Feeder Disconnect/OCPD Sizing

[aNerdyEngineer]

I'm struggling with the way to size the protection on a feeder circuit. Our typical system design will have a through-door fused disconnect switch or circuit breaker. The issue I'm finding is the definitions of 'disconnect' vs 'over current protective device'. I would interpret a through-door fused disconnect switch or circuit breaker applicable to BOTH being a disconnect and OCPD. That being said, there are two governing sizes:

1. Disconnect per UL508A:30.2.2.C, NEC:430.128, NEC:430.110, NEC:610.33 (we do cranes & hoists)
2. OCPD per UL508A:32.3.1, NEC:430.62(A), NEC:610.41 (we do cranes & hoists)

If 1) says I need a MINIMUM of 115% sizing for a disconnect and 2) says I have a MAXIMUM size per a combination of the loads, how would a combined device ever meet both of these? What am I missing?

 

[Jraef]

You use something that is above the minimum and below the maximum. I don’t see the dilemma here, you may be over thinking this.

 

[aNerdyEngineer]

Thanks Jraef, I guess I feel like there will be many scenarios where the MIN calc is greater than the MAX calc which would yield no solution. I think as a fused disconnect switch, say 60A, I'd need the 60A to be good for sizing per the 'disconnect' codes (MIN), and the actual fuses for the 'OCPD' codes (MAX). This is not possible with a CB then...

 

[olsondavidj]

Couple things here...

First, a CB isn't covered under UL 508A 30.2.2 as a disconnect. If you're using a CB for your main disconnect, you need to look at 30.2.1, which states that the CB shall not carry more than 80% of the panel load (stated the other way around: the CB must be sized for at least 125% of the load). That said, there is such a thing as a "100% rated" CB, and UL has an exception for those.

Second, re-read 32.3.1. It states that the main OCPD shall be no smaller than the LARGER of (a) or (b). So theoretically yes, sizing the OCPD using 32.3.1(a) can certainly end up with a number smaller than 115% of the load, meaning your MINIMUM disconnect is larger than your MAXIMUM OCPD, which causes the conflict you're talking about. I think it would take a large number of similarly sized loads to do this, or 1 big load and several small loads, but it can happen.

But there is a saving grace here: 32.3.1(b) optionally allows you to size the feeder OCPD based on the ampacity of the conductors on the load side.

In practice, what this means is that in the oddball scenario where 30.2 > 32.3.1(a), you would simply up-size your conductors coming from the disconnect to match the OCPD.

This can of course have a ripple effect downstream, so you'd need to re-analyze things downstream to make sure they're still in compliance.

Finally, I didn't look at your NEC references... I don't do cranes so I'm not really familiar with those parts of the code. The above analysis based strictly on UL, but I've never had a situation where UL 508A directly contradicts NEC.

On that note: UL 508A has a section for crane controls I think... double check that to make sure there aren't special requirements that override the above sections.

 

[olsondavidj]

In practice, what this means is that in the oddball scenario where 30.2 > 32.3.1(a), you would simply up-size your conductors coming from the disconnect to match the OCPD.

Sorry, I re-read this and realized that it's a bit unclear...

So let's say that 30.2 says you need a 60A disconnect, but 32.3.1(a) calls for a 45A OCPD. If you used a circuit breaker, it would have to be a 60A breaker. Therefore, to satisfy 32.3.1, you would need to up-size your load side conductors such that they have a minimum ampacity of 60A.

In the case of fused disconnects, the fusing does NOT need to match 30.2. So in the above example, you could use a 60A fused disconnect and just put 45A fuses in it.