NEC Article 430 unclarity

[stcalle]

Ok, now I've read that Article 430 on motors is one of the most misunderstood articles in the NEC. With that said, I want to bring your attention to NEC 2002 Article 430.52(C)(1) and Article 430.52(C)(1) exception no. 1.

For branch circuit protection, Article 430.52(C)(1)says "use a value not exceeding the value calculated according to value given in Table 430.52"

Then Article 430.52(C)(1) exception no. 1 says "where values for protective devices determined by table 430.52 do not correspond to standard sizes or ratings, a higher size that does not exceed the next higher ampere rating is shall be permitted"

Now it may be just me but these two articles seem to contradict themselves. The reason being that most of the time when you calculate the device rating, it won't conform to one of the standard sizes (found in Article 240). Therefore, is it correct to say that when calculating device ratings
1) if get an exact standard ampere size, you can size it that size or less.
2) if do not get an exact standard ampere size, you can size it up to the closest standard ampere rating. You can then choose to use this size or less?

thanks

 

[rbalex]

Re: NEC Article 430 unclarity

1.) Yes
2.) if do not get an exact standard ampere size, you can size it up to the next possible higher setting that does not exceed the next higher standard ampere rating.

In some cases, a breaker may be set to values between "standard" ratings. If the computation results in a non-standard size, the setting above and closest to the computed value is permitted. In no case is it permitted to exceed the next higher "standard" rating.

 

[templdl]

Re: NEC Article 430 unclarity

Bob, Re

In some cases, a breaker may be set to values between "standard" ratings.

What you pointed out is another option that is often overlooked. However, this can only apply if the breaker has a sealable setting and can not be changed or can be accessed by only a qualified person otherwise the breaker must be applied as though it was set at highest setting it can be set at. That is the breaker's highest available adjustable themal (longtime) setting is not greater than the closest higher standard rating.

 

[tom baker]

Re: NEC Article 430 unclarity

where values for protective devices determined by table 430.52 do not correspond to standard sizes or ratings, a higher size that does not exceed the next higher ampere rating is shall be permitted"
I thought the intent of this was for fuses with in-between ratings.

 

[rbalex]

Re: NEC Article 430 unclarity

Tom,

I just wanted to point out it also may apply to certain circuit breakers. It does apply to non-standard fuses.

With 240.6 now recognizing a multiplicity of non-standard ratings, "in-between" settings and ratings needed to be recognized.

Edit Add: templdl's point is also well taken.

[ March 21, 2005, 03:40 PM: Message edited by: rbalex ]

 

[tom baker]

Re: NEC Article 430 unclarity

I prefer fuses over CB's. The complaint is often fuses will let a motor "single phase", but if the fuse is sized at 125-140%, it will back up the overloads. Also the IEC overloads have a differential phase loss mechanism, they are more sensatitive on phase loss, I use the IEC overloads with NEMA contactors.

 

[coulter]

Re: NEC Article 430 unclarity

Tom -

Originally posted by tom baker:
... if the fuse is sized at 125-140%, it will back up the overloads.

I'm not sure why overloads would need backup? Have you seen a high failure rate of overload assemblies?

Originally posted by tom baker:
... IEC overloads have a differential phase loss mechanism, they are more sensatitive on phase loss ...

I've not seen phase loss relays as part of any standard thermal overload. Seen them on electronic overloads (NEMA anyway). Are you saying the standard IEC thermal overload trips on phase loss?

Originally posted by tom baker:
... I use the IEC overloads with NEMA contactors.

Is that available as part of a listed combination starter?


You have raised some interesting questions. I've got some homework to do.


carl

 

[friebel]

Re: NEC Article 430 unclarity

With all of this discussion of fuse selection versus circuit-breakers, I agree with Tom Bakers statement, where he said that he preferred fuses over circuit-breakers.
Another statement was made, saying, "why do you need a back-up for overload elements". Well, many times I have seen electricians, when they are having problems with the OL's opening up, due to an overload condition, the electrician will increase the ampere rating of the OL.
Let me give you a real life situation that we dealt with at a DuPont plant where I worked for 45 years.
We called in a Bussmann representative to give us a presentation of Selective-Co-ordination of fuses at our plant. Now I am talking about a plant at that time had 8,000 employees, and about 10,000 motors. That gives you an idea of the complexity of the task that we embarked on at our site.
With the help of the Bussmann Company we changed all of our fusing on our site to Lo-Peak dual-element fusing, and as you are aware, according to the NEC Table 430.52, the fuse-selection is 175% of the amperes on Table 430.150 for three-phase motors.
We fused all of our motors at 125%, so actually we did have a close back-up for our overloads.
Now, I will admit that their where some motors that we did increase this value.
But, can you imagine the excellent selective fuse co-ordination that we had at our site, and also better motor protection, due to overloads.
It took us better than one year to accomplish all of this changing of fusing for Selective Fuse Co-ordination.
I know that this is a very long-winded message, but if you can just bear with me I will give you an actual incident that happened after we had completed our fuse changing program at our site.
I was standing next to a Allen-Bradley motor control center that had a size 3 starter, and it controlled a 50 hp motor, 575 volts.
I heard the contactor drop out, and just a minute later a call on the Intercom system for a electrician to check the motor out.
A electrician came in and opened up the MCC door for the starter and this is what he found.
A metal nameplate tag that had been glued onto the control transformer had come loose and fallen down onto the line-side of the contactor, L1-L2-L3.
Well as you know, this was a direct short across the phases, and all that I heard was the contactor dropping out.
Had this been a one-time fuse, sized at 300 to 400 percent of Table 430.150 ampere rating for a 50 HP motor, you can imagine the damage that could happen.
I am going to mention the Bussmann engineer that helped us on this large project of Selective-Fuse-Co-ordination, and his name is Larry Eggert. Larry is still with the Bussmann Company and at the present, he is home recuperating from surgery.

 

[coulter]

Re: NEC Article 430 unclarity

Originally posted by friebel:
... Well, many times I have seen electricians, when they are having problems with the OL's opening up, due to an overload condition, the electrician will increase the ampere rating of the OL. ...

If I might ask, what is the back up for this same electrician putting in higher amperage fuses?

I promise I'm not trying to pick at you, I'm just not seeing the reasoning. I don't know how to protect against a so-called qualified electrician jacking up the overloads above the NEC limits. I say "so-called", because I don't think a journeyman or qualified maintenance electrician would do that. Well, okay, most that I work with wouldn't. And the ones that wouild, well, all of us keep a collective eye on them.


In case you haven't guessed, my preference is for MCC with listed combination starters with an MCP (mag only CB) - with CB and OLs set right up to the NEC max allowed.

This assumes we are talking industrial and the equipment was designed to run flat out. If not, then of course, run at or below design limits.

carl

 

[mc5w]

Re: NEC Article 430 unclarity

If the overload relay contact is in the neutral (old school) and there is a pilot light connected across the contactor coil, then a ground fault against the door hinge can bypass the overload relay contact.

An overload relay contact in the neutral should only be done in a self protecting motor controller where only a voltmeter probe can access the node connecting the coil to the overload relay contact.

The more modern place for an overload relay contact is in series with the stop button in the hot wire. One type of output board for Siemens programmable controllers has the outputs in pairs to facilitate doing this with reversing and 2-speed controllers.