WOW! A New Topic in the Lonely Electrical Section!

[Wolverine]

Wow, a new topic in the lonely electrical section. Why should the CE's have all the fun?

Okay, so I'm a transmission line guy and all I know about practical application of the NEC is what I learned in the two weeks I devoted to studying it prior to the PE, so I might need a little help here.

Our neighborhood pool pump motor smoked when they first fired it up for the season and the HOA pool lady emailed in a panic that it had to be replaced by next week. The electrical contractor says "the aluminum wiring caused the motor to overload and the lug connections to burn up". :th_rockon: With my skeptical toe itching like crazy, the contractor also wants to install a starter contacter and copper wiring for the three feet over to the motor to the tune of $3300. So I figured I'd better go check it out.

The motor is a 7.5HP, 230V, 1-phase, with SF=1.15, code F. It is protected by a 60A breaker. The wire to it is jacketed triplex aluminum, not sure what size that is and can't read the side since it's in conduit, but it looks plenty beefy to me (like triplex used to serve residence).

So I've never heard of "aluminum wiring" being the source of a failure (as opposed to unsufficiently sized wire, which makes no difference what the material is). I gave back my borrowed NEC book so can somebody tell me what the proper wire size and thermal overload protection is given the specs?

I suspect the motor may be rotor locked or otherwise fried, and installing a starter might be a nice addition, but the "aluminum wiring and lugs" story sounds fishy to my non-residential-expert ears.

 

[Dark Knight]

Shooting out of my belt: The aluminum does not have the conducting capacity of copper. I mean for that: #2 AL conducts less current than #2 Cu. A loose lug is always a cause of trouble because where the lug is, you will have a lot of arching and sparking (HEAT) and that is not good for the wire. I am surprised to read the motor has aluminum wires. The starting current of most motors is usually very high. Way higher than the rated current but is only momentary. I am going for my NEC and will get back to you with the rest of the info.

This is interesting Wolverine..... :th_rockon:

 

[Dark Knight]

The motor is a 7.5HP, 230V, 1-phase, with SF=1.15, code F. It is protected by a 60A breaker. The wire to it is jacketed triplex aluminum, not sure what size that is and can't read the side since it's in conduit, but it looks plenty beefy to me (like triplex used to serve residence).

Ouch. I hope that the motor is not a Locked Rotor type. If that is the case, NEC 430.251(A) asks for a disconnective of 115% of 240 Amps. That would be 276 Amps. I don't think that is the case. Is a little bit excesive for a pool motor. Wolverine, please check the type of motor. Is very important.

If the motor is not the Locked Rotor type then NEC 430.28 calls for a 40 Amps for a 7.5 HP single phase motor. 60 Amps should be enough.

Now, the next protective device we need to coordinate is the overload. Now is when the Service Factor comes to play. NEC 430.32 asks for an overload protection no greater than 125% of the FLC on the nameplate of the motor. Do we have that info?

I don't like aluminum conductors on motors but blaming them for that is not what I would do. Can we get the info of what size is the aluminun wire? From 310.16 Copper, for a 60 Amps breaker, should be #4 minimun. For a copper wire a #6 wire will be good enough. I used the 75C degree because most of the breakers are rated to that temperature.

Hope that helps.

 

[grover]

There is nothing fundamentally wrong with Al wires, if sized and installed correctly. They get a bad rap because the alloy used in homes when it first came out was brittle and cracked more easily than copper, while improper terminations caused galvanic corrosion problems. Al must be sized correctly (and will be larger than copper of the same ampacity) but unless that Al wire was inadvertantly undersized when it was installed, there's no reason to blame it.

 

[MetroRAFB]

^^^^I concur with all of the above. If the aluminum terminations weren't coated with "the gray goop" when they were done you could wind up with some of the corrosion/arcing problems that Luis described. There's nothing fundamentally wrong with aluminum wire though, it just needs to be larger than copper for the same current. I think the contractor is a bit misinformed, or maybe you only heard part of the total story Wolverine?

 

[Jahrtrix]

Hello; I'm new here and Wolverine's question gave me an excuse to say something. I have worked with

Aluminum wiring wire for many years. It requires much more care in installation and usually is sized the

next size larger when compared to copper. The "grey poop" is actually NOAlox (IDEAL) to inhibit corrosion.

I personnaly would not recommend Al. products except for transmission lines or service entrances.

Copper is much more forgiving than Aluminum in near acceptable installations. A 7.5 hp single phase

motor is not a usual stock item, the amp. rating is probably in the 40 amp area. A #6 AWG copper is

appropriate here. I agree with the contractor, but the cost???????.

Jahrtrix.

 

[Wolverine]

Good responses! Thanks, especially to Luis for doing my NEC homework for me. It confirms what I was thinking but just not confident on.

The update to the story is that I talked to one of the neighborhood old timers who said the pool contractor most likely flipped the motor breaker on and went outside for a smoke, forgetting to open the main valve, leaving the pump to suck on a vaccuum. Now that's a story that makes more sense. The other thing I could believe is the lug connections from the big AL wire to the little copper motor wires being poorly fit.

We'll probably just pull the motor, drop a new one in, and call it done. The more time I've had to think about it and based on the responses here, the more I think the 60 amp breaker is just fine for protection.

 

[Art]

I don't like Al for motors...it's fine for aerial or services...

don't have my code book, but I'm guessing 7.5hp at 240/1 is ~30A, not a small current...

so if the breaker is 60A and the wire is to code, ie, ampacity >60A, you should be fine...all you need is 125% of the motor rating...less any derating or Vd compensation/considerations...

the lugs/terminals must be rated for Al, I've seen issues when Al is used with copper terminations...in a corrosive enviornment like a pool room (chlorine) corrosion is an issue and will cause a substantial impedance, and heat up...possibly fail...I'm guessing his analysis is accurate...

my issue is his price!!! $3300 for a job that takes maybe $250 in materials and 4 to 6 hrs to do...max!

the pump will not burn out, even if cavitating (pulling air) or deadheaded...at dead head the power is extremely low (power is proportional to flow, the more GPM, the more HP required...with no flow the pump is idling) it could damage the impeller, but I doubt it...

even if it did over load, the overloads should have tripped at ~115% of FLA (35A or so), and since the circuit is rated much higher ~60A, no damage should occur (there is a method to our madness :Locolaugh: )

 

[Dark Knight]

I don't like Al for motors...it's fine for aerial or services...
don't have my code book, but I'm guessing 7.5hp at 240/1 is ~30A, not a small current...

so if the breaker is 60A and the wire is to code, ie, ampacity >60A, you should be fine...all you need is 125% of the motor rating...less any derating or Vd compensation/considerations...

the lugs/terminals must be rated for Al, I've seen issues when Al is used with copper terminations...in a corrosive enviornment like a pool room (chlorine) corrosion is an issue and will cause a substantial impedance, and heat up...possibly fail...I'm guessing his analysis is accurate...

my issue is his price!!! $3300 for a job that takes maybe $250 in materials and 4 to 6 hrs to do...max!

the pump will not burn out, even if cavitating (pulling air) or deadheaded...at dead head the power is extremely low (power is proportional to flow, the more GPM, the more HP required...with no flow the pump is idling) it could damage the impeller, but I doubt it...

even if it did over load, the overloads should have tripped at ~115% of FLA (35A or so), and since the circuit is rated much higher ~60A, no damage should occur (there is a method to our madness :Locolaugh: )

Art,

NEC calls protection for40 Amps. I have a concern with the 60 Amps breaker though. If there was something wrong with the motor and it was drawing more than the 40 Amps the breaker saw that as a load. Maybe the wires were not rated for that kind of load and also what you mention, the special considerations for Al conductors, had something to do with the problem. Al conductors are primadonas. Have to handle them with caution. Is that a common practice to install Al conductors @ swimming pools? I have no idea.

Anyways....I agree with the post that said that we, EEs, ROCK!!!!!!

 

[Art]

in general the code allows:

max to abs max of FLA...

175 to 300% for fuses, depending on type...TDDE, nonTD, etc.

250 to 400% for ITCB's, as igh as 1300%! when designed by an engineer,

motors , 100A

a single phase 230VAC 7.5 HP motor draws 40A per NEC tables, so the conductor must be at least 1.25 x 40 or 50 A...neglecting Vd and derating factors...

so you could use a 50A CB and #8 THHW (75 C) minimum...

this is only 125% of the rating, and may cause nuisance tripping (not a problem with a pump, one of the easier loads to start)

I myself would probably use a 60A (150%) and #6, well with the guidlines, and below the max of 250%

iirc Code F is low starting current, so the 50A setup may be fine too...and it's cheaper not that <5' of wire matters...

if the OL's are set-up properly ~115% of the FLA, they should trip at 46A or so, and the wire should never see 60, unless starting, and even so, the CB protects the conductor by code, so it must carry safely & without damage any current below the breakers trip point...