into effect?") constitute "making [my]self look like a fool..."?

I notice that you didn't bother to ANSWER the question, preferring instead to throw stones and brag about your expertise. I say again: http://www.amishrakefight.org/gfy/.

USMC,
I would upsize tho wire to awg 10 for future possibilities but simply use a 20a breaker and 6-20r.
In the future if 30a is necessary, you can switch out the breaker and receptacle.

Just as I suggested in http://www.garagejournal.com/forum/showpost.php?p=3557278&postcount=2; My comments in http://www.garagejournal.com/forum/showpost.php?p=3566358&postcount=16 were specificly in response to the question about using a 30A breaker (with adequate wiring, of course), presumably to avoid the cost of replacing a 20A version later.

 

[Kevin C]

It may be a somewhat "wimpy" 3HP. Per the NEC table, 3HP = 17 running Amps. So if that motor was REALLY about 2.65 HP, it could "slide by" at 15A, yet still semi-legitimately claim (by "rounding up") a nominal 3 HP.

On the contrary. I wasn't addressing the "actual running draw"; I specifically and explicitly stated that the code would require A CIRCUIT RATED FOR 25-30A. [To be pedantic about it, 21.5A would suffice; but there's no such "rating" for either wire or (AFAIK) breakers in real life, so we round up.] Silly as it might seem, what the motor actually draws is irrelevant to this.

You guessed that the running draw had to be lower to meet the NEC table or that the motor predated the table to make your scenario of what plug the OP should use fit.

Or the best one yet... The manf rounded up the HP on the motors name plate.

In other threads you are way off on HP vs current draw. In short, bad info that is not helping.

http://www.garagejournal.com/forum/showthread.php?t=227170

As I suspected... Despite the bogus advertising claims, that is NOT a "3HP" compressor. It can't be, given the "15 Amps" claim.

More bad info.... That's why I posted the HP calcs

 

[JoeFin]

On the contrary. I wasn't addressing the "actual running draw"; I specifically and explicitly stated that the code would require A CIRCUIT RATED FOR 25-30A. [To be pedantic about it, 21.5A would suffice; but there's no such "rating" for either wire or (AFAIK) breakers in real life, so we round up.] Silly as it might seem, what the motor actually draws is irrelevant to this.




Since when does asking a question ("When exactly did the current iteration of NEC-mandated motor-circuit rules come into effect?") constitute "making [my]self look like a fool..."?

I notice that you didn't bother to ANSWER the question, preferring instead to throw stones and brag about your expertise. I say again: http://www.amishrakefight.org/gfy/.




Just as I suggested in http://www.garagejournal.com/forum/showpost.php?p=3557278&postcount=2; My comments in http://www.garagejournal.com/forum/showpost.php?p=3566358&postcount=16 were specificly in response to the question about using a 30A breaker (with adequate wiring, of course), presumably to avoid the cost of replacing a 20A version later.

Step 1 - get a code book
Step 2 - read chapter 430

Its all in there

I must admit your deligent and take the time to explain to these guys what they need to know. But give credit to the 100s panel members who through collective 1000s of years of experience and numerous degrees have looked at every aspect of a SAFE functional installation