control panel disconnects

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james_mcquade

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i am reviewing prints for several control panels to built. the disconnect sizes range from 30 to 200 amp. i noticed the disconnects did not provide for any fusing, and when i talked to the designer for the project and asked him about the non fused panels, he stated that the panels were protected by the plant busplugs. i have always used fused disconnects to protect the panel wiring. i have never run across this before and i do not have the codebooks with me at present. is there any rules in the nec codebook, nfpa 79, or ul508a rulebooks that require fusing. i left this issue open with the designer and he said if he was in violation, he would be more than happy to correct the problem.

any help would be much appreciated and thanks in advance.

james
 
Re: control panel disconnects

The overcurrent protection device (OCPD)for control panels may be located in the panel or upstream from the panel. Non-fused disconnect switches on control panels are very common in my area, especially for fuse sizes larger than 200A. An OCPD in the control panel but still separated from the disconnect is also acceptable.

Control panels need proper OCPD but they do not have to be fuses.

[ December 19, 2003, 02:06 PM: Message edited by: jim dungar ]
 
Re: control panel disconnects

i have just returned from the jobsite where the control panels are buing built. my new question is shouldn't fuses or circuit breakers be used to protect smaller gauge wiring. for example, on each of these 4 control panels there are disconnects, no fusing. from the disconnecte, the power wiring is broken up into branch circuits which feeds control transformers, power supplies, and motor starters.

i understand that circuit protection can be prior to the disconnect. but a 100 amp fuse should not be allowed to protect a 12ga wire (distance averages 15 ft) rated 20 amps. even less if you derate for bundeling in the wireway. are there any violations that anyone can cite.

thanks again in advance.
james
 
Re: control panel disconnects

James,
Is there overcurrent protection at the load ends of the samller conductors?
Don
 
Re: control panel disconnects

there is overload protection before the contactors, but the only type of overcurrent protection is the plant busplug rated at 100 amps for a #12 wire. from the busplug to the disconnect is an unknown length for now, the disconnect itself is unfused, and there is 15 ft of #12 wire from the disconnect to the overload protection.

thanks for your time & keep up the good work.

james
 
Re: control panel disconnects

NFPA 79
7.2.4.2.2 Conductors sizes of 18, 16, and 14 AWG shall be considered as protected by an overcurrent device(s) of not more than a 20 ampere rating.

7.2.4.2.3 Conductors that do not extend beyond the control cabinet enclosure shall be considered protected by the load branch-circuit short-circuit and ground-fault device(s) where the rating of the protective device(s) is not more than 400 percent of the ampacity of the control circuit conductor for conductors 14 AWG and larger, or not more than 25 amperes for 18 AWG and 40 amperes for 16 AWG.

7.2.4.2.4 Conductors of 14 AWG and larger that extend beyond the enclosure shall be considered protected by the load branch-circuit short-circuit and ground-fault protective device(s) where the rating of the protective device(s) is not more than 300 percent of the ampacity of the control circuit conductors.

Also take a look at NEC Art. 430 Part VI
 
Re: control panel disconnects

harryg,
thanks for the reply. thats a good starting point for me. the conductors are inside the control panel. the load side conductors i have no problem with and i am satisfied with the way
they are wired.
again, thanks everyone & keep up the good work !!
james
 
Re: control panel disconnects

harryg,
which version of the nfpa 79 book are you refering to. i have the 1997 edition & cannot find this section.
james
 
Re: control panel disconnects

the control panels are in place, and i am still concerned about the #12 wires being protected.
as harryg pointed out about the conductor wiring,
i asked the panel designer about the #12 wire being protected with a 60 amp fuse and he responded with the statement "don't worry about it, it's only a short run". i don't consider a short run being over 2-3 ft - much less 15 ft as in the case of his wiring.
the customer is happy with the wiring, but for future references, is there any information reguarding wire length for wire protection as harryg has pointed out?

thanks in advance,
james
 
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