# 3 phase 15Hp 240V motor ? help



## Paulusgnome (Mar 28, 2009)

Assuming you meant 240V phase-to-phase, your line current calculation should come from : 
Power = V x I x Sqrt(3) x PF X Eff

You can probably take PF = 0.8 as good enough.
Efficiency Eff is likely better than 90%.

Plugging those numbers in with V = 240 and 746watts = 1Hp, I get a line current of just over 37A.


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## JRaef (Mar 23, 2009)

Nisat said:


> I do have to connect, 15 HP 3 phase, 240 V Motor for concrete mixer. The mixer manufacture has control panel with 100 Amp Breaker attached on the side . I figured ,I can used Awg 8 but not sure. Here is how I come up with my calculation.
> 
> 
> Using Table 430.250
> ...


If the starter for the mixer already has a breaker in it, and you are feeding that from a breaker in the panelboard, why are you putting in another 3R fused disconnect? It's redundant. If the breaker that you say is in the mixer starter doesn't have an external handle, then yes, you would need something to LO/TO, but it could be a non-fused disconnect.

Other than that, have you accounted for voltage drop with that #8? You don't say what the distance is, but you don't have much margin for error there, only 2.5A. It also appears you used the 90C column, the 75C column puts #8 at only 50A. I'd use #6, even without factoring VD yet.



Paulusgnome said:


> Assuming you meant 240V phase-to-phase, your line current calculation should come from :
> Power = V x I x Sqrt(3) x PF X Eff
> 
> You can probably take PF = 0.8 as good enough.
> ...


Paul,
Being you are in NZ you may not know, but for conductor and OCPD sizing we must use a standardized chart (he references Table 430.250) in our code book for motor FLC, which tends to be conservative for covering worst-case scenarios. We only use the actual motor nameplate FLA for the overload relays.


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## Nisat (Aug 5, 2013)

Yes , the mixer comes with a disconnect , but it's above ground mixer about 8ft. When did my maths, did not consider the vd, 
Yes , I end up pulling # 6.

Thank you very much for your input.


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## bobelectric (Feb 24, 2007)

Whatever happened to Chewy?


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