Ask Experts Questions for FREE Help !
Ask
    KimWilcox's Avatar
    KimWilcox Posts: 1, Reputation: 1
    New Member
     
    #1

    May 2, 2008, 06:58 AM
    Calculating load per circuit breaker
    How do I calculate how much load I can place on a nn amp circuit breaker?
    My knowns would be voltage and watts per hour.
    How to I allow for (amp draw) when a shop motor or freezer compressor spins up at initial start?
    stanfortyman's Avatar
    stanfortyman Posts: 5,598, Reputation: 279
    Electrical & Lighting Expert
     
    #2

    May 2, 2008, 11:04 AM
    Quote Originally Posted by KimWilcox
    How to I allow for (amp draw) when a shop motor or freezer compressor spins up at initial start?
    You don't. The breaker takes care of that.
    tkrussell's Avatar
    tkrussell Posts: 9,659, Reputation: 725
    Uber Member
     
    #3

    May 4, 2008, 07:15 AM
    A circuit breaker needs to be higher in rating than the load of the unit being served.

    For a standard single phase motor, nameplate data, such as wattage or amps cannot be used for these calculations, only the horsepower rating is used, and for a single phase motor, refer to the amperages listed in the NEC Table 430.148.

    A standard inverse time circuit breaker feeding standard single phase motor can be no larger than 250% of the running load in amps of that motor.

    The branch circuit wiring must be th minimum of 125% of the running load.

    So, for example, Table 430.148 lists a 1 HP motor at 120 volts at 16 amps.

    16 x 1.25 = 20, so #12 wire minimum is allowed.

    16 x 2.5 = 40, so a 40 amp maximum circuit breaker is allowed.

    The 40 amp breaker is now rated to handle the "initial start" ,or called Locked Rotor Amperage. The wire is sized to handle the running load.


    A 40 amp breaker on #12 wire? I am sure to get some comments on this.

    A refrigerator falls under Article 440, and the breaker is sized differently, due to the inherent higher Locked Rotor Amps that all refrigeration compressors have. Here the nameplate current, and not horsepower is used.

    The wire is still to be a minimum of 125% of the running load, and the circuit breaker can be a 175% of the running load, up to a maximum of 225% if necessary to start the unit.

    So, with my example of a 16 amp unit, the wire can still be #12 copper wire, but the breaker can be a 30 amp CB at 175%, or a 40 amp CB at 225%, which under normal conditions, should not be needed..
    stanfortyman's Avatar
    stanfortyman Posts: 5,598, Reputation: 279
    Electrical & Lighting Expert
     
    #4

    May 4, 2008, 08:54 AM
    Quote Originally Posted by tkrussell
    A 40 amp breaker on #12 wire? I am sure to get some comments on this.
    I'll comment that you are totally correct.

    Now I'll give my opinion.
    I am of the strong opinion that Articles 430, 440, 630, etc have NO place in a residence. The rules that allow us to use "non-typical" breaker/wire sizes do NOT apply when wiring a RECEPTACLE in a residence.
    That's right, you are wiring a receptacle, NOT a welder, refrigerator, or compressor motor.


    Like I said in my original reply. If you have a tool or appliance that can plug into a standard 15A/125v receptacle the circuit breaker IS designed to handle ANY startup load that appliance or tool might have.

Not your question? Ask your question View similar questions

 

Question Tools Search this Question
Search this Question:

Advanced Search

Add your answer here.


Check out some similar questions!

Maximum load on 20 amp circuit breaker [ 14 Answers ]

I am aware of certain normal maximums. I guess I'm looking for somebody to tell me that I'm pushing things too far, or not. I want to hook up a portable air conditioner (Sunpentown WA-1220E) which is rated at 950 watts (and, mysteriously, at 9.0 amps) along with an HP 4Si printer, which is rated...

Theory of vacuum Circuit Breaker & sf6 circuit breaker [ 15 Answers ]

I want to know about the basic Theory of vacuum Circuit Breaker & sf6 circuit breaker

Replacing Circuit Breaker with Higher AMP Breaker [ 8 Answers ]

Excuse me, I did look at the code book as is suggested in your Sticky. The NFPA may answer this question, but I am not knowledgeable enough to recognize it: Our offices are upstairs in an older (renovated home) building with modern wiring. We have 4 computers, 3 printers, and miscellaneous with...

Calculating circuit load [ 4 Answers ]

In calculating the load for a residential general purpose circuit that will have lights on it, can I use the wattage of fluorescent energy saving bulbs instead of the incandescent ones, since that is what I will be using? It doesn't sound likely, but thought I would ask anyway.

Circuit breaker load [ 2 Answers ]

I'm remodeling my basement, & putting in a lot of elec'l outlets. Wondering how many outlets, or how do I figure how many, I can put on one ckt brkr. I'm using no. 14 Romex & 15 amp brkr. Tom :confused:


View more questions Search