Circuit breaker as a safety switch?

[jkim780]

Roof Top Unit; reading from unit name plate.
MCA:249A MOCP:250A.
400A C/B as a safety disconnect.

Questions:
1. Maximum Overcurrent Protection device(Inverse time cb)is 250A while Minimum circuit ampacity 249A? Does this make sense? I am trying to contact unit mfgr. if this is right number.

2. 400A c/b mounted on the unit as a safety disconnet. Can you use a higher amp. rating c/b as a safety disconnet while you will have a OCPD at the panel without coordination? Sales rep. told me that this is not sized for trip so it was ok. This doesn't seem right but I can't point out what. Can someone tell me what's wrong.

Thanks.

 

[iwire]

Re: Circuit breaker as a safety switch?

The MOCP must be higher than the MCA, something is definitely wrong with the label.

The 400 amp breaker could be used as a service switch as long as somewhere in the circuit there is an OCP that matches the corrected tag.

A unit with an MCA of 294 could very likely have an MOCP of 400 amps.

 

[GENEM]

Re: Circuit breaker as a safety switch?

I believe you will find this is correct. If you try and size the conductors,with the temperature coeffecient for a rooftop which can go up to 60c or higher depending on your part of the country, you will come up with above the mocp for the unit. The purpose of the oversize disconnect breaker is the length from the unit as well as temperature. If it was sized for the unit it would trip before the feed breakers and quite possibly with compressor start-up.

 

[iwire]

Re: Circuit breaker as a safety switch?

Originally posted by GENEM:
The purpose of the oversize disconnect breaker is the length from the unit as well as temperature.

What does the length have to do with the overcurrent protection?

 

[GENEM]

Re: Circuit breaker as a safety switch?

I am probably getting myself in trouble here but I will try--if someone can do it better please do. The oversize breaker used as a disconnect is many times a replacement for a large disconnect switch and has little to do for overcurrent protection. With a breaker installed that close to a heavy starting load such as a compressor with very little wire resistance, the breaker will most likely sense it as an overload-especially when the breaker is heated to over 100 degrees ambient. Have you ever shorted a circuit and "rattled the pipes" and not popped the breaker due to the voltage drop caused by wire resistance? Or listened to a heavy load starting up do the same? The closer the short is to the breaker the faster the breaker heats up due to less resistance in the circuit. That's about the best I can do--sorry if I can't explain it better.

 

[ron]

Re: Circuit breaker as a safety switch?

What if the short circuit fault current at that Roof Top Unit is 20,000A. A standard non-fused disconnect does not have a withstand rating of much more than 5 or 10kA. The installer had an enclosed CB laying around with the proper AIC rating, and installed it at the Roof Top Unit. Of course he could have installed a fused disconnect switch and accomplished the same thing.
I'm just guessing, but it's possible

[ August 06, 2005, 06:30 AM: Message edited by: ron ]

 

[sceepe]

Re: Circuit breaker as a safety switch?

1. Maximum Overcurrent Protection device(Inverse time cb)is 250A while Minimum circuit ampacity 249A? Does this make sense? I am trying to contact unit mfgr. if this is right number.

Yes, I see it all the time. Never stopped to figure out why. But it is not uncommon. I would guess it has something to do with supplimental heat Vs. compressor inrush current. They use different diversity factors to calculate MCA and MOCP. Depending on the numbers MOCP can come out to be same as MCA.

2. 400A c/b mounted on the unit as a safety disconnet. Can you use a higher amp. rating c/b as a safety disconnet while you will have a OCPD at the panel without coordination? Sales rep. told me that this is not sized for trip so it was ok. This doesn't seem right but I can't point out what. Can someone tell me what's wrong.

I would put a 250 Amp HACR beaker in the panel feeding the unit and a 400 non fused disconnect at the unit and call it good.

 

[iwire]

Re: Circuit breaker as a safety switch?

Originally posted by sceepe:

1. Maximum Overcurrent Protection device(Inverse time cb)is 250A while Minimum circuit ampacity 249A? Does this make sense? I am trying to contact unit mfgr. if this is right number.

.....................I would put a 250 Amp HACR beaker in the panel feeding the unit and a 400 non fused disconnect at the unit and call it good.

This makes absolutely no sense to me, can you explain it?

 

[augie47]

Re: Circuit breaker as a safety switch?

294?? I saw 249

I certainly see no problem contacting the unit mfg., but , in inspecting I've seen the MOCP and MCA clsoe very often on the mfg.s nameplate.

In regard to the 400 amp breaker....the contractor had one surplus from another...beats buying a 400 amp switch

 

[sceepe]

Re: Circuit breaker as a safety switch?

iwire,
one of us misread the original post. I thought he said the MCA was 249 amps not 294. If its 294, then I agree with you something is very wrong.

 

[iwire]

Re: Circuit breaker as a safety switch?

Originally posted by sceepe:
iwire,
one of us misread the original post. I thought he said the MCA was 249 amps not 294. If its 294, then I agree with you something is very wrong.

Ah thanks, I misread the post.

It makes much more sense to me now.

 

[tx2step]

Re: Circuit breaker as a safety switch?

Originally posted by jkim780:
2. 400A c/b mounted on the unit as a safety disconnet. Can you use a higher amp. rating c/b as a safety disconnet while you will have a OCPD at the panel without coordination? Sales rep. told me that this is not sized for trip so it was ok. This doesn't seem right but I can't point out what. Can someone tell me what's wrong.

The sales rep is right -- there's nothing wrong with using a 400A CB for this application. It's no more unsafe than using a 400A non-fused safety switch as the disconnect. You could use a 1,000A CB or switch if you wanted to -- all it does is turn off and lock off the power supply locally at the RTU -- nothing more. Of course, the 1,000A might be larger and cost more than the RTU.