30 amp plug for motor, 50 amp OCPD?

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Howard Burger

Howard Burger

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Location
N of Anchrage, AK
New construction, major grocery chain. Commercial meat grinder, 8.5hp, comes with L15-30 plug on #8 awg SO cord. We're going to run an SO cord with connector from fused disconnect to the provided cord plug so that the unit can be disconnected and moved for cleaning.

I figure OCPD size to be 67.1 amps, max. The panel schedule calls for 50/3 breaker.

Lead journeyman says can't have 30 amp cord plug on 50 amp breaker (thinking possibly of 210.21.B.1?); my thought was that the cap is OK because the 50 amp breaker is for startup current, and the wire is protected by the fused disconnect, which I figured should be 37.5 amps (data plate shows 30 amps).

Should we put a bigger plug on the cord? To add confusion to the issue, the plans call for #10 wires from the panel.

(My amps figures comes from extrapolataing T450.250 since it jumps from 7.5hp to 10 hp; I got 26.84 for the amperage for an 8.5hp motor.)
 
poke around in 430. wire size is developed by mutiplying nameplate current by 1.25 for a single motor. in this your figure of 26.84 gives you 33.55. thus # 10 is ok for this using the table in 240.4(G) which directs you use to the table in 240.4(G) for specific conductors. this table reroutes you to section 430 again where in section 430.6 it again reroutes you to 310.15(B) which tells you that ampacities for conductors rated 0-2000 volts SHALL be as specified in table 310.16. This is the section that specifically allows you to disregard 240.4(D) for small conductors. This is a kind of confusing labyrynth of the code that gets you to a final result. There are a lot of inspectors out the who do not understand this nuance of the code as it applies to motors and say air conditioning equipment. Hope I have helped and not confused. Your disconnect size is also calculated at 1.15 x the full load current which results in 30.86 just slightly more than the 30 amp d. I would assume that if the plans call for a 30 amp dsic. the calculation was based on something a bit less than the 26.84 amps that you have developed.see 430.110(A).
 
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I think Howard points out an excellent "field problem" that many of us have encountered.
It oftentimes seems the factory approach to appliance cord and plug situations places us at odds with the NEC.
I agree with Bob that there is no Code provision to "over fuse" a receptacle (other than where specifically noted) and to change the plug would be "altering a listed device".
Still one wonders if a 30 amp breaker will handle the motor.
Interesting dilemma.
 
Could go with 50 amp cord caps. Take off the 30 and replace it with a 50. Give the manufacturer a call..
 
tlhank you

tlhank you

Thank you all for your help. I've been moved to another project, but I'll email this link to the lead jm. Stew, I'll look into the path you spelled out - thanks. Using the NEC sometimes reminds me of a road sign I saw posted - here I think - that said 'NEC ahead' and then showed roads tying themselves in knots. Gotta find that and paste it to my code book cover..
 
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