Over rated wiring?

[rsdoran]

rta, whatever the supply is fused at is what the wire should be rated for, that is in NEC but I would have to look it up to be specific.

The 125% I quoted from my copy of NFPA Electrical Standards for Machinery, I am sure it is also covered in NEC but I would have to look it up to give the specific Article.

 

[Lancie1]

Ron,

You are correct, the wire must be sized to carry the trip rating of the fuse or breaker. That is NEC Article 240-3, Protection of Conductors. As you said earlier, if the starting current of a motor requires a larger breaker or fuse, then due to 240-3, the wire must also be proportionaly larger, even though the normal motor running current is much less.

The 125 percent ampacity rule for single motor conductors is NEC Article 430-22(a):

430-22. Single Motor:
(a) General. Branch-circuit conductors that supply a single motor used in a continuous duty application shall have an ampacity of not less that 125 percent of the motor's full-load current rating as determined by Section 430-6(a)(1).

In general, if the fuse or breaker is sized large enough to handle the motor starting current, and Article 240-3 is followed, then Article 430-22 is also satisified, as the wire will usually be more than 125% of the motor FLA.

 

[Bruce99]

wire size?

Wire size is determined by the load. The fuse size is determined by the load. The code gives MINIMUM wire sizing for each application. If you wish to run #10 for a 7.5 amp load that is OK but you will not fuse the circuit for #10 wire. You would use a 10 amp time delay Class J fuse. The fuse is to protect the circuit. The code also gives fuse ratings as maximum allowable for this reason determinded by LOAD. If you over size your wires and fuse for the wire size I can see a lot of fires happening. It is important to size to the load. Motor loads, transformers etc have to have inrush to deal with, but the code allows for this. Do not size to a fuse or breaker. This is thinking backwards. What load do I have, what do I need to protect this and what is needed to carry this load. I got this pounded into my small brain over and over in Second year. The fuse size in a motor circuit can be a lot larger than the required conductor ampacity. Fuses protect wires by interupting short circuit energy before damage is done.

 

[Lancie1]

Bruce,

Wire size is determined by the load...

I mildly disagree that wire size is directly determined by the load. Under the US National Electrical Code, the wire size is determined by the fuse or circuit breaker size. The wire MUST be large enough to handle all the current let pass by the fuse. Then you ask, is not the fuse size determined by the load? Yes, but it is not a direct relationship. There are ifs, ands, and buts involved. For motors, the fuse or breaker winds up being LARGER than the full load motor current, therefore the wire will also have a larger capacity than the motor load. These rules have the same force as law and I gave up fighting with them long ago.

The fuse size in a motor circuit can be a lot larger than the required conductor ampacity.

If it were, the wire will burn up and start a fire long before the fuse blows. If I designed like this in the US, then I would be breaking the law and subject to legal action, fines, maybe even jail time, if it caused a fatality or extensive property damages.

 

[AutomaticLeigh]

I don't know anything about the NEC code, but the situation in the UK would seem to be a little different.

For a motor, it is recognised that sizing the current capacity of the wire to the fuse is uneconomic. Therefore, the fuse is there for short circuit protection, and an additional overload relay and contactor must be provided for overload protection. That leads us to the thorny subject of coordination between the fuse and overload, which I'll duck.

For short circuit protection, the critical parameter is let-through energy, which will be quoted by the fuse manufacturer as I^2 * t, because it is assumed that all the energy is absorbed by the cable. The wiring regs give a formula for calculating the permissible I^2t for a given construction and cross sectional area.

For control cicuits, I think the size is more related to mechanical strength than the current, which in many cases is tiny. Once you've seen panels with half the components hanging from the wires, thicker looks better.

 

[rsdoran]

y'all gotta be kidding. I dont care what is on the circuit...THE FUSE PROTECTS THE SYSTEM.....ie the WIRE is part of the system.

Economics are not part of it. There are guides but the wire MUST be capable of carrying the current allowed by the fuse etc....it does not matter what the load may be on the circuit.

 

[AutomaticLeigh]

Economics are not part of it.

Actually, economics is ALL of it. The reason codes exist is to give guidance on the absolute minimum amount of copper you can get away with without setting fire to something.

 

[Bruce99]

Bruce,

I mildly disagree that wire size is directly determined by the load.

The minimum wire size is determined by the code. It is calculated from what the load draws. The code then says the wire must be capable of carring 125% or whatever over this. Whatever percentage over the load is used, will be the maximum overload protection the motor can have.

The wire MUST be large enough to handle all the current let pass by the fuse.

This is what I said. The load determins this.

If it were, the wire will burn up and start a fire long before the fuse blows.

This is why the code requires overload protection on motor circuits

 

[ushidayo]

Bruce99 had it right, read his posts very carefully. Overcurrent protection (fuse or circuit breaker) is for circuit protection, i.e. the cable to the motor. Overload protection (fuse or overload device) is for motor protection. The cable must be sized for the motor (taking into consideration installation method, volt-drop, ambient temperature). The Overload protection must be sized for the motor. The overcurrent protection must be sized for the cable. Then add to this coordination between the overcurrent and overload devices.

Just because the motor control panel is normally designed before the cable installation design, doesn't mean that the ratings chosen for the overcurrent device are correct. If the overcurrent device has been oversized, consider changing it for something more suitable for the real installation.

If the rule of thumb to oversize everything is used, you cannot expect to be competitive in industry.

 

[rta53]

y'all gotta be kidding. I dont care what is on the circuit...THE FUSE PROTECTS THE SYSTEM.....ie the WIRE is part of the system.


Economics are not part of it. There are guides but the wire MUST be capable of carrying the current allowed by the fuse etc....it does not matter what the load may be on the circuit.

Ron,

Not wanting to get into an ongoing argument about this and I am not speaking as a rookie. Been doing this about 30+ years. However, would you not agree that motor loads have somewhat different requirements than other types of loads. If I am wiring a 3-phase motor, overcurrent protection to that motor is provided by the overload relay/heaters while Branch-Circuit Short-Circuit protection is provided by fuses or a circuit breaker. The NEC requires conductors to be sized at 125% of full load current. The overcurrent device is going to protect these conductors from being overloaded. However the code allows for the fact that you have to be able to start the motor and even the percentages given in Table 430.52 for fuses and breakers can be increased quite substantially if necessary. But I am not required to increase the wire size if I sized it properly to begin with.

When you are protecting other types of loads, generally speaking, you do work backwards form the overcurrent device size as stated in Article 210.3. However 210.1 states that this article covers branch circuits "except for branch circuits that supply only motor loads, which are covered in Article 430". Protection for motor circuits has allowances that are not given to other loads.

 

[Rube]

The debate has gone awry!! The post was originally about oversizing. If the wiring is correct as far as the NEC goes, you don't have to oversize it you can just build it correctly. Doesn't matter whether it's control or load wiring.