500 THHN with 600A ?

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electricaldoc

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We pulled wire for a 300 hp air compressor last night, and I ran into a couple of questions.

1 we used a 600A 480V 3phase breaker, but pulled only 500mcm THHN wire. Now this actually fed a disconnect that was to be fused at 400A. The problem I have is 500 THHN is not rated for 600A, and if 400 amp fuses were ever up sized to the full potential of 600A this would cause a problem.

Would this fall under 240.21(B)4 taps over 25ft? Or did we just make major code violation? An electrician on different shift made the call on wire size, and had already had 200ft of this wire shipped in for the job. Each leg had to be 60ft long so no way to parallel them plus when he quoted it boss told him we only need one set of 500. We had three men in there on double time to make pull before compressor would be set at 7:00am this morning. So no way to change mind then.

2 One of the electricians wanted to land the grounding conductor on a grounding box connector or bushing. This would have meant no grounding lug in disconnect, and when they landed the wires on load side the grounding conductor for load side would have also been connected to grounding bushing. I did not like this idea and since I made the land I added a grounding lug.


Now are both of these a code violation, or am I wrong?
 
Re: 500 THHN with 600A ?

If i'm not mistaken, the FLA for a 300 hp, 480V compressor is 361 amps. So the feeder needs to be 125% of FLA, which is 451 amps. So the 500 MCM appears to be a code violation. You'de need either: 700 MCM's or 2 sets of 4/0.

Also, I'd be surprised if the 400 amp fuse at the disconnect will start the compressor, unless its trip curve is such that it would allow the motor inruch current of approx 2200 amps.

But, in any case, I don't think that you'd need a fuse at the compressor. Does the compressor have overloads to protect it? if so, then you won't need the fuse.

Hope this would help.
 
Re: 500 THHN with 600A ?

Oops, I would need to correct myself, Since the motor compressor is a varying duty, then you could size the conductors down to 110% Table 430.22(E), which is 386 amps (based on 361 FLA). And depending on FLA namplate rating, It would seem that 500MCM could be ok if the nameplate FLA is less than 361.

One more item, the 600 amp breaker is not intended to protect the feeder against overload, it's there, however to protect against short-circuit and ground fault.
 
Re: 500 THHN with 600A ?

Regarding question #2, the grounding of the equipment grounding conductor to only a bonding bushing would be acceptable if the KO's in the breaker enclosure where punched for the size of the conduit installed. Since the feeder is over 250 volts bonding would be required if the enclosure contained concentric or eccentric KO's. See 250.97.
 
Re: 500 THHN with 600A ?

electricaldoc
Have you been able to actually start the compressor?
Do you have the name plate ratings?
Can you identify your motor in table 430.22E?
 
Re: 500 THHN with 600A ?

Varying duty for a continuous duty motor is a conductor size that is 200% of motor full load current. The 110% is for a short time duty motor and I have never ever seen that on an air compressor.

The best practical solution is to put six 500 KCM copper in the same conduit plus a redundant equipment ground. This will make having the wires the same length easier and you can reuse the wire you already have. This will give you 600 amps which will allow you to put in a 1.25 service factor motor or a bigger motor if it turns out that the 300 HP motor is too small.

I have opened up a wall enough times ( once ) to know why the British require redundant ground wires in all metal conduits in all occupancies. This is also almost always a job specification for all occupancies here in Ohio. Our customers like grounded.
 
Re: 500 THHN with 600A ?

I have not see compressor yet nor due I know FLA. Remember a different electrician on another shift did all calculations and supply ordering. We just pulled Wire and landed it. The concern is 500 MCM on a 600 A breaker even if it does go to 400A fuse. Wouldn't this still be code violation? What if some one replaces 400A fuses with 600A fuses know 500 MCM on 600A load? :eek:
 
Re: 500 THHN with 600A ?

Originally posted by electricaldoc:
We just pulled Wire and landed it. The concern is 500 MCM on a 600 A breaker even if it does go to 400A fuse. Wouldn't this still be code violation? What if some one replaces 400A fuses with 600A fuses know 500 MCM on 600A load? :)
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Re: 500 THHN with 600A ?

Originally posted by iwire:
By the way "MCM" is not used anymore, the term is Kcmil. :)
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Why? When did this change occur? I don't usually work with wire sizes that require that terminology so hence my unawareness of the change.

I like MCM better. And does anyone actually say "KCMIL" in the field? :p Because that's the way I learned it. Good reason, right? :roll:
 
Re: 500 THHN with 600A ?

Originally posted by peter d:
I like MCM better. And does anyone actually say "KCMIL" in the field? :confused:
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I am not sure what code cycle it was changed but it was Kcmil in the 99 code.

It makes more sense to me as K = 1000.

Yes when I order wire I ask for 350 Kcmil.
 
Re: 500 THHN with 600A ?

That makes sense, as the '99 started the transition to the Metric system and its associated nomencalture. (Don't get me started on that topic. :D
 
Re: 500 THHN with 600A ?

AT work today got new compressor info. it has FLA of 385A @460V. We are using 480V, and it is pulling 372A. 385A X125% = 481.25A too much for 500KCM. What I am still not understanding here is every one still looking at FLA. I was understanding that you used FLA X 125% to figure out what size OCPD and conductor size would be needed, But if you install a 600A breaker than conductors had to be rated at 600A or more? I mean I can not run a motor that has FLA of 5A and say 5A X 125%= 6.25A so 16 or 18 AWG will handle motor. Then hook it to a 20A breaker?
SO regardless of motor FLA if I have 600A breaker conductor must be able to handle 600A. Correct?
P.S. The fuses the compressor company installed to handle this laod was 600A. So now we do have 600A breaker going to 600A fuses with 500KCM between them. Also electrician for the compressor company ran two 350KCM in parallel on bottom of disconnect. So we have 500KCK feeding two 350KCM in parallel. This all seems very wrong to me. :confused:
 
Re: 500 THHN with 600A ?

The wire could be substantially smaller then what we are used to seeing on an overcurrent protection device.

430.22a - size wire 125% OF FLA

430.52(C)(1) & its table - the motor's short circuit protection device may be sized between 150% to 1100% depending on the type of motor and the type of protecton (duel-element fuses, non-time delay, inverse time breakers, etc.)

This device is only providing short circuit, not overload protection.
 
Re: 500 THHN with 600A ?

electricaldoc
You compressor can pull 4 to 6 times its FLA when starting. In order for the OC device to handle this additional load the code allows you to install a larger device. For standard motors the code allows 250% of the FLA for the size of the OC device. This has noting to do with the conductor size. You may see an air conditioner
compressor with #12 conductor and a 40 amp breaker.
In order to size the conductor for a continuous load, the code requires you to use 125% of the FLA of the motor .

[ March 28, 2005, 04:01 PM: Message edited by: bob ]
 
Re: 500 THHN with 600A ?

Originally posted by electricaldoc:
AT work today got new compressor info. it has FLA of 385A @460V. We are using 480V, and it is pulling 372A. 385A X125% = 481.25A too much for 500KCM.
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Where did you find the FLA of 385?

Was it a label on the unit as a whole and did it say "MCA" or "Minimum circuit Ampacity" 385?

500 Kcmil copper is rated 380 amps.

If the MCA is 385 amps the 500 kcmil copper is 5 amps undersized. An MCA rating already includes the 125%.

Originally posted by electricaldoc:
What I am still not understanding here is every one still looking at FLA. I was understanding that you used FLA X 125% to figure out what size OCPD and conductor size would be needed, But if you install a 600A breaker than conductors had to be rated at 600A or more?
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No, you do not need 600 amp rated conductors for this.

For conductors feeding motor loads the conductors are based on the load and can be rated much smaller than the breaker or fuse protecting them.

This part is important!

On a 'normal' branch circuit the breaker or fuse provides overcurrent, short circuit and ground fault protection.

With a circuit feeding a motor the breaker or fuse only provides short circuit and ground fault protection, It does not provide overcurrent protection

The conductors are protected from an overcurrent condition by the motor overloads typically located beneath the starter.

Originally posted by electricaldoc:
I mean I can not run a motor that has FLA of 5A and say 5A X 125%= 6.25A so 16 or 18 AWG will handle motor. Then hook it to a 20A breaker?
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Actually you might be able to do just that.

Originally posted by electricaldoc:
SO regardless of motor FLA if I have 600A breaker conductor must be able to handle 600A. Correct?
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Nope, that is incorrect.

Originally posted by electricaldoc:
P.S. The fuses the compressor company installed to handle this laod was 600A. So now we do have 600A breaker going to 600A fuses with 500KCM between them. Also electrician for the compressor company ran two 350KCM in parallel on bottom of disconnect. So we have 500KCK feeding two 350KCM in parallel. This all seems very wrong to me. :confused:
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The two sets of 350s are more than adequate.

The only question that remains is if you read an FLA (Full Load Amps) or an MCA (Minimum Circuit Ampacity) from the label.

As a last example you have an HVAC unit, from the units label you see the following info.

Minimum Circuit Ampacity 25 amps

Maximum Over Current Protection 45 amps

Given that information you could use 12 AWG copper conductors and a 45 amp breaker.

This would be perfectly code compliant.

[ March 28, 2005, 04:33 PM: Message edited by: iwire ]
 
Re: 500 THHN with 600A ?

I got the FLA off of the name plate on the compressor.

Thanks every one I see my error now. The differance between normal branch circuit and motor overcurrent protection.
:)
 
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