Motor branch circuit sizing, MOCP and mfg. supplied plug...

[sbrehler]

Hi all,

I have a client who wants me to wire up some equipment for his wood shop (dust collection, compressor, etc.). I'm hoping that If I explain one scenario then I can apply it to the other equipment... None of the equipment has documentation on branch circuit/OCP. Only wiring configurations. I'm looking to get some input on my calcs and what the mfg. had in mind when they installed a 20A plugon one of these pieces of equipment.

Compressor (motor #1): 5 HP/230V = 28A FLC at 230V (per table 240.248) *125% = 35A

Branch circuit condutors = 40A @ 60'C = #8THHN

MOCP = 28A FLC * 250% (per table 430.52) = 70A

Can someone tell me why the manufacturer installed a 20A/250V plug (NEMA 6-20P) on the compressor? It would seem to me that this is way undersized for the branch circuit that I calculated.

Also, I know that the Max. OCP can be up to 250% of the motor's FLC. But should I necessarily size it that large? I know it is to protect the conductors...the overload protection is for the motor. Is there any reason to start at a lower OCPD and go up from there? I often hear of people (electricians - they are people, right? ) who talk of that, but I've never understood why.

I am running from a 110/220V panel upstairs with Inverse-time breakers and am going to install plug and cord disconnects for some of the equipment and non-fusible disconnects for others. Some of the dust collection equipment needs to be rolled away to make emptying the bins easier. Am I to understand the in each circumstance that the disconnects need to be rated at not less than 115% of the FLC (430.110 (A))?

I don't typically do this kind of work, but would like to know if I'm on track. No, I'm not a handyman - NC licensed, limited class.

I appreciated any and all input...

Scott

 

[Jraef]

Hi all,

I have a client who wants me to wire up some equipment for his wood shop (dust collection, compressor, etc.). I'm hoping that If I explain one scenario then I can apply it to the other equipment... None of the equipment has documentation on branch circuit/OCP. Only wiring configurations. I'm looking to get some input on my calcs and what the mfg. had in mind when they installed a 20A plugon one of these pieces of equipment.

Compressor (motor #1): 5 HP/230V = 28A FLC at 230V (per table 240.248) *125% = 35A

Branch circuit condutors = 40A @ 60'C = #8THHN

MOCP = 28A FLC * 250% (per table 430.52) = 70A

Per standardized "cheat sheets" that motor control mfrs used to give out, for a motor with 28FLA, recommended CB = 70A, or recommended Time Delay Dual-Element fuses = 45A, recommended wire size based on 75C rating can be #10, (assuming you have a motor starter with an OL relay set for 28A), but adjust for distance and voltage drop. I would probably use #8.

Can someone tell me why the manufacturer installed a 20A/250V plug (NEMA 6-20P) on the compressor? It would seem to me that this is way undersized for the branch circuit that I calculated.

Because they were a nincompoop? NEMA 6-20 is rated for a maximum of 2HP. So putting that on a 5HP motor would be "illegal". Most likely the mfr did not install that plug, someone did that later, because that's all they had to plug in to.

But... are you reading 28A FLC on the motor NAMEPLATE, or are you reading "5HP" on some piece of marketing fluff attached to the compressor and getting the 28A from the NEC table? Because if you are not aware, the "HP" used to market consumer goods like compressors is not actually the same as what "HP" means in the real industrial world. For the consumer market, they can get away with saying crap like "develops" 5HP, meaning when the motor is overloaded, you can kind of sort of call it 5HP by looking at current, power factor and voltage before something trips, and they are allowed to use that scam to sell it. But what they CAN'T lie about is the actual FLC number stamped onto the motor nameplate. If that motor NAMEPLATE says 28A, then the 20A plug is dangerous. If the motor nameplate FLC is saying something like 12A or 1500W, then it's a 2HP motor in reality, so the NEMA 6-20 is allowed.

Also, I know that the Max. OCP can be up to 250% of the motor's FLC. But should I necessarily size it that large? I know it is to protect the conductors...the overload protection is for the motor. Is there any reason to start at a lower OCPD and go up from there? I often hear of people (electricians - they are people, right? ) who talk of that, but I've never understood why.

Because if the OCPD is too close to the bone, you run the risk of nuisance tripping on starting current.

I am running from a 110/220V panel upstairs with Inverse-time breakers and am going to install plug and cord disconnects for some of the equipment and non-fusible disconnects for others. Some of the dust collection equipment needs to be rolled away to make emptying the bins easier. Am I to understand the in each circumstance that the disconnects need to be rated at not less than 115% of the FLC (430.110 (A))?

Correct, although ALSO rated for the HP involved. If you select them for their HP rating, you will automatically meet the other requirements.

I don't typically do this kind of work, but would like to know if I'm on track. No, I'm not a handyman - NC licensed, limited class.

I appreciated any and all input...

Scott

Live and learn.

 

[retirede]

I spent most of my career designing control systems for large compressors. Manufacturers of small "tank mounted" compressors use magic horsepower. You need to look at the nameplate FLA...forget what "horsepower" is says. Chances are you have a motor that is in the neighborhood of 1-1/2 HP.


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[jusme123]

Hi all,

I have a client who wants me to wire up some equipment for his wood shop (dust collection, compressor, etc.). I'm hoping that If I explain one scenario then I can apply it to the other equipment... None of the equipment has documentation on branch circuit/OCP. Only wiring configurations. I'm looking to get some input on my calcs and what the mfg. had in mind when they installed a 20A plugon one of these pieces of equipment.

Compressor (motor #1): 5 HP/230V = 28A FLC at 230V (per table 240.248) *125% = 35A

Branch circuit condutors = 40A @ 60'C = #8THHN

MOCP = 28A FLC * 250% (per table 430.52) = 70A

Can someone tell me why the manufacturer installed a 20A/250V plug (NEMA 6-20P) on the compressor? It would seem to me that this is way undersized for the branch circuit that I calculated.

Also, I know that the Max. OCP can be up to 250% of the motor's FLC. But should I necessarily size it that large? I know it is to protect the conductors...the overload protection is for the motor. Is there any reason to start at a lower OCPD and go up from there? I often hear of people (electricians - they are people, right? ) who talk of that, but I've never understood why.

I am running from a 110/220V panel upstairs with Inverse-time breakers and am going to install plug and cord disconnects for some of the equipment and non-fusible disconnects for others. Some of the dust collection equipment needs to be rolled away to make emptying the bins easier. Am I to understand the in each circumstance that the disconnects need to be rated at not less than 115% of the FLC (430.110 (A))?

I don't typically do this kind of work, but would like to know if I'm on track. No, I'm not a handyman - NC licensed, limited class.

I appreciated any and all input...

Scott

...the overload protection also protects branch circuit conductors

 

[Dennis Alwon]

Earth to Scott are you home-- don't make me call you....

 

[sbrehler]

Sorry for the delayed response. We got socked with the storm yesterday and lost power... thanks for the replies.

Everything I'm getting is off the nameplate...the one on the motor, not the one stuck on the tank. It says, HP: 5, Amps: 19.5. See attached pic...

Does this still sound out of whack for this motor? Almost 20A @ 5HP ("developed HP"?) By my reckoning 746W*5 = 3,730W. Quite a bit over what a 20A plug would take. But, then again, perhaps that is only to handle the in-rush?

This is brand new equipment, right out of the box. The controller and plug are stock from the factory.

I don't see anything regarding the terminal temps, so I guess I need to go 60'C on branch circuit conductors...right?

I guess that what I'm getting at is: why would the mfg. install a 20A receptacle when by code I'm going to install a #8 w/ a 70A breaker on it! Am I missing something, or is the mfg. off their rocker.

Dennis, I'm trying to be good!

 

[Jraef]

As I said, a NEMA 6-20 device is UL listed to a MAXIMUM of 2HP at 240V. So either they didn't put the plug on it and someone else did, or they are nincompoops. Either way, that plug cannot be used legally. If you hook it up, you are on the hook.

 

[GoldDigger]

As I said, a NEMA 6-20 device is UL listed to a MAXIMUM of 2HP at 240V. So either they didn't put the plug on it and someone else did, or they are nincompoops. Either way, that plug cannot be used legally. If you hook it up, you are on the hook.

The 2HP maximum is based on safe interrupting capability rather than just static current carrying, yes?
So the nameplate FLA is not, in this case, the limiting factor?

 

[Jraef]

The 2HP maximum is based on safe interrupting capability rather than just static current carrying, yes?
So the nameplate FLA is not, in this case, the limiting factor?

Essentially, yes.

See my previous response to a similar question: http://forums.mikeholt.com/showthread.php?t=164752

 

[Dennis Alwon]

I have had 2 issues with Baldor motors and I have only hooked up 2 of them. They were both for a pool cover and we had issues with the built in relays and the wiring to the external switch. They used the neutral wire for the switching and when I called them they said it was safer that way and when I showed them it wasn't they said "We didn't think about that". Anyway my confidence in them is not good.

I assume the motor is listed-- It looks like CSL but I can't really make it out. Not sure how they get it listed with a cord and plug like that. I would call them but I bet you get a dumb response. I am sure they are thinking the motor draws 19.5 amps so the 12/2 cord is rated 25 amps thus we are good. No thought about the cord cap or the our side of the install.

 

[Jraef]

I have had 2 issues with Baldor motors and I have only hooked up 2 of them. They were both for a pool cover and we had issues with the built in relays and the wiring to the external switch. They used the neutral wire for the switching and when I called them they said it was safer that way and when I showed them it wasn't they said "We didn't think about that". Anyway my confidence in them is not good.

I assume the motor is listed-- It looks like CSL but I can't really make it out. Not sure how they get it listed with a cord and plug like that. I would call them but I bet you get a dumb response. I am sure they are thinking the motor draws 19.5 amps so the 12/2 cord is rated 25 amps thus we are good. No thought about the cord cap or the our side of the install.

That motor has a UR (UL Recognized) stamp on it, meaning the motor is not listed by itself, it must be listed as part of an assembly. Not unusual for an OEM motor. But what would be unusual would be for UL to list the entire assembly using a cord cap not rated for the motor size.

Most likely the unit is UL listed for hard wire connections, and someone else slapped the cord and cap on it.

 

[ADub]

How far is the run? #8 seems like major overkill for that little guy


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[iwire]

Just install the circuit to match the plug and it will work fine.