wire size

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getwyred

Member
Just a quick ?, can you use the 75deg.C column in 310-16 if you termination points are rated for 75deg.C :confused: with the amperage being under 100 amps.

Thanks
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: wire size

If the wire, terminations and equipment are OK for 75 C you can use that column.

NM must use the 60 C column regardless of the markings.
 

getwyred

Member
Re: wire size

Well what I am trying to figure out for sure is I am wiring up a spa for a friend and was planning on using 3 # 8's and a 10 ground which is good for 50 amps in th 75 deg. column, but the wiring diagram that came with the spa says do not use smaller than # 6 awg can you help me with this, the run is only about 30 feet.

Thanks James

[ April 08, 2004, 12:24 AM: Message edited by: getwyred ]
 

getwyred

Member
Re: wire size

What I am trying to figure out is if there is something I am not seeing in the code, or if the mfg is making their own code. not whether I should run 6 or 8 " don't want to void their warranty".
 

roger

Moderator
Staff member
Location
Fl
Occupation
Electrician
Re: wire size

Do as Todd says and follow the Manufacturers specs, if you do not, you will indeed void the waranty.

Roger
 

ryan_618

Senior Member
Re: wire size

Well what I am trying to figure out for sure is I am wiring up a spa for a friend and was planning on using 3 # 8's and a 10 ground which is good for 50 amps in th 75 deg. column, but the wiring diagram that came with the spa says do not use smaller than # 6 awg can you help me with this, the run is only about 30 feet.

Thanks James
Hi James. If this is a residential spa, they are probably assuming that you would be using type NM cable. Are you? If so 334.80 is going to limit you to the 60 degree column (as Bob mentioned).
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: wire size

I just ran a voltage drop calculation, using the free calculator on Mike Holt?s home page. You would not have a voltage drop problem with #8. But the NEC is focused on minimum requirements for safety. Anyone, from the manufacturer to the designer to the contractor to the owner, and even the local AHJ, can insist on more strict standards than are contained in the NEC. You could ask the manufacturer why they require #6, but knowing the ?why? would not change the fact that it is their requirement. You could disregard their requirement, and stay within the NEC, by using #8, but you would do so at your own risk. I don?t recommend it.
 

getwyred

Member
Re: wire size

Thanks for the input, one more thing that the code book doesn't answer and the guys at work don't seem to know. I would like to know why romex can only be sized according to the 60deg. chart. Just curious, they didn't go over some of this stuff in the apprenticeship.

thanks again
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: wire size

I would like to know why romex can only be sized according to the 60deg. chart.
Part of the reason is that NM is often installed in areas where it will be surrounded by thermal insulation that will limit the heat dissipation from the cable. There are some tests that show if you surround the NM with insulation (an example would be blown insulation in an attic or wall cavity) and load the cable to its maximum permitted 60?C ampacity, that the conductor temperature will actually exceed 90?C.
Don
 

tom baker

First Chief Moderator
Staff member
Re: wire size

And recall the reason why (exactly what Don posted) we went from NM to NM-B in 1982. NM was overheating. It wasn't a problem with Knob & Tube or early NM as buildings were not insulated. The ampacity of a conductor is based on its ability to dissapate heat.
 

peter d

Senior Member
Location
New England
Re: wire size

I have run into this too. I called a spa manufacturer to ask why all conductors must be #6, specifically the neutral. I explained that if you operate the spa at 240 volts, most of the load will be L-L, with hardly any neutral load.

Well, the person I spoke to didn't have a clue what I was talking about and told me exactly what the manual said: "Use minimum #6 conductors." :roll:

So much for resource conservation.
 

peter d

Senior Member
Location
New England
Re: wire size

Charlie B,

Anyone, from the manufacturer to the designer to the contractor to the owner, and even the local AHJ, can insist on more strict standards than are contained in the NEC.
I thought the AHJ can only enforce the minimum NEC requirements. I realize that in this case the manufacturer's requirements trump the NEC, but how could an AHJ insist on more than the NEC minimums?
 

bennie

Esteemed Member
Re: wire size

A public official can not enforce a manufacturers requirements. This action would be a conflict of interest. Manufacturers will be paying inspectors to promote their product.

Inspectors can only enforce the laws adopted by the jurisdiction. Manufacturers instructions are not laws.

I think it would be a stretch to void a warranty, with wiring in compliance with the NEC, and not the manufacturers specs. I doubt if it would hold up in court.
 

charlie b

Moderator
Staff member
Location
Seattle, WA
Occupation
Electrical Engineer
Re: wire size

Originally posted by peter d: I thought the AHJ can only enforce the minimum NEC requirements. . . . how could an AHJ insist on more than the NEC minimums?
You are right. I should have put it the way Bennie did:
Inspectors can only enforce the laws adopted by the jurisdiction.
The jurisdiction can enact laws that require more than the NEC. If they do, then the Inspector, acting as the AHJ, and enforcing the local laws, can thereby enforce requirements beyond those of the NEC.
 

iwire

Moderator
Staff member
Location
Massachusetts
Re: wire size

Inspectors can only enforce the laws adopted by the jurisdiction. Manufacturers instructions are not laws.
Isn't it the manufacturer who knows if the terminals are good for 60 C or 75 C?

If the manufacturer knows the terminals are 60 C then the NEC would require 6 AWG for this 50 amp branch circuit.
 
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