Marathon installs like any standard electric tank water heater — same footprint, same connections, same code requirements. No special accommodations for the composite tank; the install procedure is identical to a Rheem or AO Smith electric tank swap.
Electrical requirements
| Model | Voltage | Element wattage | Breaker | Wire |
|---|---|---|---|---|
| MR40/50/55/66/75/85/105 (Standard) | 240V | 4500W upper + lower | 30A double-pole | 10/2 NM |
| MHD40/50/75/85 (Heavy Duty) | 240V | 5500W upper + lower | 30A double-pole | 10/2 NM (some installs 8/2) |
| MRH38/50 (Lowboy) | 240V | 4500W upper + lower | 30A double-pole | 10/2 NM |
Critical electrical specifics
- Dedicated circuit — only the water heater on this breaker, per NEC 422
- Service disconnect within sight per NEC 422.31(B). Most installs use a 30A disconnect switch mounted within 50 feet
- 208V not recommended — Marathon is rated 240V. Operating at 208V (some commercial buildings) drops element output by 25% and stresses thermostats
- Verify ground — bonding lug must connect to system ground
Plumbing requirements
- ¾" NPT cold inlet and hot outlet
- Shutoffs on both inlet and outlet (cold supply shutoff is the primary; outlet shutoff aids service)
- Dielectric unions where copper meets the steel cold inlet nipple — Marathon\'s composite tank doesn\'t need dielectric protection, but copper-to-galvanized connections elsewhere in the plumbing do
- Expansion tank required on closed-system installs (post-meter check valve, pressure regulator, or backflow preventer)
T&P relief valve
- Marathon ships with T&P valve installed; verify orientation
- Discharge tube: ¾" copper or CPVC; minimum 4" off floor, no traps, terminate within 6" of safe surface
- No threaded cap on discharge end — must be open
- Do not relocate T&P valve to side or other location — keep at factory location
Drain pan
- Required on installs above living space OR indoor installs not in a basement floor drain area
- 22" or 24" plastic or galvanized drain pan
- ¾" PVC drain line from pan to floor drain or condensate pump
- Pan drain must terminate at safe location (sump, floor drain, exterior)
Clearances
- 0" rear and side clearances on most installs (Marathon composite outer jacket is rated for direct contact)
- 24" front for service access
- 12" top minimum for plumbing connections
- No combustion air requirement — electric tanks don\'t consume room air
Specific Marathon considerations
- Lighter than equivalent steel tanks — Marathon MR50 weighs ~85 lb dry vs ~140 lb for an equivalent steel tank. Easier handling during install
- No anode access port — composite tank doesn\'t have one. Don\'t look for it
- Standard ¾" element ports — Marathon elements use industry-standard threading, interchangeable with mainstream tank elements at equivalent wattage
- Standard thermostat mounting — clip-on thermostats interchangeable with mainstream electric tank thermostats
Lowboy install (MRH38, MRH50)
- Short-height tank (~33" tall) for mobile homes, crawl spaces, under-counter
- Larger diameter than standard tank — verify clearance in install location
- Standard 240V 30A electrical and ¾" plumbing
- Common application: mobile home replacement of old narrow lowboy
Heavy Duty install (MHD series)
- Same external dimensions and connections as Standard residential
- 5500W elements draw slightly more amperage at startup (~26A vs ~21A for 4500W)
- 30A breaker still adequate; some installs use 8/2 wire for headroom
- Service disconnect same as Standard
Permits
Most US jurisdictions require permit for water heater replacement. Like-for-like swap (electric for electric) is rarely inspected if a homeowner permit. Change of fuel (gas to electric) requires permit, inspection, and possibly service panel upgrade for adequate amperage.
DIY vs contractor
- Like-for-like electric tank swap: homeowner-capable with basic plumbing and electrical skills. Marathon ships ready-to-install
- Change of fuel (gas tank to Marathon electric): contractor work — service panel evaluation, 240V circuit add, possible service upgrade
- Mobile home install: often DIY with the Lowboy MRH model
Bottom line
Marathon installs exactly like a mainstream electric tank — same code requirements, same connections, same procedure. The lightweight composite tank is slightly easier to handle. Skip all anode-rod-related install steps; Marathon doesn\'t have one.