300.20 (B) and Knob and Tube

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mphillipps

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300.20 (b) states that individual conductors pass through a single opening. When splicing K&T together with NM, I've always passed the K&T (hot & neutral) conductor and loom through two separate clamps with the NM passing through a third. Recently I've been asked to clamp the two K&T wires through a single opening. With a little bit or reading, I understand this is to prevent inductive current and heat.

My issue with 300.20(B) and K&T is that the hot and neutral are clamped closely together and if they were in separate holes there would be less chance of arching and the insulation of the loom would prevent inductive current.

My questions are: Does 300.20(B) apply to the splicing of K&T to NM? If yes, is there a preferred technique the splice K&T with NM? For example using a plastic box.
 
While the code requires both of the K&T conductors to go through the same opening, it was never done that way on an original K&T installation.

The inductive reactance is not a real issue at the current levels used with K&T. In fact it is not a real issue until the current exceeds 200 amps. The Canadian Electrical Code equivalent to our 300.20(B) does not apply below 200 amps.

That being said, it is a violation of the rules in the NEC to run the conductors through separate openings, no matter what the current level is.
 
Put conductors in separate holes then cut a slot between the holes and you will satisfy all the requirements;)
 
Currently I'm using metal boxes. The wire and loom are currently sized and routed to enter the boxes at 90º (separate sides of the box). My understanding of the code is, if I were to replace the metal boxes with plastic and use two separate holes, I'd be in compliance with the code and the K&T would be much better routed?

True or False?
 
iwire said:
300.16 requires that K&T enter through seperatly bushed holes in the enclosure.
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How does that work...there are no provisions in the code that actually permit a rule in a Chapter 1-4 article to modify another rule in a Chapter 1-4 article?
 
mphillipps said:
Currently I'm using metal boxes. The wire and loom are currently sized and routed to enter the boxes at 90º (separate sides of the box). My understanding of the code is, if I were to replace the metal boxes with plastic and use two separate holes, I'd be in compliance with the code and the K&T would be much better routed?

True or False?
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What is your justification for "better routed"?

First thing with K&T is the open conductor needs to be either in free air or attached to a support insulator, second is it needs to be in a tube where passing through framing/other building components, third is where it does not comply with the previously mentioned items, it usually needs to be in flexible tubing or "loom" - often common at outlet boxes or other points of termination, the tubing can touch the tubing from the other conductor of the circuit.

Next best thing when it comes to routing conductors is to keep conductors close as practically possible to lower EMF's, which is part of why we are supposed to run all conductors of same circuit in same raceway or cable - but that rule most likely came about sometime after K&T became "legacy" instead of current common installation methods.
 
ActionDave said:
I have always wanted to know if this means you are allowed to solder NM to existing K&T?
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Well, you would have to make sure that the joint was mechanically sound before the solder was added, which was probably the case when K&T was the only way to go. It is certainly required by current code for new soldered joints in legacy systems.
Are there any heat problems in trying to solder NM with a hand soldering iron you heat up with your blowtorch?
 
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