electrical question

paul_79

i have a question and can't get a definitive answer. i have a chiller with a 150amp circuit breaker back at the main panel. [ 3 phase 460 volt ] 1/0 wire to a fused disconnect then on to the chiller.

But i can't get an answer to the question why use a  fused disconnect not a regular disconnect.  i see no purpose for the fuses. if you have any electrical code or reason i would appreciate the answer. i will be changing the disconnects this week because of hot lugs and wires burning and would rather put in regular disconnects instead of the fused ones i have.

icesailor

Fused Disconnects:

I'm not an electrician.

That said, I would be wondering why you get so much heat at the fused disconnects. I think it might be a very good idea to remove them if they are tripping out. That way, you can use the length of the wire to act as a heat sink and maybe get rid of some of the excess heat between the panel and the load.

I've seen 30 amp water heater breakers used as disconnects at the heater, get so hot that if you spit on them, they would sizzle and they were drawing over 30 amps. When the load should have been less than 18 amps. And the breaker didn't trip in the panel.

I would prefer to have a fuse as close to the load as possible.

But I'n not an electrician. I just fix them when I can.

SWEI

what kind of fuses?

Unless the breaker has an inverse time curve, those fuses might need to stay.  Regardless, the disconnect should not be overheating unless it was improperly sized in the first place, or there is something amiss in the utilization equipment.  Replacing it without first understanding the cause of the overheating is a very bad idea.



Remember that branch circuit protection is sized to protect wires, not utilization equipment.

EngineerinBoxford

NEC

The requirement for fuses is most likely driven by the words "maximum fuse size" on the chiller nameplate. It could also be because a suitable HACR rate circuit breaker was not readily available or the feeder breaker was already installed at the time that the chiller arrived, Article 440 of the NEC covers Air Conditioning and Refrigeration equipment. IF the nameplate of the chiller does not mention fuse size but rather mentions HACR breaker size or both, and the breaker installed in the feeder panel is HACR rated then a fused disconnect switch can be replaced with a fuseless one. The underlying cause of the overheated wire(s) and lug(s) should be identified and corrected before replacement so that you do not have to keep doing this. Use and amprobe to identify the actual current being drawn on each leg as a start. Is the damage on only one leg? If so, and the current is not imbalanced then suspect loose or dirty hardware at the melting lug. When you perform the replacement, make sure that all terminals are tourqued to the manufacturers requirements.

Garth

paul_79

electrical question

the reason for the change is the lugs were corroded and the wires got hot and burnt  so new wires [ 2/0 ] and new disconnect,  the odd thing was the fuses in the disconnect were 175 amp fuses. The 1/0 wires are rated at 150 amps for the 70 c. temp. rating. The R.L.A. of the chiller is 135 amps.  The disconnect has to be within site of the chiller by code so i can't remove it. 

SWEI

breaker sizing

135 RLA x 125% = 168.75A minimum OCPD size.



175A is the closest standard breaker size, and would be the likely choice unless the chiller manufacturer specified a different MCA.  If the breaker has not been nuisance tripping, it's probably sized OK.  If measured starting and running amps are within range, it's probably working OK.  If either of those is high and the breaker does not trip (check the curve and use a stopwatch) it may need replacement.



In what kind of environment is the disconnect located?  If environmentally-induced corrosion caused the overheating, you might want to examine (and possibly upgrade) the enclosure rating.

paul_79

electrical question

the disconnect is  200 amp rated square D.  the area around is not hot or corrosive. but why a fused not non fused.

SWEI

breakers and fuses

Sorry - re-read your original post and saw that there is a 150A breaker in the panel.  It's actually undersized, unless the manufacturer specified an MCA of 120A or less.



Fuses are not required by code, unless the wires between the disconnect and the load are smaller than the upstream breaker would allow.

Ironman

Or...

If the breaker is not "HACR" rated, then fuses are required.

SWEI

right

Hence my original comment about the inverse time curve (typically specified on larger breaker like this.)

paul_79

electrical question

so from what calculations get from you is at 135 amps x1.25 = 168 amps for continuous

load rating and my nec. book says 1/0 rating is 150 amps at 75 c  then this wire job was  undersized from the beginning.

paul_79

electrical question

or my  thinking is that the 135 amp  divided by the wire rating of 150 amp = 90% continuous rating  and the wiring is ok 

Techman

Fuses? Breakers?

It is all up to the chiller mfrg nameplate info. If the nameplate says "Max Fuse Size" then fuses are required either @ the "loadcenter" or the disconnect. If the nameplate states "Max Fuse OR HACR Circuit Breaker size" then the disconnect can be non-fused or in a slightly different setup the disconnect will be required to be fused or circuit breakered. then the next  is "Maximum rating of overcurrent protective device" which can be fused ,non-fused ,HACR breaker or standard breaker.This maybe slightly confusing so the best way is consult w/ a good electrician in your area for the latest updates to the N.E.C.and your prevailing local electrical codes.The overheating is a different problem and as others have said ,find out why it is getting hot.What type of  insulation is on the 1/0 wires? 60*C ,75*C or 90*C ? What is the phase to phase voltages and amps? And I think the C.B. is good for only 80% of the C.B. rating so that 150a breaker is good for 120a .

clammy

fused over non fused

Paul .i always use fuseable disconnects where they are required the reason being is that i have seen contactors that where totally pitted and carboned up and would blow the fuses in the disconnects while with  the non fused disconnect the breaker would not trip and the wires would over heat some times burning or degrading the insulation and the compessors leads and termials .I always thought of it as a cheap insurance even though larger fuses are not cheap it is cheaper then a compressor burn out and can give you a hint of possible electrical promblems out side of the equiptment like low incoming voltage, high amp draw or electrical grid power draws from the building that are imbalanced .Hope that helps peace and good luck clammy

SWEI

wire sizing

Determines breaker size, and both carry 80%.  You can get 100% rated breakers, but then you still have to upsize the wire to 125% of that.



#1/0 THHN has a maximum ampacity of 170A @90C, but since the breaker terminals are almost certainly rated @75C, the ampacity drops to 150F.  Use the 90C rating for temp & multi-conductor deratings but the max after all derating is still limited to the lowest in the chain, usually the terminal temp rating.  #2/0 ampacity is 175A @75C so you have the correct wire size now.  Change out the branch breaker to a 175A unless the chiller manufacturer specifies otherwise.

Techman

Fuses/Breakers

The "reason" for all of this is something called  a"short circuit test" and if the piece of equipment passes the "short circuit current" test using a fuse or a breaker .

paul_79

power

thanks for the information. here are some pictures of the burnt disconnect wires

SWEI

zow

That's some pretty significant damage - its the wire aluminum, or did all of that actually sputter off the terminal?



Oh, and the breaker is an I-Limiter, just what you want for a motor-driven compressor.   Undersized, as previously mentioned.  It's possible that the current-limiting curve on the breaker interacted in an unpleasant way with the motor, but given that all the damage is on one phase I'd be more inclined to suspect a problem in the starter or even the motor itself as the trigger.

paul_79

electrical question

zow is right on. i am  going to have the compressor megged and checked over from head to toe. this was copper wire and it melted at the lug.

Garfield

Bad connection

Most likely just a high resistance connection from an improper install. The breaker, fuses, overloads etc would trip first if it were a load issue. You can probably just replace the lug..... You won't know until you pull the lug off and see the extent of the damage. Fuses may not be required but they have a higher interrupting rating than breakers and may have been specced. As far as fuses or breaker undersized this is not a problem if they're not tripping.

Techman

Nametag

what does the nametag info say?

paul_79

electrical

this is the disconnect panel name tag. oh and here is a picture of a pilot who was having a bad day. he took off and heard some noise,then he circled around to land and check out the noise. well as he was touching down his prop started to hit the runway that is when he realized he had not put his landing gear down. after he skidded down the runway on the belly and damaged the prop , engine, ect. well the bad luck just would not stop. they lifted the plane up and put the gear down and were towing it back to the hanger the left gear collapsed and damaged the previously undamaged wing. the guy could not catch a break. no one was hurt in this incident

Techman

Chiller

 nametag electrical spec's.