Diaelectric fitting Dielectric Unions - Copper to Steel Pipe

  • POST a QUESTION or READ FAQs about the requirement for using dielectric (dieelectric or diaelectric if misspelled0 fittings when connecting copper to steel piping in buildings and the requirement for a jumper wire across dielectric fittings when grounding a plumbing system.

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Dielectric plumbing connectors:

Building Code & Practical Requirements for Dielectric fittings when connecting copper to steel piping. They're not just dialectic. This article describes the building code and practical requirements for a dielectric fitting when connecting copper piping to steel pipes or steel fittings such as at the top of a water heater.

We explain the effect of galvanic corrosion at copper to steel pipe connections and we include notes on the recommendation for a jumper wire across dielectric fittings or water meteres to assure that a plumging system is properly grounded for electrical safety.

The FAQs section of this article includes questions & answers about the dielectric fitting debate, concerns for fitting leakage, and notes that a brass nipple may be permitted to connect copper to steel in some building code jurisdictons.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Galvanized Steel Pipe to Copper Pipe Connections - Use a Dielectric Fitting to Avoid Corrosion

Water supply piping connection: copper to galvanize (C) Daniel FriedmanIn most jurisdictions plumbing codes require the use of a dielectric fitting when joining steel piping to copper or other metals; some plumbing jurisdictions permit use of a 6-inch brass nipple in this location as an alternative to diaelectric fittings. Why?

When connecting iron or galvanized iron pipes to copper in buildings, often galvanic corrosion and ultimately plumbing leaks will occur at the meeting of these two dissimilar metals.

Using a diaelectric fitting or an approved brass fitting to connect these two metals, or more commonly, using plastic or bronze fittings at the joint between these two metals will avoid future corrosion and leaks.

The photo (left) shows a galvanized iron union used to connect copper to galvanized iron. In the upper image you can just make out the black bronze ring built into this plumbing connector to avoid corrosion where the copper presses against the galvanized iron.

OPINION: in our experience having inspected thousands of homes, we find that original plumbing in older homes rarely made use of a dielectric fitting or even a brass nipple at copper to iron or steel piping - and corrosion is often found at one or several such connections.

In new construction where more careful code compliance and enforcement are applied, dielectric or brass fittings are more common. Some plumbers opine that "lined" nipples sold by some manufacturers for water heater hookup reduce the galvanic corrosion risk at those connections. - Ed.

Water chemistry affects the rate of galvanic corrosion

Diaelectric fitting How do we explain that in some buildings we see direct copper-to-iron pipe connections with no corrosion? Luck? Maybe.

But the corrosivity of the water is probably a factor in how rapidly copper-to-galvanized pipe connections will corrode and leak.

Spelling note that may help some searches: it's not dialectic pipe fittings, but dielectric pipe fittings.

Teflon tape or pipe dope may slow the rate of galvanic corrosion

Another reason we may not always see rapid galvanic corrosion at copper to steel pipe connections is the use of teflon tape (or less-so, pipe dope or sealant compound) that provides a thin non-metallic contact surface between the two dissimilar metals.

Nevertheless, both to comply with plumbing codes and to avoid leaky water pipes, use an approved diaelectric coupling or an approved brass nipple (if local codes permit) when connecting copper to steel pipes or fittings or when connecting copper pipe to the steel fitting at a water heater.

More about the galvanic scale and corrosion between dissimilar metals is at GALVANIC SCALE & METAL CORROSION.

Jumper wires are required to ground plumbing across dielectric fittings

Jumper wires needed at non-conductive pipe fittings (C) Carson Dunlop Associates

At ELECTRICAL GROUND SYSTEM INSPECTION we discuss the need for a jumper wire around interruptions in the metal piping system, such as where a diaelect4ric fitting is installed or where a water meter interrupts continuity.

As Carson Dunlop Associates' sketch shows at (A), if the building plumbing includes dielectric fittings or non-conductive sections of piping (such as plastic piping) then the building plumbing system may not be safely grounded.

A jumper wire is installed to assure that the building metal plumbing pipes are safely grounded to earth. The purpose of the ground jumper is to ground the building plumbing, not to use the building plumbing as a grounding system.

That is to say, we're making the plumbing safe, we're not using it to provide a ground for the electrical system.

Spelling note that may help some searches: it's not dialectic pipe fittings, but dielectric pipe fittings.

Manufacturers of Dielectric Unions / Fittings

Watts Dielectric Union example

Discussion & debate about dielectric fittings

READER OPINION: On your web page ELECTRICAL GROUND SYSTEM INSPECTION, near the bottom, is a question "jumper across dielectric fittings is asking for galvanic corrosion." Your reply to the question concludes with the comment "Also I am nor sure which jumper you saw, but connecting a ground between similar metals ought not create the concern youncite."

That same page includes an illustration [shown just above in this article - Ed] showing this very connection: copper pipe joined to galvanized pipe through a dielectric union with a jumper wire around the dielectric union.

The unilateral recommendation that this connection should be jumpered is concerning: if the union is required to protect the integrity of the pipe and whatever fluid (or gas?) flows inside it, then jumpering around the union defeats that protection and may lead to accelerated failure of the pipe. I think additional consideration on this topic is warranted. Regards, G.H. 7/10/2013

Editor's Reply:

Thank you GH for the comment, and for being interested in this episodic debate about using dielectric fittings and jumper wires around them as well as understanding the effects of galvanic corrosion.

I think we may be mixing two different concerns, so I summarize my understanding below and invite your comment if I'm mistaken.

The diaelectric union has the purpose of preventing corrosion that occurs between two dissimilar metals, such as copper pipe joined to galvanized pipe in a water supply system. In such circumstances the corrosion occurs at the point of contact between the two dissimilar metals.  The diaelectric fitting includes an insulator, typically of plastic, to prevent the two metals from coming into direct contact.

A problem is that the diaelectric union may in some locations prevent the building's metal plumbing system from being safely grounded.

The jumper wire is installed to assure electrical continuity as a safety feature for the building's electrical system. E.g. if a toaster, plugged in, falls into a sink of water, a good electrical pathway between metal plumbing connected to the metal sink (or its metal components) and earth, assures that current flows to ground through that safe path, thus blowing a fuse or tripping a circuit breaker, thus protecting the building occupant from possibly fatal shock.

The ground wire connectors and ground wire do not themselves become points of corrosion in the piping system - at least not at any I've seen, most likely because the wire-to-pipe connectors use an intermediate metal, such as brass, between the copper pipe and (say) galvanized pipe.

We could and should include authoritative citations on a topic such as this - something for which we both should be alert and that I'd be pleased to add to the article.

I would add, though it's a bit too esoteric to imbed in the article, that stray electrical currents, combined with improper electrical grounding can have strange and damaging effects on a building's mechanical system. For example, improper neutral or ground wiring at a building can cause current to flow where it is not expected, causing damage. I found costly damage to an internal refrigeration coil in a water to air cooling system was ultimately traced to improper connections in the electrical panel.

Reader Comment:

While I'm certainly not an expert on the topic, I think the assumption that galvanic corrosion will occur only at the point of contact between the two metals is flawed. Consider for example the "lasagna cell" battery: an uncoated steel baking pan containing a lasagna (or other salty food serving as electrolyte) and covered with conventional aluminum foil. Holes will form through the aluminum foil in the places where it contacts the lasagna -- not around the edges where the foil is touching the steel.

I recently found myself in a sprinkler (irrigation) supply shop. A customer at the counter held a length of galvanized pipe with (part of) an elbow at one end. He explained that it was used in a water feature connecting underground piping to an electric pump above ground and had "rusted out" twice, each time lasting less than a year. There was basically nothing left of the sidewall of the elbow; it had been dissolved away from the outside (I could conclude this because in one area the elbow metal was gone, exposing the threads on the end of the pipe inside it).

There's no way it could have corroded so quickly without a boost from something like galvanic corrosion. But it didn't corrode at a connection between dissimilar metals; it was the side of the elbow that was eaten out. Anyway, I think he mentioned that beyond the elbow (continuing underground) was PVC pipe so there probably was no dissimilar connection in that area at all.

In the case of the wire jumper around the dielectric union, I expect it won't help any to use a brass or bronze clamp -- both are alloys of copper and are very near copper in the anodic index (see "galvanic corrosion" page on wikipedia). For that reason I believe the corrosion will still happen.

If not so, I suppose we'd be using brass or bronze couplers or unions to join the pipes and skip the business with the plastic isolator in the dielectric union. I believe your experience (not seeing corrosion at the location of a jumper around a dielectric union) supports my suggestion that the corrosion is likely to happen somewhere out of sight -- specifically, at a point where the steel pipe contacts an electrolyte (something wet, probably soil).

I do understand (and agree with) the concern about having the plumbing systems grounded for occupant safety. To that end, I'm trying to figure out how to accomplish both mechanical and electrical protection. It seems to me that the galvanic corrosion problem occurs when copper and steel are both in an electrolyte and a conductive path exists between them -- anything that looks conceptually like a copper ground rod and a buried steel water pipe, even buried apart from each other, but with some electrical bonding between them.

Given that we're talking about a plumbing joint of copper and steel, one of these is the supply pipe buried outside and the other is the plumbing through the building interior. Suppose it's copper in the dirt and steel through the building. I guess there's no problem here (and in fact no dielectric union needed) because there's no steel immersed in electrolyte with the copper. But if it's the other way round with buried steel supply, then I think it's imperative that the steel be electrically isolated from all buried copper.

We can assume that the electrical system contains buried copper either in the form of a ground rod at the same building or at another installation nearby (and connected via the neutral wires and ground-bonding at the main service panels).

So, in this case, I think I would isolate the buried steel with a dielectric union and install a bonding wire from the copper pipe back to the electrical panel -- I wouldn't connect wire to the buried steel at all. This building would have to get its grounding some other way, maybe through ground rods, plates, etc.

As I said in the beginning, I don't claim to be an expert on the topic. I'm genuinely interested in your thoughts, and if you have electrical inspectors you consult with regularly, it'd be interesting to hear their thoughts on the matter as well.

Regarding the damage to the refrigeration coil traced back to improper connections in the electrical panel -- that sounds fascinating. If you have the time to write, I'd love to read more about the details of the problem and especially how in the world you managed to find the cause. That must have been a treat to figure out! Ugh!


I appreciate the discussion but it seems we disagree about the locus of galvanic corrosion in plumbing systems and perhaps you didn't know that building codes require both dielectric fittings (or in some locales a brass nipple) connecting copper to steel pipes in a building as well as jumper wires around dielectric fittings when needed to assure safe electrical grounding of the plumbing system.

The jumper wire around a dielectric fitting, properly connected with approved (typically bronze) clamps to the copper and steel piping and a copper ground jumper will not itself promote corrosion at the pipe connection nor nearby.

Plumbing texts as well as field experience point out that corrosion occurs at and/or very close to (micros to millimeters) the point of contact between dissimilar metals that are further apart on the galvanic scale, such as copper and steel. Long-standing and successful plumbing practice was to connect via an intermediate metal such as brass - as the galvanic corrosion problem does not appear at, for example a copper to brass connection (there are hundreds of millions of such connections in both plumbing and heating systems today).

Diaelectric fittings, also referred to as diaelectric unions or DEUs, are required by the model plumbing codes adopted by most states and provinces, though in some U.S. states plumbing codes permit the use of brass fittings in those locations instead.

I would agree that there are episodic debates in the plumbing community about using diaelectric fittings,  out of concern that some leak and others may not work as well as advertised.

Example Code Citations for the Requirement for Dielectric Connectors Between Copper & Steel Pipes

Section 316 of the California Plumbing Code is amended by adding Subsection 316.2.5 to read as follows:

316.2.5 All connections between ferrous and nonferrous pipe shall be made with a six-inch red brass nipple or a dielectric union.

Ord. 559 § 2 (part), 2010: Ord. 543 § 2 (part), 2007: Ord. 499 § 2 (part), 2002: Ord. 463 § 2 (part), 1999. Formerly 15.16.170

Other building citation code sources where the requirement for a dialectric plumbing fitting is cited:


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