wire calculation

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Re: wire calculation

If there are motor loads involved - especially larger motors, then it is conceivable that a serious analization of Art. 430 rules on minimum ampacity of feeder/service conductors & their maximum protection, would allow (3) 500KCMIL per phase.
 
Re: wire calculation

Hi Taco, First of all the conductors must have an ampacity capable of carrying the load as calculated in accordance with Art. 220. Service conductors are not provided with "overcurrent" protection, they are provided with "overload" protection. An overcurrent device is used for this purpose and located at the termination of the service conductors. Where as overcurrent protection is provide where the conductors receive their supply (except for tap rules). See 230.90(A), Exc. No. 2. However, since the load will be greater than 800A, the exception is not applicable. So it looks like your going to have to go with 600 Kcmil conductors in separate raceways or cable.
John C.
 
Re: wire calculation

Let me clarify my statement above just a bit before I get attacked.

If you desire a full 1200A feeder or service, then yes, you need (3) 600 per phase.

What I was saying is that the allowance that is frequently used on motor branch ckts., (where the OCP may be higher than the conductor ampacity) is also applicable to feeders & services.

For services - 230.90(A) EX.1 & EX.3 give insight.

It is conceivable that a certain scenario would allow (3) 500 per phase terminating at a 1200A
service OCD.

A very frequently asked question on this Forum is "what is the NEC Art. 220 calculated load ?"
 
Re: wire calculation

He might be thinking of the part of the code where you go up to 800 amps and the next size to the next size can be used. I believe thats...ummm ;)
 
Re: wire calculation

Well lady, you said it.
I'll call you wrong. Not trusting (2) 500KCMIL with 800A OCP is just wasting good material & labor costs.
 
Re: wire calculation

The 500 kcmil is not a problem and the conductor damage curves show that the 500 kcmil is well protected with the multiples of 400 ampere overcurrent devices. The problem is with the devices that the 500 kcmil terminates into. Overcurrent devices, especially circuit breakers, use the mass of conductor for a heat sink. When a circuit of 4 - 500 kcmil is used for a 1600 ampere circuit instead of 4 - 600 kcmil, the overcurrent device does not have enough copper for a heat sink and will not operate properly.

If you look at the proposals and comments for CMP-10 for last cycle, you will see more of what I am talking about. I was trying to make a change to permit multiples of 500 kcmil to be used for up to 1600 amperes if the overcurrent device had been listed for the purpose. :D
 
Re: wire calculation

Lady, 500 kcmil is rated for 380 amperes and can not be damaged under overload conditions because the overcurrent protection will open before any damage to the conductors could happen. In other words, it doesn't really matter what you do with the conductors or how they are loaded, the overcurrent device will protect them.

The rules in 240.4(B) are really for the overcurrent device. Take a look at my previous post. :D
 
Re: wire calculation

Originally posted by charlie:

The rules in 240.4(B) are really for the overcurrent device. Take a look at my previous post. :)

PS Charlie, not disputing you, just trying to see why it would help using 500kcmil, and I know 600kcmil is an odd ball size, but wouldn't it provide the proper ampacity - 1260A opposed to 1140 A for a 1200A service?

[ April 22, 2005, 04:51 PM: Message edited by: lady sparks lover ]
Click to expand...
 
Re: wire calculation

Lady,

The ampacity of 500 kcmil THHN copper conductors, with 600v insulation, in an ambient temperature of 86?F, with not more than 3 current carrying conductors in the raceway, is 430A. Changing any of these conditions of use will change the ampacity.

Most breakers have lugs rated at 75?C which requires that the ampacity be reduced to 380A. This (as I understand it) is required to protect the breaker, not the conductor. Article 240.4(b), by allowing the next larger breaker to be used, implies that breakers are not damaged by the small increase in current that results.

Edit: So if you believe that breakers can be damaged by the additional current, and choose to change the specs on a project accordingly, I respect that. Just don't expect a contractor, once he has won a bid, to oversize the conductors on his own.

[ April 22, 2005, 05:25 PM: Message edited by: sirdle ]
 
Re: wire calculation

Lady, what Sirdle said. Trust me (you can always trust a guy who says, "Trust me."), you can't hurt the conductors. :D
 
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