Sizing fuses in a fused disconnect

[pedorrero79]

Hello all,

I need a little bit of help clarifying my thought process on this. When sizing the fuses for a fused disconnect, should the fuse(s) be the same size as the breaker protecting the branch circuit?

For instance, my ME is installing humidifiers that have an FLA of 61.3A and the OCP, per the manufacturer, is 80A. My understanding is that these are considered a continuous load. However, we have to provide a fused disconnect at the unit. Normally, when we provide disconnects, the disconnect is sized for 115% of the FLA, correct? If so, that leads me to believe that we'd have a 100A disconnect, but what size fuses should be installed? I'm thinking 80A, to match the OCP on the branch circuit - any smaller, and we'd run the risk of nuisance trips, any larger, and it won't effectively protect the circuit. Am I right in thinking this?

I'll be honest, I have nothing concrete to back up my thought process on this, so if anyone here can point me to a code reference that will lead me in the right direction, I'd appreciate that. BTW, I'm in CT where we're still on the 2005 NEC.

Thanks in advance,
Andy

 

[renosteinke]

While fuses and circuit breakers are both referred to as 'overcurrent protective devices,' they are very different things. They do not get sized the same, and it's very common for a circuit to have the two sized differently.

To make things even more confusing, there are two distinct types of fuses, and each is sized differently.

Finally, different types of loads have the overcurrent devices sized in different ways.

First, let me take the example of a "30 amp circuit." You have to ask: what is it for? If you're powering a clothes dryer, you need #10 wire, and you would size the breaker and fuse the same way, for 30 amps. You'd use a 30 amp disconnect. Pretty straightforward.

Now, let's change the load to an air conditioner. Chances are, #12 wire would be allowed, and a 40-amp breaker. This is to allow for the moment of high current that the unit draws on start-up ..... a concern not shared by clothes dryers. It all comes down to what is on the nameplate.

Let's look at a simple motor load. Just for fun, we'll make it a 1-1/2hp, 115v motor. The NEC tells me I need to size the wire for 20 amps, so it's #12. Yet, my nameplate (for example) might say only 13.6FLA. To allow for the large current required at start-up, I have to use a breaker or fuse a LOT larger than 13.6 amps. Or, even, 15 amps.

If it's a breaker, I'm allowed 2.5 x 13.6, or 34 amps. Since the next standard size is 35 amps, you can use a 35-amp breaker.

If it's a fuse, I have two choices: a 'fast' fuse or a 'time delay' fuse. A 'fast' fuse can be as large as 3 x 13.6, or 40.8 amps. Next standard size is a 45-amp 'instant-trip' fuse.

If I choose the far more preferred 'dual element' or 'time delay' fuse, I use 1.75 x 13.6, or 23.8. This would be a 25-amp fuse.

In any case, a motor would need a disconnect that was rated for 1-1/2 horsepower, not any particular amperage.

These figures come from various code sections, the main one being table 430.52 What is important to note is that the sizes of the different OCPD's are different for each type.

Now, you might still be worried about 'protecting' a 13.6-amp motor with a 35 amp breaker. Well, that's why we have 'heaters,' or overload relays, as part of motor starters. Being very slow to respond, we can set these overloads very, very close to the actual current drawn by the motor; they might even be less than the FLA. That's where 'professional judgement' come in.

 

[petersonra]

Why would you want a fused disconnect at all? A non-fused disconenct would make a lot more sense most of the time.