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Joined: Apr 2012
Posts: 5
New Member
I need to replace a 350 amp I-line with a 225 but in searching online with the vast number of breakers out there and different price how do you which is what you need. All I can find is type LA and current rating no catalog # on it. It is for a new roof top unit.

Thanks Shamrock

Joined: Jul 2001
Posts: 599
JBD Offline
You need to know:
Current Rating
Number of poles
Voltage of system
Available fault current

When in doubt, you should at least match the specifications of an existing breaker (hopefully they are correct for the application)

The current family offerings would be a QD @240V and a JD at 480V.
The previous families were Q2 @240V and KA @480V.

Joined: Jun 2004
Posts: 1,273
You've an option: modern breakers have adjustable ratings. (Limited to specific models, of course.)

I'd simply call up SqD and talk to their tech. What would take you hours is something he'll know off the top of his head... and he's free to you.

Embedded 'free' technical assistance is why you pay so much for such breakers.

You also have the option of dropping in a fuse set -- cut in a safety switch -- and are you CERTAIN that it's 225 A that you need? Will you have to set a safety switch/ disconnecting means ANYWAY? Can this be downsized to the next lower frame size?

[ Disconnecting switch frame sizes: 30, 60, 100, 200, 400, 800, 1200... BTW, they do get huge. ]

[ Disconnecting switch voltage thresholds: 240 VAC, 600 VAC ]

If the conductors are already pulled, and sized for 350 A, then you don't need a new breaker at all. Just downsize your safety switch fusing. 225A fuses are the same frame size as 350 A fuses -- in the same class.

( J type fuses require that you fiddle with the safety switch fuse mount screws. That should take no more than a few minutes to complete. )

Again, stay out of the engineering profession if at all possible. There's no upside -- it's all downside.

You don't ever get paid for your efforts -- but will entirely take the heat if you're wrong in any way.

Last edited by Tesla; 07/22/12 01:46 PM.

Joined: Apr 2012
Posts: 5
New Member
Thanks Tesla,

I did have the chose of cutting in a dis but that would have required putting in a 400 dis and it would have had to be a nema 3r and the existing wire was 250 mcm which wasn't correct to start with. So I don't know if something had changed since original wiring, but I would like to walk away know I did something correct.


Joined: Jun 2004
Posts: 1,273
Actually 250 mcm sounds like it might be correct IF the load is covered by 430.

There is a strong tendency to over wire motor loads.

Under 430 a 50 Amp C/B can protect a #12 conductor.

If your calculations show that you're needing only 250A at the C/B -- AND you're dealing with a motor load, then your conductors seem fine.

It's looking like the prior EC may have been building to engineered specs.


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