I am getting ready to install a Dimension One spa that the manufacture calls for using #6-3 awg wire and 240 Volt - 50 Amp with a GFCI breaker and disconnect.
I had the house pre wired for a hot tub and 240 Volt service. They installed a 50 Amp breaker in the main box and ran about 40 feet of #8-3 G AWG Type NM-B 600V wire to a disconnect box. I know I need to change the disconnect box to a GFI at 5-10 feet from the tub and will run #6 AWG UF wiring from the GFI to the tub.

My question, is the #8-3 wire from the main box to the GFI going to be OK? I talked with the building inspectors office and he sent me a page from the NEC which said the wire was OK for 50 amps (Table 310.16) 2002 edition.

Would a 50 Amp breaker be installed with 8-3 wire, why not use 6-3 wire?

According to NEC 334.80 the ampacity of NM cable (including NM-B) is determined using the 60ºC temperature rating column of table 310.16, which would mean that #8 is 40A and #6 is 55A. So for a 50A breaker it should be #6.

There is a lot of confusion about this subject, and this is part of the code that is often misinterpreted by a lot of people including electricians and inspectors. The confusion stems from the fact that NM-B cable's conductors are rated at 90º... but this doesn't matter, according to 334.80, the 60º column is to be used for all NM cable. I was dinged on an inspection because of this once. I thought for sure that the inspector was wrong but after I looked into it further, I found out that he wasn't. Anyone who doubts this, check out the first sentence in this document (notably what's in parentheses and the chart on page 2) from Southwire: http://appprod.southwire.com/ProductCatalog/XTEInterfaceServlet?contentKey=prodcatsheet6


Here is NEC 334.80:

334.80 Ampacity. The ampacity of types NM, NMC, and NMS cable shall be determined in accordance with the 60ºC (140ºF) conductor temperature rating. The 90ºC (194ºF) rating shall be permitted to be used for ampacity derating purposes, provided the final derated ampacity does not exceed that for a 60º (140ºF) rated conductor... Don't think that because it doesn't specify NM-B that it doesn't apply. NM-B is still considered type NM and there are actually types NMC-B (aka UF, 340.80 says 60º also) and NMS-B too.